Starry Night 7, eclipses and ... Elmo

Like most two years olds, my daughter loves Elmo. Something about his high-pitched voice and ever-present smile I guess.

Parents innately learn to tune him out, probably to moderate the risk of going insane listening to that same shrill voice on reruns day after day.

One episode that sticks out in my mind, today more than ever perhaps, puts Elmo in a room with none other than David Beckham. The short ends after Elmo, who pesters Becks to teach him to dribble a soccer ball on his head, learns a new word... Persistence.

Persistence is an extremely important quality, and some might say a disappearing quality these days. It goes hand-in-hand with patience I think, something we can all agree is becoming scarce in this age of on-demand everything.

Persistence, as in the case of Elmo and Becks, often pays off.

Over the past few weeks, I've been involved in an exchange over the accuracy of Starry Night's eclipse predictions. One of our "power users" (who shall remain nameless for now) reported what he believed to be an issue with our calculations. They just didn't match up exactly with what NASA (and other government sites) were reporting.

The discrepancy was small; in fact, the timings were exact, to the sub-second but the sizes of the Earth's shadows (both the umbra and penumbra) were too small by a very small but noticeable fraction.

Normally no-one would even notice this, but in this case it changed our prediction for the April 4th, 2015 eclipse from a Total (as reported by NASA) to a Partial eclipse. That made all the difference to a few people.

I sit down at my 'puter. "I'll figure this out", I say to myself confidently:

  • Were we perhaps using an old or imprecise value for the Earth's diameter? Nope. That was in agreement with the USGS.
  • Were we taking the oblateness of the Earth into account? Yup. A precise calculation there too.
  • Was this somehow rounding or another error in calculation? Nope. Everything seems to check out.

After a few hours of this, I checked with my confrere, Bill (our SkySafari developer) to see if they had the same discord with NASA predictions. They did! And even better (worse, it turns out) both Starry Night, and SkySafari, two completely independently created applications, gave almost exactly the same result!

That sealed it for me. Other matters were more pressing. NASA could be wrong too (note that it's not insane to believe this, the pages we were referring to were intended for the public, maintained by a summer student and weren't necessarily vetted by anyone), or could have used slightly different values for the radius of the Earth. Were THEY accounting for the oblateness? Doubts and lack of information made it difficult.

After presenting my findings to Keiron, our head of support, I let the issue go.

Lots of other stuff to work on.

Again, the power user, and Keiron came back to me. Why were our predictions different? We claim arcsecond precision in our planet and moon positions. How could we make that claim if this discrepancy existed?

Again, I chatted with Bill. He said he had been talking with Keiron too. I Don't know what this annoying user thinks he knows that we don't. What could it be? "The Earth's atmosphere?" Says Bill. I dunno. Does NASA take it into account? It's not clear from their site. How can we compare if they don't say? The discrepancy is so small.

Lots of other stuff to work on.

Again, an email from Keiron. "Aren't you busy?" I think to myself. I open it, a curt reply, ready at my fingertips. This time, the message comes with a link.

"Do you think the Danjon rule explains the difference?" he asks:

http://www.eclipsewise.com/oh/oh-help/LEshadow.html

...

Holy s*&t!

...

Not just a clear explanation of what, how, and by how much the Earth's atmosphere contributes to the shadow, but authored by Fred Espenak himself, The Godfather of eclipse predictions! 

This page on the eclipse, with nicely outlined parameters showed something called "Shadow Enlargement":

http://www.eclipsewise.com/lunar/LEprime/2001-2100/LE2015Apr04Tprime.html

... with a link to a clear explanation of what the Danjon shadow rule is, and its value.

It turns out that the Earth's atmosphere contributes around 1/85 (just over 1%) to the diameter of the Earth's shadow... an effect that we were NOT taking into account in Starry Night or SkySafari... but are now! (SN version 7.1.2+, SkySafari ).

Starry Night has been around for almost 20 years in some form or another, and we have never taken this into account, until now.

We could all learn a little from this power user and Keiron (and Elmo for that matter). It pays to be persistent! 

Thanks for keeping at it guys. Starry Night is that much better for it.

After a few hours of this, I checked with my confrere, Bill (our SkySafari developer) to see if they had the same discord with NASA predictions. They did! And even better (worse, it turns out) both Starry Night, and SkySafari, two completely independently created applications, gave almost exactly the same result!

That sealed it for me. Other matters were more pressing. NASA could be wrong too (note that it's not insane to believe this, the pages we were referring to were intended for the public, maintained by a summer student and weren't necessarily vetted by anyone), or could have used slightly different values for the radius of the Earth. Were THEY accounting for the oblateness? Doubts and lack of information made it difficult.

After presenting my findings to Keiron, our head of support, I let the issue go.

Lots of other stuff to work on.

Again, the power user, and Keiron came back to me. Why were our predictions different? We claim arcsecond precision in our planet and moon positions. How could we make that claim if this discrepancy existed?

Again, I chatted with Bill. He said he had been talking with Keiron too. I Don't know what this annoying user thinks he knows that we don't. What could it be? "The Earth's atmosphere?" Says Bill. I dunno. Does NASA take it into account? It's not clear from their site. How can we compare if they don't say? The discrepancy is so small.

Lots of other stuff to work on.

Again, an email from Keiron. "Aren't you busy?" I think to myself. I open it, a curt reply, ready at my fingertips. This time, the message comes with a link.

"Do you think the Danjon rule explains the difference?" he asks:

http://www.eclipsewise.com/oh/oh-help/LEshadow.html

...

Holy s*&t!

...

Not just a clear explanation of what, how, and by how much the Earth's atmosphere contributes to the shadow, but authored by Fred Espenak himself, The Godfather of eclipse predictions! 

This page on the eclipse, with nicely outlined parameters showed something called "Shadow Enlargement":

http://www.eclipsewise.com/lunar/LEprime/2001-2100/LE2015Apr04Tprime.html

... with a link to a clear explanation of what the Danjon shadow rule is, and its value.

It turns out that the Earth's atmosphere contributes around 1/85 (just over 1%) to the diameter of the Earth's shadow... an effect that we were NOT taking into account in Starry Night or SkySafari... but are now! (SN version 7.1.2+, SkySafari ).

Starry Night has been around for almost 20 years in some form or another, and we have never taken this into account, until now.

We could all learn a little from this power user and Keiron (and Elmo for that matter). It pays to be persistent! 

Thanks for keeping at it guys. Starry Night is that much better for it.

Again, I chatted with Bill. He said he had been talking with Keiron too. I Don't know what this annoying user thinks he knows that we don't. What could it be? "The Earth's atmosphere?" Says Bill. I dunno. Does NASA take it into account? It's not clear from their site. How can we compare if they don't say? The discrepancy is so small.

Lots of other stuff to work on.

Again, an email from Keiron. "Aren't you busy?" I think to myself. I open it, a curt reply, ready at my fingertips. This time, the message comes with a link.

"Do you think the Danjon rule explains the difference?" he asks:

http://www.eclipsewise.com/oh/oh-help/LEshadow.html

...

Holy s*&t!

...

Not just a clear explanation of what, how, and by how much the Earth's atmosphere contributes to the shadow, but authored by Fred Espenak himself, The Godfather of eclipse predictions! 

This page on the eclipse, with nicely outlined parameters showed something called "Shadow Enlargement":

http://www.eclipsewise.com/lunar/LEprime/2001-2100/LE2015Apr04Tprime.html

... with a link to a clear explanation of what the Danjon shadow rule is, and its value.

It turns out that the Earth's atmosphere contributes around 1/85 (just over 1%) to the diameter of the Earth's shadow... an effect that we were NOT taking into account in Starry Night or SkySafari... but are now! (SN version 7.1.2+, SkySafari ).

Starry Night has been around for almost 20 years in some form or another, and we have never taken this into account, until now.

We could all learn a little from this power user and Keiron (and Elmo for that matter). It pays to be persistent! 

Thanks for keeping at it guys. Starry Night is that much better for it.

Partial solar eclipse on October 23rd

On the evening of October 23rd a partial solar eclipse takes place, ideally timed for North Americans to observe in the evening. The Moon's first contact with the Sun takes place at 4:25 PM CDT, and the moment of greatest eclipse is at 5:31 PM. These times are for an observer located in Minneapolis, MN; for other parts of the US, the times will vary slightly. Anyone wishing to view the eclipse should view it through special solar observing filters or by projecting the Sun's image. Don't ever stare directly at the Sun, even with sunglasses, and especially not through a telescope!

To simulate the partial solar eclipse in Starry Night 7 look in the Find Panel (the right sidebar) under "Today's Sky" and click on the listing for "Partial Solar Eclipse". Enjoy SN7!

Will you be viewing the partial solar eclipse today?

Double Stars around Boötes

On a May evening many years ago, I made my first exploration of the night sky. The only star pattern I could recognize was the Big Dipper, but with a star chart in a book, I used that to discover the bright star Arcturus in the constellation Boötes.

The curve of the Big Dipper's handle leads to Arcturus, the brightest star in the kite-shaped constellation of Boötes. Surrounding Boötes is an amazing variety of double stars. Credit: Starry Night software

The trick to learning the constellations is to begin with the stars you know, and use them to identify their neighbors. This same technique, known as "starhopping" is the key to discovering all the wonders hidden amongst the stars.

Start, as I did, with the Big Dipper, high overhead as the sky gets dark at this time of year. The stars that form the Dipper’s handle fall in a gentle arc, and if you project that arc away from the Dipper’s bowl, you come to a bright star. This is Arcturus, the third brightest star in the night sky, and the brightest star in the northern sky. Only Sirius and Canopus, far to the south, are brighter.

Arcturus is bright in our sky for two reasons, first because it is relatively close to us, 38 light years away, and secondly because it is inherently a bright star, much brighter than our Sun. Though larger and brighter, it is a slightly cooler star than our Sun, so appears orange to our eyes.

Although Boötes is supposed to be a ploughman in mythology, its pattern of stars most resembles a kite, with Arcturus marking the bottom of the kite where the tail attaches. Notice the little dots over the second "o" in Boötes: this indicates that the two "o"s are supposed to be pronounced separately, as "bow-oo’-tees," not "boo’-tees."

Once you have identified Boötes, you can use its stars to identify a number of constellations surrounding it. Between it and the Big Dipper are two small constellations, Canes Venatici (the hunting dogs) and Coma Berenices (Bernice's hair). To Boötes left (towards the eastern horizon) is the distinctive keystone of Hercules. Between Hercules and Boötes is Corona Borealis (the northern crown) with Serpens Caput, the head of the serpent, poking up from the south.

Although most stars appear to our unaided eyes as single points of light, anyone with access to binoculars or a telescope soon discovers that nearly half the stars in the sky are either double or multiple stars. Some of these are just accidents of perspective, one star happening to appear in the same line of sight as another, but many are true binary stars: two stars in orbit around each other, similar to the stars which shine on the fictional planet Tatooine in Star Wars.

Every star labeled on this map of Hercules, Boötes, and Ursa Major is a double star, worth exploring with a small telescope. Some, like Mizar in the Dipper’s handle, can be split with the naked eye. A closer look with a telescope shows that this is really a triple star. Others require binoculars or a small telescope. Some of the finest are Cor Caroli in Canes Venatici, Izar (Epsilon) in Boötes, Delta Serpentis, and Rho Herculis.

One of the joys of double star observing is the colour contrasts in some pairs. Others are striking for matching colours and brightness. My favorites are stars of very unequal brightness, which look almost like stars with accompanying planets.

Also marked on this chart are three of the finest deep sky objects: the globular clusters Messier 13 in Hercules and Messier 3 in Canes Venatici, and the Whirlpool Galaxy, Messier 51, tucked just under the end of the Big Dipper’s handle. You will probably need to travel to a dark sky site to spot this galaxy. A six-inch or larger telescope will begin to reveal its spiral arms, including the one that stretches out to its satellite galaxy, NGC 5195.

Orion and His Friends and Enemies

On winter evenings, the sky is filled with bright stars, more than at any other time of the year.

On winter evenings, Orion dominates the sky, surrounded by numerous striking constellations, all decorated with brilliant stars.  Credit: Starry Night Software

Central in the southern sky is the constellation of Orion the Hunter. Along with the Big Dipper, this is probably the most easily recognized constellation, and the starting place for a stargazing adventure.

We apologize to our readers in the southern hemisphere, where it is summer. But even in the south, Orion dominates the sky right now. Turn the chart upside down, and everything we say will still apply.

Orion itself sits astride the celestial equator, half way between north and south celestial poles. This makes Orion an “equal opportunity” constellation, well seen everywhere on Earth except at the poles.

The main figure of Orion is a large rectangle of four bright stars, including two of the brightest stars in the sky, Betelgeuse at upper left and Rigel at bottom right. These four stars represent the shoulders and knees of a might hunter.

The thing that most people notice first is the diagonal line of bright stars right in the middle of the rectangle, which represent the giant’s belt, worn at a jaunty angle. Hanging from his belt are three stars representing his sword.

If you’re located at a dark sky site, you will be able to see more details in Orion. His rather small pointy head is represented by a triangle of stars. His right arm raises a club and his left arm raises something towards Taurus the Bull. Some legends have this as a lion’s skin, others as a shield.

I like to see Orion as a superhero beset by evildoers on all sides, but also with friends and allies.

Taurus, to his upper right, is marked by a bright red star, Aldebaran, in the midst of the cluster of stars known as the Hyades. A bit higher is a second cluster, the Pleiades. Both clusters are easily seen with the naked eye. Orion is shielding himself from the Bull with his lion’s skin.

Below Taurus, to the right of Orion, is a meandering stream of stars which early astronomers saw as the river Eridanus. This river meanders below the southern horizon for most people in the U.S.A., but those in southern Florida and Texas may catch a glimpse of its destination, the first magnitude star Achernar.

Above the horns of Taurus is Auriga the Charioteer, marked by Capella, the sixth brightest star in the night sky. I see him as the cavalry riding to Orion’s rescue.

Above and to the left of Orion is the constellation Gemini, the Twins, with its two bright stars Castor and Pollux. Currently this is where the planet Jupiter is located, outshining all the stars. So which is Castor and which is Pollux? I remember them because Castor is closest to Capella, both starting with a “C,” while Pollux is closest to Procyon, both starting with a “P.”

Orion, like all good hunters, is accompanied by his two hunting dogs, big and small: Canis Major and Canis Minor. “Canis” means “dog,” “major” means large, and “minor” means “small.”

Each dog contains one bright star: Procyon in Canis Minor and Sirius in Canis Major. There is only one brightish star besides Procyon in Canis Minor, making it more like a hot dog than a real dog. Canis Major is more like a real dog, sitting up with a head, body, and two hind feet. Sirius and Procyon are the first and eighth brightest stars in the night sky, and among the nearest to the sun at 8.6 and 11.4 light years distance respectively.

Between the two dogs is a faint constellation with a long name: Monoceros. “Mono” means “one” and “ceros” means “horn,” so Monoceros is a unicorn. Although it lacks any bright stars, it is one of the richest constellations in deep sky objects, because an arm of the Milky Way lies in this direction.

What is beneath Orion’s feet? Usually called Lepus the Hare, I like to think of this as Monty Python’s Killer Rabbit, yet another foe for our hero to vanquish.

Everything I’ve described can be seen with the unaided eye, even in fairly light polluted skies. If you have binoculars or a small telescope, there are incredible riches to be discovered, such as the clouds of glowing gas in Orion and Monoceros, the star clusters of Taurus, Auriga, Monoceros, and Canis Major, and the galaxies of Eridanus.

Venus Shines at its Brightest

This week Venus will be shining at its brightest, low in the southwestern sky just after sunset. Venus’ brightness is the result of geometry.

At 2 p.m. EST on Friday December 6, Venus will be shining at its brightest. Look for it in the southwestern sky just after sunset.  Credit: Starry Night Software

As Venus moves around the Sun, closer to it than the Earth, we see it illuminated from all angles.  This causes it to pass through a series of “phases” similar to the moon. When it is on the far side of the Sun, called “superior conjunction,” it is fully illuminated from our point of view, and we see it as a “full Venus.” It is 100 percent illuminated but far away, only 10 arc seconds in diameter.

When Venus is at “greatest elongation,” farthest from the Sun in our sky, as it was on November 1, we see it as a “half Venus.” When it passes between Earth and the sun, as it will on January 11 2014, called “inferior conjunction,” it is illuminated from behind, just like the new moon.

The brightness we see from Venus depends on two things: its phase and its distance from us.  It should be brightest at its “full” phase, like the Moon, but at that time it is at its furthest from us. At “half” phase, as it was on November 1, only half of it is illuminated, but it is much brighter because it is much closer.

As Venus nears inferior conjunction, its illuminated portion shrinks down to a narrow sliver. This causes it to fade in brightness. But it is also getting closer to us, which makes it brighten.  This week, these two factors balance out, and we will see Venus at its very brightest. It is neither “half Venus” (50 percent illuminated, 25 arc seconds in diameter) or “new Venus” (0 percent illuminated, 60 arc seconds in diameter), but somewhere in between. In fact it is 26 percent illuminated and 41 arc seconds in diameter. This is the “Goldilocks point” when distance and  phase combine to produce the greatest brightness.

This week Venus will shine with a brightness of –4.9 magnitude, on the upside-down brightness scale that astronomers use. It is based on the brightest stars being magnitude 1 and the faintest stars visible being magnitude 6. Thus the brighter the object, the smaller its magnitude number. 

Astronomers extended this scale into the negative for really bright objects.  The brightest star in the night sky, Sirius, is magnitude –1.4. The full moon is –12.7 and the sun is –26.8. So Venus this week will be considerably brighter than Sirius, but nowhere near as bright as the moon. It is bright enough to cast shadows, when observed on a moonless night from a dark location.

Even though Venus is the brightest object in the night sky other than the moon, surprisingly few people have seen it in its current apparition. That’s because at this time of year the ecliptic, the path of the planets across the sky, makes a very shallow angle with the horizon in the northern hemisphere. Although Venus is very bright, it is also very low in the sky, so is often blocked by clouds or buildings.

This week, find yourself a location with a low southwestern horizon and look for Venus. Watch it as it slowly sets, and see if you can see it change color from white to orange to red as it nears the horizon, just as the sun and moon do.

Did you know that you can see Venus in daylight? The best time to look for it will be on Thursday this week. Look for the narrow crescent moon in the afternoon sky above and to the left of the sun. Use that to locate Venus, just below the moon. You may need binoculars to first spot it, but once you know where it is relative to the moon, it’s very easy to see.

Comet ISON at Perihelion

Astronomers all over the world are training their eyes and telescopes on Comet ISON as it approaches its closest distance to the sun, called perihelion.

At noon on Wednesday November 27, Comet ISON should be visible at the plotted location in the view of the SOHO satellite’s LASCO C3 camera. The next day it will pass perihelion at 1:44 p.m. EST. If it survives, it will be moving towards the top of the LASCO field. Credit: Starry Night Software

Perihelion will occur at 1:44 p.m. on Thursday November 28, Thanksgiving Day in the U.S.A.

Rather than the traditional football game, we’d suggest you watch ISON’s progress around the sun instead.

While some predictions suggest that the comet may be visible in daylight, we’d rather you didn’t risk your vision by staring at the sun. It might be possible to block the sun from view with a well placed chimney or lamp post, but there is a better way, and it’s 100 percent safe.

There are several satellites in orbit around the sun which are trained on the sun all the time. One the oldest and still one of the best is SOHO, the Solar and Heliospheric Observatory.

SOHO was launched in 1995, and carries an array of cameras pointed at the sun. For our purposes, the most interesting one is the Large Angle and Spectrometric Coronagraph #3, or LASCO C3 for short. This shows a field of view about 32 degrees wide. The sun itself is blocked by a disk at the end of a stalk, its diameter marked by a small white circle. Because SOHO is in space, there is no atmosphere to scatter the sun’s light, so we can see the stars surrounding the sun, just as if we were watching a total solar eclipse. Actually, the view is much better than at a total eclipse, with stars down to about 7th magnitude visible.

Very early in SOHO’s history, astronomers realized that they could observe comets passing very close to the sun, called “sungrazers.” To date, more than 2400 comets have been discovered by careful skywatchers scanning SOHO’s LASCO C3 images. The images from LASCO are refreshed regularly about an hour apart, and relayed immediately to this web page:

http://sohowww.nascom.nasa.gov/data/realtime/c3/512/

Whenever you check this page, you will see the most recent image from space, usually not more than an hour or two old. Go there right now, and you will see the sun’s current coronal activity and, in the background, the stars on the far side of the sun. Over the next few days, if all goes well, you will see Comet ISON’s progress around the sun. It should enter LASCO C3’s field of view at the right side on Tuesday November 26.

ISON will pass just below the sun on Thursday and then move upwards, leaving the field on Saturday afternoon. In our graphic, the yellow circle shows the LASCO C3 field of view, the gray parabola is ISON’s path, and the stars of Scorpius are in the background. ISON is shown in its position on Wednesday November 27.

It’s impossible to say in advance exactly what we will see in the next few days. By watching the comet’s progress in the LASCO camera, you will participate in a great astronomical adventure.

Will the comet break up? Will it continue on to be one of the brightest comets in history? As of this writing, no one knows, but by watching it on LASCO, you will know as soon as anyone.

The Galaxies of Autumn

As the Earth moves in its orbit around the sun, new constellations are revealed in the east as the old ones disappear into twilight. The bright stars of the Summer Triangle will soon be replaced by the even brighter stars of Orion and his winter companions.

Use this chart to locate the galaxies of autumn. Credit: Starry Night Software

During this period of transition, we find ourselves looking outwards from our own galaxy into intergalactic space. This is one of the two best times of the year to hunt for galaxies.

The stars of autumn are relatively faint, as we are looking out away from the disk of our galaxy. To see these fainter stars you may need to travel away from your urban or suburban home to seek out the darker skies of the countryside.

After twilight falls, look eastward to see the Square of Pegasus, formed by four second magnitude stars, about the same brightness as the stars of the Big Dipper. As the Square is rising in the east, it will appear more as a diamond than a square. The leftmost star of the diamond, Alpheratz, marks the head of Andromeda. Two streams of stars extend to the left from Alpheratz, pointing towards Perseus, just below the "W" shape of Cassiopeia.

These constellations provide the framework of relatively nearby stars in our own galaxy, the Milky Way, through which we look locate our neighboring galaxies beyond.

Most people have never seen another galaxy. In fact, most people today have never seen the galaxy in which we live, because the widespread glow of light pollution blocks the Milky Way's faint light. It’s only on rare occasions of major power failures that city dwellers get to see the Milky Way.

To locate our nearest galaxies, look for the middle star in the lower chain of Andromeda, Mirach. Just above it is a fainter star in the upper chain. Draw an imaginary line from Mirach to this second star, and extend it the same distance. This will take you to the Andromeda Galaxy.

What will you see? With the unaided eye, probably nothing, unless you are at a very dark location. However, with ordinary 7x50 or 10x50 binoculars, you will see a tiny faint glowing cloud. Oddly enough, if you point a telescope at this cloud, you will probably see less than with binoculars. This is a case of less being more: the wide field of view of the binoculars sets off the view of the Galaxy perfectly.

This faint glowing cloud may not seem impressive, until you realize that its light has been traveling for more than two million years to reach your eyes. When that light began its journey, our ancestors were just a bunch of small primates wandering on the plains of Africa.

Go back to Mirach and its companion, but this time, follow the line in the opposite direction. This will lead you to an even fainter cloud, the Triangulum or Pinwheel Galaxy. This is a smaller galaxy than Andromeda, but located at about the same distance from us. Even though this is one of the brightest and nearest galaxies in our sky, it is unusually difficult to see. That’s because it is almost at right angles to our line of vision, so we are seeing it in plan view. As a result its feathery edges blend into the background, and we have no sharp edge to catch our eye.

Usually you need binoculars to see the Triangulum Galaxy, but sharp eyed observers at very dark locations have managed to see it with the naked eye, making it the farthest object that the human eye can see unaided.

Partial Eclipse of the Moon

There will be a partial eclipse of the moon on Friday October 18. The moon will pass through the edge of the Earth’s shadow, ending up partially in the shadow of the Earth.

The moon passes through the edge of the Earth’s shadow at dusk on Friday October 18, as seen from eastern North America.  Credit: Starry Night Software

If you look closely at any shadow cast by the sun, say the one cast by your hand on a piece of paper, you will notice that the edge of the shadow isn’t sharp. That’s because the sun is not a point source of light. It is a disk, so the shadows it casts are slightly fuzzy. The solid dark part of the shadow is called the “umbra,” Latin for “shade.” The fuzziness is called the “penumbra,” Latin for “almost shade.”

When a shadow is cast by a nearby object, the penumbra is very slight. When the shadow is as far away as the moon is from the Earth, the penumbra is quite wide.

The graphic shows the situation at maximum eclipse at 7:50 p.m. EDT on Friday night, October 18. The inner circle is the umbra, the outer circle the penumbra. The moon makes it part way into the penumbra, but misses the umbra completely, hence this is a “partial penumbral eclipse.”

For observers in Africa, Europe, and western Asia, the eclipse will occur in the middle of the night when the moon is high overhead. The partial shading will be visible as a slight reddish dimming of the normally bright full moon.

For observers in North America and South America, maximum eclipse will be around the time of moonrise, which is also the time of sunset. This will make the eclipse difficult to see, because we will be looking for a slight dimming of a moon which is already dimmed by passing through a great deal of the Earth’s atmosphere.

The farther east and north you are located, the better your chances of seeing this eclipse. For example, in New York City, the moon will rise at 5:59 p.m. EDT, and will be at an altitude of 20 degrees at maximum eclipse. In Chicago, moonrise is at 5:54 p.m. CDT and the moon’s altitude only 9 degrees at maximum eclipse. In Los Angeles, moonrise is at 6:09 p.m. PDT, more than an hour after maximum eclipse, so the chances of seeing the eclipse are zero.

For observers in North America, the effects of the moon’s shadow will be most pronounced on the lower right corner of the moon. The shadow will probably be more pronounced in photographs than with the naked eye, so this is a good opportunity to get out your telephoto lens and photograph the rising moon. Remember that maximum eclipse will be at 7:50 p.m. EDT and 6:50 p.m. CDT.

Triple Eclipses on Jupiter

A rare planetary viewing event occurs this week when three of Jupiter’s largest moons cast their shadows simultaneously on the planet below them: three solar eclipses at the same time.

Three of Jupiter’s moons cast their shadows simultaneously on the planet beneath them on Friday night and Saturday morning, October 11 and 12. Picture the sun coming over your left shoulder, causing the three moons on the left to cast their shadows on the planet to the right, causing eclipses of the sun in three different locations on Jupiter. Credit: Starry Night Software

An eclipse of the sun happens when a moon casts its shadow on the planet below it. For observers located where the shadow falls, the sun appears to be completely blocked by the moon, and they are able to see the prominences and corona around the occluding moon.

In the Earth-moon system, the Earth is a fairly small target and its moon is far away, so it’s rare for the moon’s shadow to fall on the Earth. On average it happens about twice a year, and the diameter of the shadow on the Earth’s surface is quite small, only a couple of hundred miles in diameter. To see a solar eclipse, you need to be in exactly the right place, the place where the moon’s shadow falls.

Because our moon’s orbit is tilted with respect to the Earth, most of the time the moon’s shadow passes above or below the Earth, and no eclipse occurs. Only about twice a year do the three bodies line up exactly so that the moon’s shadow touches Earth. This next happens on November 3.

With Jupiter, the situation is different. Jupiter has four large moons: Io, Europa, Ganymede, and Callisto. These are relatively close to Jupiter and Jupiter is much larger than Earth. As a result, the shadows of Jupiter’s moons cross its face very frequently. The innermost moon Io causes an eclipse on Jupiter once every 1.8 days (42 hours). Even the outermost moon, Callisto, traveling much more slowly, should cause an eclipse every 17 days. In fact it does so less frequently because, like our moon, sometimes its shadow passes above or below Jupiter.

So, if eclipses on Jupiter happen very often, why don’t we see more of them? The timing has to be exactly right. Io may cause an eclipse on Jupiter every 42 hours, but the eclipse itself lasts only a little over 2 hours. Also, Io’s shadow is very small; you need a telescope with at least 90mm aperture to see it. If you aren’t looking for it, you probably wouldn’t see it at all, it’s that small. Also, because of the Earth’s rotation, Jupiter is below the horizon half the time, and often lost in the daylight sky.

If the shadows of its moons fall so often on Jupiter, what are the chances of two shadows falling simultaneously? Pretty good, it turns out. This is especially true because there is a resonance between the orbits of Jupiter’s satellites. Europa’s period of revolution is almost exactly twice that of Io, and Ganymede’s almost exactly four times. Only Callisto doesn’t keep step with the inner satellites.

As a result, double shadow transits usually happen in a group. The current group started with a double transit of Io and Europa’s shadows on September 28 and will continue every few days until November 13.

The rarest of all shadow transit events is when three shadows cross Jupiter’s face simultaneously, and this will happen this coming Friday night, October 11, stretching on into Saturday morning, October 12. Because of Jupiter’s present location, this event is mainly visible in the eastern part of North America. The event is in progress when Jupiter rises around midnight on the east coast, but is completely over by the time Jupiter rises on the west coast. Here are the times of the events to look for in Eastern Daylight Time—some events occur before Jupiter rises:

11:12 p.m. Callisto’s shadow enters

11:24 p.m. Europa’s shadow enters

12:32 a.m. Io’s shadow enters: all three shadows visible

1:37 a.m. Callisto’s shadow leaves

2:01 a.m. Europa’s shadow leaves

2:44 a.m. Io’s shadow leaves

Keep an eye on the moons themselves, because they will also begin to cross Jupiter’s disk: Io at 1:48 a.m. and Europa at 2:02 a.m. There’s an added bonus in the Great Red Spot also transiting at this time. It helps to visualize the moons’ movements in three dimensions, with the sun seeming to come from over your left shoulder. Sometimes you get an almost three-dimensional effect with the moons casting their shadows on the planet beneath.

If you miss this event, there will be three double shadow transits later this month visible over most of North America, on October 16/17, 18/19, and 25/26. In each case, the events mostly occur after Jupiter rises around midnight on the first date, so that’s the night you should mark on your calendar. Remember that the date changes at midnight.

Starhopping to Uranus

Most of the planets in the solar system hover close to the sun, which illuminates them and makes them among the brightest objects in the sky. The outer planets, Uranus and Neptune, are far from the sun and catch few of its rays, making them dim and hard to find.

The planet Uranus is currently located in the dim constellation of Pisces. Although just visible with the unaided eye, Uranus is brighter than any of the stars in this area, so is relatively easy to locate with binoculars. Credit: Starry Night Software

Uranus, for example, reaches opposition with the sun on Thursday October 3. Directly opposite the sun in our sky, it is at its closest and brightest, yet it is only just barely visible to the unaided eye in a dark moonless sky. For most people, a binocular will be an essential tool for spotting Uranus.

This is done through a technique popular with amateur astronomers called “starhopping.” We use the brighter stars as guideposts to locating faint or distant objects. In this case, we start with the most prominent group of stars in the autumn sky: the four stars that form the Square of Pegasus. Although not among the brightest stars, these are all good second magnitude stars, about as bright as the stars in the Big Dipper.

Many people who go looking for the Square of Pegasus miss it because it is so large. The four sides of the Square are roughly 15 degrees long, about the length of the handle of the Big Dipper. Look for them in the east as the sky gets dark around 9 p.m. At that time the Square is rising, so is tilted over, making it more of a diamond shape than a square.

Now we begin our “starhop.” We start with the two stars that form the bottom of the square (if you’re in the northern hemisphere). Make that one side of a south-pointing equilateral triangle. The southern point of the triangle marks the “Circlet,” part of the dim constellation of Pisces. This is an oval of dim stars, more easily seen in binoculars than with the unaided eye.

To the left of the Circlet is a chain of stars, part of the chain that binds the two fish of Pisces together. The first two stars in this chain are a bit brighter than the rest. About half way between these two stars, and a bit to the south, you should find Uranus.

How will you know you have found it? First of all, although dim, Uranus is brighter than any of the stars in this area. Secondly, it has a distinctive blue-green color, quite unlike any star. Finally, if you plot its position relative to the stars, and check again in a few days, you will find that it has moved to the right.

Of course, the acid test is to point a telescope at it. Most newcomers to astronomy are surprised at how small any of the planets appear in a telescope, but Uranus is smaller still, a mere 3.7 arc seconds in diameter, about a tenth the size of Jupiter. Unless you have a very large telescope, it will appear as nothing more than a tiny blue-green pinhead.

Autumn Monsters

As the constellations of summer depart from our sky, they are replaced by what are often called "the watery constellations." These include normal sea creatures like fishes and dolphins, and even Aquarius carrying a water jug.

The constellations of autumn include some strange monsters like Capricornus, the Sea Goat, Cetus, the Sea Monster, and Pegasus, the Flying Horse. Credit: Starry Night Software

Among these watery creatures are some strange creatures which we would call monsters: strange combinations of parts of unrelated animals.

The first to appear is Capricornus, the Sea Goat. Seen in the lower right of our chart looking southward on an autumn evening in the northern hemisphere, he combines the front end of a goat with the rear end of a fish. Most people would be hard pressed to see either a goat or a fish in this large triangular group of stars. I see it more as a tricorn hat turned upside down. The front end of the goat, to the right, is marked by two wide double stars, Algedi and Dabih, a fine sight in binoculars. “Algedi” or “Al Giedi” is Arabic for “the goat.” The rear end of the fish is marked by Deneb Algiedi, which translates from Arabic as "the tail of the goat."

Much of our knowledge of ancient astronomy, along with mathematics and other sciences, has been passed down to us by medieval Arab scholars. In the process many of the old star names were translated into Arabic. As a result, astronomers learn a bit of Arabic. "Deneb" is Arabic for tail, so turns up in many star names in constellations derived from animals. The most famous is Deneb in Cygnus, marking the tail of the Swan.

"Al" is Arabic for "the" and turns up in many scientific words like "algebra," "alcohol," and "alkali."

In the lower left corner of our chart we find another monster, Cetus. Modern astronomy books usually translate this as "the whale," but our chart shows a much stranger creature. it has the head of a dragon, webbed feet, and a fishy tail. This tail is marked by one of the few bright stars in this part of the sky, Deneb Kaitos. With our new knowledge of Arabic, we can translate this easily as "the tail of the whale."

Buried in the heart of Cetus is a remarkable star called Mira, which means “wonderful” in Latin. This was discovered by David Fabricius in 1596 to be a star which varies in brightness, one of the first variable stars to be discovered.

Flying high above these watery creatures is yet another monster, a horse with wings called Pegasus. This is probably one of the most familiar mythological creatures, so familiar that most people never think of how strange a flying horse would be. The celestial flying horse is marked by four fairly bright stars forming an almost perfect square, the Square of Pegasus.

When I first went looking for Pegasus in the sky, I made a common beginner's error. Because I was using a small star chart, I looked for a small square of stars in the sky, and totally missed it. The constellations in the sky are much larger than they appear on star charts. So look for a really large square of stars.

Actually, only three of the four stars in the Square are part of Pegasus. The star in the upper left corner is Alpheratz, actually part of the constellation of Andromeda. But that is another story.

The September Equinox

Most people know that something called the “equinox” happens twice every year around March 21 and September 21, but don’t really know what that means. Here are the real facts about the equinox.

The sun crosses the celestial equator on September 22 at 4:44 p.m. EDT. On this date the sun rises due east and sets due west, and the day and night are of equal length. Image Credit: Starry Night Software

The Earth moves in two different ways. First, it spins on its polar axis, a line through the north and south poles, once every 24 hours, causing the alternation of day and night. Secondly, it moves in its orbit around the sun once every 365 1/4 days, causing the annual cycle of seasons. The equinox occurs when these two motions intersect.

Because the Earth is very massive, its mass has an enormously powerful gyroscopic effect. For this reason, its poles always point in the same direction, although a major earthquake can cause tiny wobbles in this axis. Most importantly, the Earth’s motion around the sun has absolutely no effect on the direction the poles are pointing, which has very important consequences for Earth’s seasons.

Astronomers mark the positions of objects in the sky relative to the Earth’s poles of rotation: those are the red lines you see in the chart. The most important line is the celestial equator, which divides the sky into northern and southern hemispheres.

The Earth’s pole of rotation is tilted 23.4 degrees relative to the plane of its orbit. This tilt is always towards the same point in the sky, called the celestial pole, no matter where in its orbit around the sun the Earth happens to be. This tilt has the effect on the surface of the Earth that the sun appears to move across the sky at an angle to the celestial equator. This is marked by the green line in the chart, called the “ecliptic” because eclipses happen along this line.

Twice a year, the sun crosses the celestial equator, moving from the northern hemisphere to the southern hemisphere or from the southern hemisphere to the northern hemisphere. These two crossings are very important for the inhabitants of Earth, because they mark the change in the direction the sun’s rays fall on the Earth.

Specifically, on September 22, the sun will move from the northern hemisphere to the southern hemisphere. It will pass overhead everywhere along the Earth’s equator on that date. It will rise exactly in the east and set exactly in the west. After that date, the sun will shine more on the southern half of our planet, and less on the northern half. Summer will be over in the northern hemisphere, and winter will be on its way. Also, winter will be over in the south, and summer will begin.

The sun will continue on its path southward for the next three months, reaching its southernmost point on December 21, the date called “solstice.” In the northern hemisphere, the days will become shorter, the nights longer, and the temperatures colder, all the result of the sun’s being south of the celestial equator.

It’s always important to remember that this is part of a cycle, and that after December 21 the sun will start moving northward again, and spring will be on its way.

First Quarter Moon

The next few nights are the best this month to observe the surface features of the moon. The sun will be rising along the center line of the moon, casting the mountains and craters in high relief.

Image Credit: Starry Night Software

The sun will be rising along the center line of the moon, casting the mountains and craters in high relief.

Beginners in astronomy are often surprised that the best time to study the moon is not at full moon, which will occur on Thursday the 19th this month. At full moon the sun is high overhead in the center of the moon, and the surface looks like the desert at high noon. The best time is when the sun is falling obliquely, casting long shadow, which is what you see at the first and third quarters.

Third quarter is a bit of a problem because it occurs when the moon is in the dawn sky. First quarter, on the other hand, occurs when the moon is high in the sky at sunset, perfect for evening observing.

The moon is so close to us that you don't really need a telescope to study it. Even without any optical aid, the major features of the moon can be clearly seen, if you take the time. In fact, you can see more detail on the moon with your naked eye than you can see on any of the planets with a powerful telescope.

When you look at the first quarter moon, you are seeing a sphere lit by the sun from the right side (in the northern hemisphere; left side in the southern hemisphere). The surface of the moon varies in reflectivity, so that you see a pattern of light and dark. The lighter areas are the older mountainous regions, mostly on the southern half of the moon; the darker areas are younger lava flows, mostly in the north.

Early observers mistook these for seas and oceans, and named them accordingly. Later astronomers realized that the moon is an airless waterless body, and that its “seas” are dryer than the driest deserts on Earth. The water which has been discovered recently on the moon is buried deep beneath the surface.

If you examine the moon more closely in binoculars or a telescope, your attention will be drawn to the “terminator,” the narrow band down the middle of the moon where dark meets light. This is where the sun is rising.

Ordinary binoculars can give a surprisingly detailed view of the moon, especially if they are steadied by mounting them on a tripod. Many binoculars have a tripod socket hidden under a small cap in the hinge between their two halves, but you will need a small L-shaped adapter to connect this to a standard camera tripod. You will be amazed at the improvement a tripod mount makes to the view.

The 10x50 size binocular is the most popular among astronomers. Its 10 times magnification and 50mm objective lenses provide excellent views of both nearby objects like the moon and distant objects like star clusters and galaxies. In recent years more powerful binoculars have become widely available at amazingly low prices. The 15x70 size gives particularly good views of the moon, but must be mounted on a tripod.

Because the sun is rising over the terminator, even the slightest variation in topography is exaggerated by the rising sun. Look especially for craters and isolated mountains on the plains. With the extra magnification afforded by a small telescope, you can actually watch the sun rise over craters in real time.

With a good map of the moon you can become familiar with the “geography” of the moon. More than a thousand craters bear the names of famous astronomers of the past.

Neptune reaches Opposition

This week, on August 27, the planet Neptune reaches opposition.

On August 27 Neptune reaches opposition in Aquarius, making it visible all night. Credit: Starry Night Software

When a planet is in opposition, it lies directly opposite the sun in Earth’s sky. It is highest in the sky when the sun is lowest, which is local midnight. When Daylight Saving Time is in effect, this is close to 1 a.m. local time.

Because Neptune is directly opposite the sun. it rises at the same time as the sun sets, and sets at the same time as the sun rises, so is visible all night.

Now that Pluto has been demoted to “dwarf planet” status, Neptune is the most recent planet to be discovered, on September 23, 1846, and the farthest planet from the sun, at an average of 2,798,310,157 miles (4,503,443,661 km.)

As planets go, Neptune is extremely dim, requiring at least a binocular to become visible. Even in a powerful telescope, it is a tiny blue-green disk with no detail to be seen. Unless you look carefully, you could easily mistake it for a star.

In fact, that happened several times before Neptune’s official discovery. Most famously, Galileo twice observed Neptune while studying Jupiter’s moons in 1612 and 1613, but mistook it for a star both times.

After the discovery of Uranus by William Herschel in 1781, mathematicians calculated the possible location of another planet farther from the sun, but no one looked seriously for it until 1846. The first to actually spot it was German astronomer Johann Gottfried Galle on September 23.

Because it takes Neptune 164 years to circle the sun, it spends an awfully long time in any one constellation. For example, Neptune has been in Aquarius since January 24 2011 and won’t move on into Pisces until May 22 2022. In fact, it has only just completed its first trip around the sun since its discovery, and is again very close to the spot where it was discovered.

Since Neptune is so far from the Earth, it presents too small a disk to be studied well with even the largest telescopes. The only good view we’ve ever gotten of Neptune was in 1989 when the Voyager 2 probe passed within 2740 miles (4400 km.) of Neptune’s cloud tops. At that time it recorded two large blue spots in Neptune’s atmosphere, apparently similar to the Great Red Spot on Jupiter. Voyager 2 also confirmed the existence of a faint ring around Neptune, a ghostly echo of Saturn’s ring system.

The Summer Triangle

Have you ever wished you knew more about the stars overhead? It’s easier than you think.

Lie back on a warm summer night and look straight up. You’ll see three bright stars: Vega, Deneb, and Altair. These mark the corners of the “Summer Triangle” and are your guides to the three constellations of Lyra, Cygnus, and Aquila. Credit: Starry Night Software

All you need to do is lie back on a warm summer evening and look up towards the zenith. It will help if you can find a spot free from light pollution on a night when the Moon isn’t in the sky.

The first thing you will notice is that some stars are brighter than others. The brightest stars are said to be “of first magnitude” and there are three that should leap out at you. The brightest is Vega, almost directly overhead at 10:30 p.m. Daylight Time this week. Next brightest is Altair, down towards the southern horizon, and third is Deneb, off towards the northeast. These three form the “Summer Triangle” and are as characteristic of the summer sky as Orion is of the winter sky.

Since these three stars appear to be about the same brightness, you might think that they were all about the same distance away, but you’d be wrong. Stars come in many colors and brightnesses, and sometimes a very distant, very bright star can look as bright as a very close, relatively dim star. Vega and Altair are both relatively close to the Sun, 25 and 17 light years away, but Deneb, which rivals them in brightness, is a whopping 3300 light years away, making it one of the farthest objects you can see with your unaided eye. In fact, Deneb is an absolutely brilliant star, but so far away that its brightness is greatly dimmed by distance.

As you continue to watch the sky, you may begin to see patterns in the fainter stars. The human brain always tries to find patterns in random shapes, as when we look for the shapes of animals in the clouds. The same is true for stars.

Stars are pretty much randomly distributed across the sky, but from time immemorial humans have grouped them into patterns which we call constellations. Each of the three stars in the Summer Triangle is a member of such a constellation.

If you look closely at Vega, for example, you may notice a small parallelogram of stars just to its south. This reminded ancient astronomers of the musical instrument the lyre, so they named this group of stars Lyra. If you have binoculars, use them to take a closer look at Vega and the stars nearby. Even a small binocular is enough to confirm one of Galileo’s first discoveries when he turned his telescope on the sky: many stars which appear single to the naked eye turn out to be double or multiple with a bit of magnification. Several of the stars near Vega are obvious doubles, even with only 6 or 7 times magnification.

Now take a closer look at Deneb. It stands at one end of a chain of bright stars stretching to the south of Vega. There is a second shorter chain of stars which crosses the first chain at right angles. Different cultures have seen different patterns. The ancient Greeks saw this as a swan and named it Cygnus. Deneb is the tail of the swan, the short chain marks the swan’s wings, and the long chain its outstretched neck, with a brighter star Albireo at the head. Others see this as a Christian cross, and call it the Northern Cross, to distinguish it from Crux, the Southern Cross.

If you are under a dark country sky, you will see that the swan is flying along the faint silvery Milky Way. This is the glow from millions of distant stars, too far away for the individual points of light to be resolved. This was another of Galileo’s discoveries.

Altair marks the head of a different bird: Aquila the Eagle. This has broader wings than the swan, and a distinctive curved tail.

Look for some of the smaller constellations in this part of the sky, in particular Delphinus the Dolphin, one of the few constellations which actually resembles its namesake.

Don’t be dismayed if you can’t readily see these patterns. Sometimes the objects the ancients saw in the sky owed more to imagination than to reality.