On May 09, 2016, amateur astronomers with small telescopes and solar filters will be able to observe Mercury’s tiny silhouette moving slowly across the face of the Sun. Astronomers call this event a “transit”.Read More
On Friday, January 8, the planet Venus will appear to pass just 5 arc minutes north of the planet Saturn, That is a mere one-sixth of the diameter of the moon, a small enough distance to fit in the eyepiece of a powerful telescope.
This will be a rare opportunity to see two planets at the same time in a telescope’s narrow field of view. In a lifetime of observing the skies, I have seen such a close conjunction of two planets only two or three times. With the naked eye, sharp-eyed observers will be hard pressed to separate the two points of light.
Unfortunately for observers in North America, the point of closest conjunction will occur at 11 p.m. EST, while the planets are below the local horizon. For a skywatcher in New York, for example, the planets won’t clear the eastern horizon until 5 a.m. EST, at which time they will have separated so that they are 17 arc minutes apart, or slightly more than half the diameter of the moon. Even so, they will still fit in a telescope eyepiece.
Of course, this conjunction is something of an optical illusion. The two planets aren’t anywhere near each other in space, but merely appear close together from our perspective here on planet Earth.
Venus is currently on the far side of the Sun from Earth, 1.22 astronomical units distant (1.22 times the average distance from the Earth to the sun), so it appears similar to a gibbous moon.
Saturn is 10.79 astronomical units from Earth, nearly 9 times farther away than Saturn.
The two appear almost the same angular diameter: 14 arc seconds for Venus, 15 arc seconds for Saturn, yet in reality Saturn is actually almost 10 times the diameter of Venus. So Saturn’s greater distance balances out is larger size, and the two appear almost the same from Earth.
The most striking difference between the two is their difference in brightness. Venus is magnitude –4.0 on the upside-down brightness scale astronomers use, while Saturn is only magnitude +0.5, 4.5 magnitudes (or almost 100 times) fainter than Venus. This difference is mainly due to Venus’ closeness to the sun (0.72 astronomical units) compared to Saturn’s (9.55 astronomical units).
When you look for the twin planets just before dawn on Saturday morning, the first thing you will notice is Venus shining brightly in the southeastern sky. You will have to look closely to spot “tiny” Saturn just above and to the right of it.
If you own a planetarium program like Starry Night that lets you travel to other planets, check out the view Saturday morning from Saturn. You will see Earth and Venus in a close conjunction, Venus a narrow crescent from Saturn’s perspective, and Earth a rounded gibbous shape, since despite being close in the sky, they are actually on opposite sides of the sun, and lit by it quite differently.
Next Tuesday or Wednesday marks a major turning point in the annual cycle of the seasons. The sun reaches its southernmost position in the sky, resulting in the shortest day of the year in the northern hemisphere, and the longest day of the year in the southern hemisphere.
The solstice gets its name from the apparent stop (“stice”) in the motion of the sun (“sol”). This was carefully recorded by the earliest astronomers; monuments like Stonehenge are thought to have been used to mark the extreme positions of the sun in the sky. The December solstice has long marked the beginning of the new year, and it’s mainly because of slippage in our calendar that it now occurs eleven days before the “official” New Year, January 1.
Solstice day is a day of celebration in many cultures. The Romans knew this as “Saturnalia,” and the early Christians adopted this date to mark the birth of Christ, so that they could celebrate without drawing the attention of their Roman masters.
You’ll notice I said, “Tuesday or Wednesday.” That’s because, although the time of solstice is exactly the same everywhere in the world, because of our local clocks it falls on different days in different places. The exact time of solstice this year is December 22 04:48 coordinated universal time, the time used by astronomers and pilots everywhere.
In England, where the prime meridian lies, the solstice will occur a 4:48 a.m. GMT on Wednesday, December 22. Similarly, it will occur in Europe and Africa in the early hours of Wednesday morning.
In North America, we subtract a number of hours from UTC to get our local times. In most of eastern North America, we are on Eastern Standard Time, and subtract 5 hours, so the solstice falls at 11:48 p.m. on the previous day, Tuesday, December 21. The farther west we go in North America, the earlier the solstice occurs in the evening, so that on the Pacific coast it occurs at 8:48 p.m. PST.
Remember, these are all exactly the same time in the broader scheme of things; local times are just vagaries of the way we handle time around the world.
The graphic shows the sky as it would appear at the time of solstice from a location where the solstice occurs at noon, which this year would be Vietnam. Through the magic of Starry Night software, we have turned the blue sky transparent and have added the two coordinate systems we use to mark positions in the sky.
The red line across the sky marks the celestial equator, half way between the celestial north and south poles. It shows 18h on the meridian because we measure right ascension, the celestial equivalent of longitude, from the vernal equinox, exactly 9 months ago.
The green line marks the ecliptic, the path that the sun appears to follow across the sky. The sun is at its southernmost position, at a declination (celestial equivalent of latitude) of exactly 23 degrees and 26 minutes, which just happens to be the exact angle at which the Earth’s poles are tilted from the ecliptic.
The height of the sun above the horizon on solstice day depends on your latitude on the surface of the Earth. Where I live in southern Canada, it hangs very low in the southern sky, barely 22 degrees above the horizon. For anyone north of the Arctic Circle, it never rises at all. Even in the southernmost continental United States, it’s barely 42 degrees above the horizon, less than half way up the sky.
In the southern hemisphere, the situation is reversed. The December solstice marks the longest day in the year, combined with the shortest night. Longer days mean more hours of sunshine and warmer weather.
You may have noticed that I have avoided using the words “winter” and “summer.” That’s because, even though the sun’s position in the sky is responsible for our seasons, it does not match exactly with the seasons as we experience them. That’s because there is a lag of about six weeks between the astronomical season markers, solstices and equinoxes, and the actual seasons.
The Earth is a complex ecological system, and it takes a while for the sun’s movement to effect the temperature. Sometimes the astronomical markers are referred to as “the first day of…” which makes some sense, if the seasons were exactly 3 months long. However, the lengths of the seasons in any particular location tend to vary depending on local conditions. Hence the old joke about the Canadian seasons: eleven months of winter and one month of bad sledding.
Next Monday, December 7, the planet Venus will do a vanishing act behind the moon.
In the course of its monthly journey around Earth, the moon often passes in front of stars, and occasionally planets.
Next Monday it will pass in front of the brightest of them all, the planet Venus, in what is called a lunar occultation.
This will be visible, weather permitting, all over North America. The only catch is that it occurs in broad daylight.
I’m often surprised that many people don’t know that the moon is visible in the daytime sky; in fact my son first spotted it when he was about 2 years old. That mainly says that very few people bother to look closely at the sky. On Monday, the moon will be a slender crescent only a few days away from new moon, so may be a bit harder to spot, though if you start observing at dawn you should have no trouble following it up into the blue sky.
Venus is also visible in daylight provided you know exactly where to look. On Monday, its closeness to the moon will help you spot it.
The time the occultation will occur depends on your exact location. For example, in New York City, Venus will disappear behind the moon at 12:43 p.m. EST and reappear at 1:51 pm. In Chicago the occultation lasts from 11:20 a.m. CST until 12:35 p.m. In Los Angeles, the times are 8:06 and 9:56 a.m. PST. The easiest way to get the exact times for your location is with a planetarium program like Starry Night or SkySafari.
It was observations of occultations by the moon of stars and planets that first told us that the moon has no significant atmosphere. Stars simply wink out and in as they pass behind the moon; planets show no fuzziness as they disappear behind the moon’s edge.
Starry Night gives you the opportunity to travel to other places in the universe, and view events from a different perspective. Our second image today shows the view of this occultation from the planet Venus at 12:35 p.m. EST. Instead of an occultation, we see what is called a transit, as one object (the moon) passes in front of another (Earth). The moon is moving from right to left in this view.
The occultation is already over on the east coast, to the right of the Moon, and has yet to begin on the west coast, to its left. The surface of the moon looks strange because we are seeing its “far side,” the side permanently turned away from Earth. This side has more mountains and craters and fewer open flat plains than the side we usually see.
One of the first things every new moon watcher learns is that, when observing the moon, timing is everything.
You might think that the best time to look at the moon with binoculars or a small telescope is when the moon is full, because it looks so big and bright. Well, that’s usually the worst time of the month for observing the moon. That’s because at full moon, the sun is high overhead on the moon’s surface, making it look like the desert at high noon. Far better times are at first and last quarters, when the sun is shining obliquely across the moon’s terrain, casting mountains and craters into high relief.
That’s what makes this week the perfect time to begin your exploration of our nearest neighbor in space. First quarter will be on Thursday night, November 19, meaning that the moon is one quarter of the way around the Earth in its monthly journey. The light will be hitting the terminator, the boundary between light and shadow, very obliquely, making the moon exactly half illuminated.
Because the moon is a world in its own right almost as large as the planet Mercury, and because it is so close to us, its surface geography is know almost as well as that of the Earth. Its surface has been mapped for hundreds of years, and most of its features have been named. Everyone who studies the moon soon becomes familiar with its major features and their names.
What catches everyone’s attention first at this phase of the moon is the three gigantic craters almost exactly in the centre of the moon. Like almost all the craters on the moon, these are named for famous astronomers of antiquity.
The northernmost and largest of the three is Ptolemaeus, named for Claudius Ptolemy. His name was given pride of place on the moon because he was probably the greatest astronomer of antiquity. His book, the Almagest, was written in the middle of the second century AD, and summarized all of astronomical knowledge of his time. It was written in Greek, but is known by its Arabic title because it only survived into the modern day in an Arabic translation.
The crater Ptolemaeus is 95 miles (153 km) in diameter, and is characterized by a wide flat floor pockmarked by many small craterlets.
The middle crater of these three is Alphonsus, named for Alphonso El Sabio, king of Castile in what is now Spain in the 13th century. Besides being a king, he was also an astronomer and a musician, and compiled the Alphonsine tables which were used to calculate planetary positions until the time of Kepler. This crater is a bit smaller than Ptolemaeus, but is more of a typical lunar crater with a central peak and complex system of rilles on its floor. This crater became famous in the 1950s when a Russian astronomer claimed to have seen a volcanic emission in it.
The third crater Arzachel is smaller still, 60 miles (97 km) in diameter, but with a larger central peak.
Just to the north of these three craters is the broad arc of the Apennine mountain range. This range is 370 miles (600 km) long and has peaks as high as 16,000 feet (5,000 m) in height. At its northern end is Mount Hadley, near which the astronauts of Apollo 15 landed in July 1971.
To the west of the Apennines is the broad expanse of the Mare Imbrium, the so-called “Sea of Rains.” Early astronomers had no idea how dry and airless the moon was, and its “seas” are dryer than the driest deserts on Earth. This is the second largest “mare” on the moon, exceeded only by the Oceanus Procellarum to its south.
The Mare Imbrium is a huge flat plain 720 miles (1160 km) across, with an area of 320,000 square miles (830,000 sq km). It contains many small mountain massifs and individual peaks. The lunar Alps are to its north, enclosing the beautiful crater Plato, 63 miles (101 km) in diameter. Its almost flat floor contains a number of small craterlets which are used by lunar astronomers to test the resolution of their telescopes. How many can you see?
Near the south pole of the moon, look for the gigantic crater Clavius, 140 miles (225 km) in diameter, and containing a whole series of craters in its interior graded in size. The largest of these is large enough to warrant a name of its own, Rutherford, 32 miles (51 km) in diameter.
With these landmarks learned, you can begin your exploration of the moon’s finer details. What I like to do myself is to start at the moon’s north pole and work my way down towards the southern highlands along the terminator, using a good lunar map to identify the topography.
Most of the features I’ve described can be seen with ordinary binoculars. Even the smallest telescope will reveal hundreds of smaller features. The moon is generally a good target for photography, because it is so bright that long exposures are not needed. You can even put your smartphone up to the eyepiece of your telescope and get remarkably good photos that way.
Autumn is the favorite season for many skywatchers. You can get your last look at summer stars and, if you stay up late, your first look at winter stars. Best of all, it gets dark earlier and the night temperatures are still comfortable.
Our graphic this week shows a panorama of the sky looking south around 8 p.m., just after the sky becomes fully dark. After the change to standard time this weekend, this will be the view at around 7 p.m.
Looking towards the west, at the right in the graphic, you can see the familiar constellations of summer. Sagittarius and the core of the Milky Way Galaxy are setting in the southwest, while the summer triangle: Deneb, Vega, and Altair, shines overhead.
It’s not too late to revisit some of the popular summer objects: double stars Albireo and Epsilon Lyrae, the globular clusters in Hercules and Ophiuchus, the Ring Nebula in Lyra, and the bright nebulae and clusters of Sagittarius.
Looking south, the upside down triangle of Capricornus rides high. Its rightmost star, Algedi, is a naked-eye double. Above Capricornus, just to the left of Altair, is the tiny constellation Delphinus, the dolphin, one of the few constellations that actually looks like its name. It’s worth exploring the region between Altair and Albireo, where you will find two of the finest deep sky objects: Brocchi’s Cluster, popularly called “the coat hanger,” and the Dumbbell Nebula, one of the largest and brightest planetary nebulae.
Many of the constellations in the autumn sky have watery associations. These include Capricornus (the sea goat), Delphinus (the dolphin), Aquarius (the water bearer), Pisces (the fish, plural), Piscis Australis (the southern fish, singular), and Cetus (the whale). Most of these are lacking in bright stars, with the exception of Pisces Australis which contains the first magnitude star Fomalhaut, the first star to have one of its planets directly imaged by the Hubble Space Telescope.
Although Aquarius is dim in terms of stars, it contains a number of fine deep sky objects, including the globular cluster Messier 2, and two fine planetary nebulae, the small bright Saturn Nebula snd the huge Helix Nebula. The latter is probably the planetary nebula closest to the sun, about 700 light years distant, and as a result is very large in size, almost as large as the moon. Because of its large angular size, its light is spread out over a wide area, making it very hard to see. You will need a narrow band filter on your telescope to spot it.
Off to the east, the Square of Pegasus dominates the sky. This consists of three stars in Pegasus with the fourth corner of the square being marked by Alpheratz, the brightest star in the constellation Andromeda, which trails away to the northeast.
Right in the upper left corner of the graphic are the two largest and brightest galaxies in our neighborhood, the Andromeda Galaxy and the Triangulum Galaxy. These are located symmetrically on either side of the second pair of stars eastward from Alpheratz in Andromeda.
The Andromeda Galaxy (to the north) is large and bright. If you live in a city, you will need binoculars to see it, but sharp-eyed observers in the country, including myself, have spotted it with their unaided eyes. The Triangulum Galaxy is almost as large, but nowhere near as bright as Andromeda. It is best seen in small binoculars. Oddly, it is very hard to see in the narrow field of a telescope because its dim light is spread across such a large area.
Finally, in the northeast you can see the first of the winter stars, the bright star Capella in Auriga and the brilliant Pleiades Cluster in Taurus. Soon Orion will arrive in the east, in the words of Robert Frost:
You know Orion always comes up sideways.
Throwing a leg up over our fence of mountains,
And rising on his hands, he looks in on me…
Stay up until midnight, and you will see him, too.
On Monday morning, October 26, Venus will be at its greatest elongation west, the farthest it can get from the sun as seen from Earth.
Venus has been dominating the morning sky for the last two months, and reaches its greatest angular separation from the sun on the 26th: 46 degrees west of the sun. Remember, directions in the sky are the reverse of directions on the surface of the Earth because we are looking outward rather than inward, so west is to the right in the sky if you live in the northern hemisphere.
The inner planets, Mercury and Venus, never stray far from the sun in Earth’s sky. Mercury was at greatest elongation west, 18 degrees from the sun, last Friday, October 16, and is now moving back towards the sun. Venus is still moving away from the sun, but will pause and reverse directions on the 26th. Mars and Jupiter, with orbits outside that of Earth, both appear to be moving steadily westward relative to the sun.
As a result, Venus and Jupiter will pass very close to each other on the 26th, within 1.1 degrees.
Venus is by far the brightest of the two, at magnitude –4.6, while Jupiter is nearly 3 magnitudes fainter at magnitude –1.8. Mars, which is also nearby in the sky, comes a distant third in brightness at magnitude +1.7, more than 3 magnitudes fainter than Jupiter and 6 magnitudes fainter than Venus.
This close conjunction of Venus, Jupiter, and Mars is best observed with the naked eye or binoculars. In a telescope, Venus will appear as a perfect miniature quarter moon, because at greatest elongation it is lit by the sun exactly from its left side. It is 24 arc seconds in diameter, slightly smaller than Jupiter at 33 arc seconds, but much larger than Mars at 4 arc seconds.
Venus appears brightest because it is both close to the sun and close to Earth. Jupiter is much larger in actual diameter, but is far from the sun so only lit dimly. Mars, in between in distance from the sun and small in size, is currently very far from Earth, so appears even dimmer.
To put them in perspective, even though grouped close together in our sky, the three planets are at vastly different distances. Venus is closest at 0.69 astronomical units, Mars next at 2.2 a.u., and Jupiter farthest at 6.0 a.u., nearly ten times farther than Venus. An astronomical unit is the average distance between Earth and the sun.
Many photographers have been taking advantage of the close approach of these three planets to photograph them in the dawn sky, though the different brightnesses present a challenge in exposure. As always, we welcome your pictures of this beautiful event.
Sometimes a whole bunch of different sky events happen over a very short period, giving skywatchers a chance to witness many unusual events in a few days. The next week or two is a case in point.
Uranus at opposition
First, the planet Uranus is just past opposition, so it is visible all night long. It’s also in a location where you can spot its dim glow rather easily. You will need a binocular to bring out the faint planet from its surrounding stars
Look at the chart above, and you will see how to spot Uranus using an easy bit of star hopping.
Start with visualizing an equilateral triangle with one side marked by Hamal in Aries and Algenib, the lower left star of the square of Pegasus. The third point of the triangle is Alrescha, Alpha Piscium.
Although only fourth magnitude, Alrescha is the second brightest star in the dim constellation of Pisces. It marks the knot which joins the two chains of stars attached to the two fish of Pisces. Follow the lower chain of stars to the right under the square of Pegasus to find Uranus.
The stars that make up this chain do not have names, only Greek letters. Starting at Alrescha, they are Nu, Mu, Zeta, and Epsilon. You don’t need to know their names, just look for Uranus between the last two, a bit to the south of the line joining them.
In binoculars, Uranus will look like a blue green 6th magnitude star. In a powerful telescope, it will show as a tiny blue-green disk 3.7 arc seconds in diameter.
Planetary diameters are usually measured in arc seconds: the number of seconds they subtend at the observer’s eye. There are 360 degrees in a circle, 60 minutes in a degree, and 60 seconds in a minute. To give you some idea how tiny the planets appear, the sun and moon are both around 30 arc minutes in diameter, or 1800 arc seconds. The largest any planet can appear to Earthbound astronomers is Venus at its closest, which is about 1 arc minute or 60 arc seconds in diameter. Thus all the planets are typically smaller in a telescope than the largest craters on the moon.
Mercury at greatest elongation
On Thursday, October 15, Mercury will be at the farthest it strays west of the sun. Because it is only 18 degrees away from the sun, it can best be seen about half an hour before sunrise. Again it is easiest to spot in binoculars. It is about twice the angular diameter of Uranus, so will look like a tiny half moon 8 arc seconds across in a large telescope.
Mars near Jupiter
On Sunday morning, October 18, Mars will be in conjunction with Jupiter, and the two planets will be close enough together that both will fit in the same field of a small telescope. Look for Mars just above Jupiter.
Both will be on the far side of the Sun, so much smaller than they usually appear when in the evening sky. Jupiter measures 32 arc seconds while Mars is only 4.1 arc seconds, half the size of Mercury. By the time they reach opposition in spring of 2016, they will swell to 44 and 18.3 arc seconds respectively.
Double shadow transits on Jupiter
Speaking of Jupiter, there will be two interesting passages of the shadows of its moons across its face in the next two weeks. These require at least a 5-inch telescope and steady observing conditions.
Again on Sunday morning, October 18, the shadow of the moon Io will chase the shadow of Ganymede across Jupiter’s face. Both shadows will be on Jupiter from 6:42 to 8:08 a.m. EDT, but will be increasingly hard to see after sunrise at around 7:30 a.m. local time. Observers in western parts of North America will have a better view because the transits will occur between 3:42 and 5:08 a.m. PDT, long before sunrise, though Jupiter will be lower in the sky.
Exactly a week later. on Sunday October 25, there will be another double shadow transit. This one is especially interesting because both shadows enter the disk at almost the same time, 8:36 a.m. EDT or 5:36 a.m. PDT, but then cross at very different speeds, Io’s shadow taking 2 hours and 17 minutes to cross while Ganymede’s takes 3 and a half hours, because of its greater distance from Jupiter and larger orbit diameter.
This transit occurs in full daylight in eastern North America, so probably won’t be visible, but is at a much more favorable time for western observers.
All in all, planetary observers will have a busy week ahead and, as always, we welcome your images of these events.
The next two weeks are an excellent opportunity to spot the brightest of the asteroids, Vesta.
In the first six years of the 19th century, astronomers discovered four new members of the solar system. All four were small objects moving in orbits between the orbits of Mars and Jupiter. Initially they were called planets, but by mid century, enough new objects had been found in this area that they were given a category of their own, much as Pluto was reclassified from a planet to a dwarf planet. They were called “asteroids” because all were so small that they looked just like stars in the telescopes of the day. Now there are tens of thousands of known asteroids.
Vesta is the brightest of all the asteroids, ranging between magnitudes 5 and 8, and one of the largest, measuring 318 miles (512 km) across. It reached 6th magnitude at opposition on September 29, meaning that it could just barely be seen by someone with perfect eyesight at a perfectly dark site.
The rest of us have to make do with binoculars. Here’s how to find it.
The first chart shows its overall position among the constellations of autumn. The two left-hand stars of the Square of Pegasus, Alpheratz and Algenib, point southward across the circlet of Pisces to the constellation Cetus, the Whale. Look for a large triangle formed by Eta and Iota Ceti and Deneb Kaitos. The last is easy to spot because, although only second magnitude, it is by far the brightest star in this rather dim part of the sky. Eta and Iota are both magnitude 3.5, so quite a lot dimmer than Deneb Kaitos.
The second chart shows these three stars in detail, and the path of Vesta over the next two weeks. The end of Vesta’s path with the label is its position on Wednesday, September 30, and the points on the trail to the right show its position each night after that.
Vesta should be quite easy to spot, since it is about two magnitudes brighter than any of the stars along its track. Just to be sure, make a simple plot of the stars in its vicinity, and then check again a night or two later. The “star” that has moved is certain to be Vesta.
Vesta is now one of the best known objects in the solar system because it had the NASA spacecraft Dawn orbiting it for over a year (July 2011–September 2012). This is a great chance for you to see it with your own eyes.
This Sunday evening, 9/27, stargazers will see a rare supermoon lunar eclipse. If you miss it, the next one isn't until 2033! What makes this event so special?
A Full Moon
First, the moon will be full, as it always must be for a lunar eclipse to occur. This is a special full moon, all on its own, because this is the harvest moon. Traditionally, this designation goes to the full moon closest to the autumnal equinox. In two years out of three, the harvest moon appears in September, but every third year it occurs in October.
At this time of year, corn, pumpkins, squash, beans, and wild rice—the chief Native American staples—are ready for gathering; and at the peak of the harvest, farmers could work into the night by the light of this full moon.
Most of the year, the moon rises an average of 50 minutes later each night, but for the few nights around the harvest moon, the moon seems to rise at nearly the same time each night: just 25 to 30 minutes later across the United States, and only 10 to 20 minutes later for much of Canada and Europe.
Secondly, the full moon will be at its closest to Earth in all of 2015, what is known to astronomers as a perigee moon. In recent years this has become known as a “supermoon.” Perigee (meaning “closest to Earth”) occurs at 10 p.m. EDT, the moon being a mere 222,374 miles (357,877 km) from Earth.
In fact, the human eye can’t detect the 5 percent difference in size between the moon at perigee and the moon at apogee (farthest from Earth), but everyone who looks at the moon Sunday night will swear it looks bigger than usual. Partly that is because, when seen low on the horizon, the human eye and brain combine to create an optical illusion known as the moon illusion, whereby the moon (and other objects) seen close to the horizon seem larger than when seen overhead.
The moon is the same size regardless of how low or high it is above the horizon. To prove this to yourself, cut out a circle just big enough to block the moon at arm's length and use it to see for yourself that its size stays the same as it rises in the sky.
The only noticeable effect of a perigee moon is that the ocean tides will be a bit higher than usual for the day of full moon and the next three days.
A Total Lunar Eclipse
The third, and most important part of this special event, is that we will have a total eclipse of the moon. At most full moons, the sun, Earth and moon line up approximately, but because of the tilt of the moon’s orbit, the moon passes above or below the Earth’s shadow, and avoids being eclipsed.
At certain points in the moon’s orbit, sun, Earth, and moon line up exactly, and the Earth’s shadow falls across the face of the moon, and we have a lunar eclipse. This is what will happen Sunday night.
The moon’s shadow has two parts: a darker inner part called the umbra, and a lighter outer part called the penumbra. This is because the sun is not a point source of light, so its light leaks around the edge of the Earth, and results in an unsharp shadow. In passing through the Earth’s atmosphere, the light turns red or orange, so that the light that actually reaches the moon is tinted by thousands of sunsets and sunrises all around the periphery of the Earth.
One result of these multiple sunrises and sunsets is that the moon during an eclipse is often tinted red, which is the origin of the idea of a lunar eclipse being a “blood moon.” It isn’t a far stretch of the human imagination to turn this “blood moon” into a portent of disaster.
A lot has been made in the media of this eclipse being the fourth event in a foursome of total eclipses known as a “lunar tetrad.” There really is nothing unusual about four lunar eclipses in two years, since we usually average at least two lunar eclipses every year, though not all are total.
In fact, there was no tetrad of total eclipses at all, because the last lunar eclipse, on April 4, was not really a total eclipse. According to the usual way of calculating eclipses, the moon spent only 4 1/2 minutes in the umbral shadow, but recently this calculation method has been corrected, resulting in the April eclipse failing to be total at all.
This Sunday’s lunar eclipse is a true total eclipse, with the moon being in the umbra for a full hour and twenty-two minutes.
When To See It
Observers in eastern and central regions of North America will get to see the whole eclipse; those further west will see the moon rise already partially eclipsed. Observers in Europe and Africa will see the eclipse before dawn on Monday, September 28.
This brings up the question of dates and times, which often causes confusion. Even a usually reliable source like Canada’s Weather Network, got the date of this eclipse wrong.
Officially, mid-eclipse occurs on September 28 at 02:47 Universal Time, which is the same as Greenwich Mean Time (but NOT British Summer Time). Subtracting 4 hours, this places mid-eclipse in the Eastern Daylight Time zone at 10:47 p.m. on the evening of September 27; the date changes at midnight. So be sure you look for the eclipse on Sunday evening. If you wait until Monday evening, you will be a day late.
Here are the important times in Eastern Daylight Time; if you’re using CDT, MDT, or PDT, the times will be earlier by 1, 2, or 3 hours.
08:11:46 Moon enters penumbra
09:07:12 Moon enters outer edge of umbra
10:11:11 Moon completely in umbra
11:23:07 Moon begins to emerge from umbra
12:27:06 Moon completely out of umbra
01:22:33 Moon leaves penumbra
As always, we look forward to your pictures of this beautiful event.
The third of four eclipses in 2015 will occur this Sunday, September 13. It will be a partial eclipse of the sun, and unfortunately it will be visible only from southern Africa, the Indian Ocean, and parts of Antarctica.
The best view of this eclipse from an urban area will be from Cape Town, South Africa, where a maximum of 30 percent of the sun will be covered by the moon.
The eclipse begins in Cape Town just as the sun and moon are rising at 6:49 a.m. local time. Early risers will see a tiny bite out of the sun. It reaches its maximum at 7:43 a.m. By 8:50 a.m. the eclipse will be over.
This eclipse will be visible throughout South Africa, and also in southern parts of Madagascar, Mozambique, Zambia, and Zimbabwe. It will also be visible over a wide area of the Indian Ocean, and Antarctica (if you happen to be a penguin).
The best way to observe a partial solar eclipse is with a filter specifically designed for observing the sun, sold by stores specializing in telescopes. Safe “eclipse shades” are often widely available prior to an eclipse. A number 14 welder’s glass also works well, and is available from specialized welding shops. The ordinary number 12 welder’s glass sold in hardware stores does not provide adequate protection.
If you don’t have a proper solar filter, you can view the partially eclipsed sun with a pinhole camera by punching a hole about a millimeter in diameter in a piece of cardboard. Natural “pinholes” created by leaves on trees or reflections from a building’s windows will also work.
Under NO circumstances look directly at the sun, even with sunglasses, as you can quickly cause permanent damage to your eyes. If a small magnifying glass can light a fire in seconds, think what will happen to the retina of your eye by staring at the sun.
As always, we welcome your pictures of the partially eclipsed sun; a solar filter on your camera will be essential. The sensor in your camera is just as easily damaged by the direct sun as your eyes. Try to get a landmark or tree in the foreground to give a sense of scale.
Saturday, September 5, 5:54 a.m. EDT
Last Quarter Moon
The Last Quarter Moon rises around 11:30 p.m. and sets around 3 p.m. It is most easily seen just after sunrise in the southern sky.
Sunday, September 13, 2:41 a.m. EDT
The Moon is not visible on the date of New Moon because it is too close to the Sun, but can be seen low in the East as a narrow crescent a morning or two before, just before sunrise. It is visible low in the West an evening or two after New Moon.
Monday, September 21, 4:59 a.m. EDT
First Quarter Moon
The First Quarter Moon rises around 1:30 p.m. and sets around midnight. It dominates the evening sky.
Sunday, September 27, 10:51 p.m. EDT
The September Full Moon is known as the Harvest Moon or Full Corn Moon. It rises around sunset and sets around sunrise; this is the only night in the month when the Moon is in the sky all night long. The rest of the month, the Moon spends at least some time in the daytime sky.
Neptune at opposition
Tuesday, September 1, midnight
Neptune will be directly opposite the Sun in the sky, and visible all night. It is located in Aquarius but is too faint to be seen with the unaided eye. Use binoculars and a star chart from Starry Night.
Mercury at greatest elongation east
Thursday and Friday, September 3 and 4, dusk
Mercury will be well placed in the evening sky for observers in the southern hemisphere, less so for observers in the north.
Aldebaran and the Moon
Friday/Saturday, September 4/5, near midnight EDT
Observers on the eastern part of North America with low eastern horizons may be able to see the Moon occult the first magnitude star Aldebaran just after moonrise (around midnight, but check for local times). Observers in Europe will see it just before sunrise on the 5th.
Friday, September 11–Thursday, September 24, before dawn
The best time in the year to see the dim glow of the zodiacal light in the pre-dawn eastern sky, the light reflected from millions of interplanetary particles. It lies along the ecliptic (shown in green).
Partial solar eclipse
Sunday, September 13
This eclipse will be visible from southern Africa, Antarctica, and the oceans in between. Seen here is the maximum eclipse in Cape Town, South Africa.
Wednesday, September 23, 4:21 a.m. EDT
The Sun crosses the celestial equator moving southward, causing the days to grow shorter in the northern hemisphere and longer in the southern hemisphere.
Sunday, September 27, evening
There will be a triple treat for observers in eastern North America as sun, Earth, and moon align: a total lunar eclipse, the moon at its closest, and a full moon, all in one evening. At 8:12 p.m. EDT, the lunar eclipse will begin with the first faint lunar shadow creeping onto the moon. At 9:48, the moon reaches an extreme perigee, the closest it will get to Earth in all of 2015: 221,753 miles (356,877 km). Total phase of the eclipse will begin at 10:11 p.m. and mid eclipse will be at 10:47. At 10:51 it will be the instant of full moon, the largest full moon in 2015. At 11:23, the total phase of the eclipse will end, and at 1:23 a.m. the last of the moon’s shadow will leave the moon. In western North America, the moon will already be in eclipse when the moon rises. Observers in South America, Europe, and Africa will also see most of this eclipse. The illustration shows the moon just entering the umbral shadow of Earth at 9:12 p.m. EDT.
Vesta at opposition
Monday, September 28, 11 p.m. EDT
The brightest asteroid Vesta will be directly opposite the Sun and visible all night in Cetus. At magnitude 6.2, it will be right at the limit of naked-eye visibility, but easily spotted with binoculars.
Mercury is well placed in the evening twilight for the first half of the month. This apparition is more favorable for observers in the Southern Hemisphere.
Venus is now a bright object in the pre-dawn sky, reaching maximum brightness of magnitude –4.8 on the 21st.
Mars is low in the eastern twilight, moving eastward through Cancer into Leo.
Jupiter reappears in the eastern pre-dawn sky in the middle of the month..
Saturn is low in the southwest mid-evening sky, and sets in late evening.
Uranus rises in mid-evening in Pisces, nearing opposition on October 12.
Neptune is in opposition on the 1st, visible all night in the constellation Aquarius.
Mercury reaches its greatest elongation east of the sun this week on Friday, September 4.
Here we see it as we might from space, say on the International Space Station or the Hubble Space Telescope. The green line marks the ecliptic, the path the sun appears to follow over the year. Most of the planets also appear to revolve in this same plane.
The red line is Mercury’s orbit, which you can see is tilted quite a bit compared to the ecliptic. Of all the planets, Mercury’s 7 degree tilt is the most extreme. The small orange dot marks Mercury’s position on Friday, as far east of the sun as it can go.
How will Mercury look to us here on the surface of the Earth? It very much depends on where you are located.
This view of Mercury is how it will appear to me from my location close to Toronto, Canada at sunset. Everyone at a similar latitude across southern Canada, the northern United States, and most of Europe and Asia will see something very similar.
Because of the Earth’s current position in its orbit around the sun, the ecliptic makes a very shallow angle with the western horizon as seen from the northern hemisphere. So even though Mercury is as far west of the sun as it can get, at this time of year it ends up very close to the horizon around sunset, the best time to look for it. To make matters worse, because of its orbit’s tilt, Mercury is quite far south of the ecliptic at this time. As a result Mercury is barely 7 degrees above the horizon at sunset.
The situation in the southern hemisphere, here seen from southern Australia, is very different. The ecliptic makes a very steep angle with the horizon, and Mercury is south of the ecliptic, so Mercury is much higher above the horizon at sunset, 26 degrees in fact.
As a result, spotting Mercury from the northern hemisphere will be a major challenge this week, but the lucky people in the southern hemisphere will have a fine view.
This situation reverses in the spring, when northerners get a fine view of Mercury at dusk, and southerners are out of luck. It also reverses when Mercury is at elongation on the western side of the sun. That is why every year we publish a table showing which elongations of Mercury will be favorable or unfavorable, depending on which hemisphere you view it from.
All in all, even though it is very bright, Mercury is probably the most challenging planet to view. You have to be in the right place at the right time.
Mercury viewed through a telescope is a disappointment. The most you will see is a tiny disk, which goes through phases similar to Venus and the moon. But it is a great satisfaction to most stargazers to say that they have actually seen it at all.
You’ll probably be hearing a lot about a “super moon” for the next few months. This is not a term that astronomers use, but here are some facts about what will actually be happening.
Full moon is when the sun, Earth, and moon line up with the Earth in the middle. As seen from the surface of the Earth, the moon is fully illuminated. Because it is exactly opposite the sun in the sky, the moon rises in the east just as the sun sets in the west and, roughly 12 hours later, sets in the west just as the sun is rising in the east.
Because the Earth is constantly revolving around the sun and the moon is constantly revolving around the Earth, full moon is an instantaneous event, occurring when the moon is exactly opposite the sun. This week this happens at 2:35 p.m. EDT on Saturday, August 29.