Dancing Planets

Take a look at the western sky just after sunset this week, and you’ll see an amazing sight: three bright planets in close formation.


Just after sunset on Sunday May 26 the three brightest planets, Venus, Jupiter, and Mercury, will form a perfect tiny triangle in the western sky. Credit: Starry Night software.

The giant planet Jupiter is moving behind the sun, so is dropping rapidly in the western sky. Mercury and Venus, much closer to Earth, are just emerging from behind the sun, moving in the opposite direction from Jupiter. The three meet this week.

Find yourself a location with a very low western horizon. Go there just after sunset. A small 7x50 or 10x50 binocular will help, but don’t point it in the direction of the sun until it is well below the horizon.

Venus is the brightest of the three, and so should be the first that you see, just above and to the left of the point where the sun has just set.

Astronomers use an upside down scale to measure brightness. The brightest stars are magnitude 1; the faintest the human eye can see are magnitude 6. Really bright objects, like these planets, end up with negative brightness. Tonight Venus will be magnitude –4.

Once you’ve spotted Venus, look for Jupiter nearby. Jupiter will be two magnitudes fainter than Venus, magnitude –2, still extremely bright.

Finally, try to spot the third planet, Mercury, slightly fainter than Jupiter at magnitude –1. Tonight (May 22) and tomorrow night it will be just to the right of Venus, moving upward and to the left more rapidly than Venus.

Try to catch these three every night for the next week, and you’ll see them appear to circle around. On Sunday night, May 26, they will form a perfect equilateral triangle, two degrees on a side.

Mercury continues to rise and Jupiter continues to set so that by the middle of next week they will appear in a straight line with Mercury highest and Jupiter lowest, brilliant Venus in the middle.

We don’t often get to see three planets in such close proximity and see how rapidly they appear to move.

The Mountains of the Moon

When you look at the moon through binoculars or a small telescope, the first thing you notice is that the moon is divided into two distinct forms of terrain: large dark flat plains and bright mountainous highlands. Both of these are pockmarked by an enormous number of craters of all sizes.

The northern half of the moon exhibits many mountain ranges and a few isolated peaks. Credit: Starry Night software.

Early observers of the moon assumed that the large flat plains were seas, not knowing that liquid water was not available on the dry airless surface of the moon, and so named them “maria,”the Latin for “seas,”singular “mare.”“Mare”is pronounced “mah-ray,”not like a female horse. They named the lunar highlands for the mountains of Earth, not knowing that the mountains of the moon were formed by a totally different process from the mountains of Earth.

On Earth, mountains are formed by two different processes. Most mountains, and mountain ranges, are formed by tectonic action: the plates that make up the surface of the Earth bump into each other causing mountains to rise up. Other mountains on Earth are caused by volcanic action: hot magma welling up from the depths of the Earth to deposit itself on the surface as volcanoes.

The moon has neither tectonic plates nor volcanic action. Virtually all of its mountains are the result of impacts by asteroids in the distant past. Early in the moon’s history, there were many gigantic asteroids in the solar system. When these impacted the moon and planets, they formed craters far larger than the ones we see today, forming the lunar maria and leaving their rims to form lunar mountain ranges. The enormous heat generated by these impacts melted the surface material of the moon and caused it to flow, swamping some craters and mountains, which stand out now as ruins on the surface of the maria.

This chart shows some of the mountain features visible this week at the current phase of the moon, around first quarter. Several prominent craters are marked to help you get your bearings: Aristoteles, Plato, Archimedes, and Copernicus.

The mountain ranges and individual mountains are labeled with their Latin names, “montes” for mountain ranges and “mons” for individual mountains. Far over to the east are the Taurus Mountains (Montes Taurus), the landing place of the last of the manned lunar explorers, Apollo 17.

Two major mountain ranges divide the Mare Serenitatis from the Mare Imbrium: the Montes Caucasus to the north and the Montes Apenninus to the south. Where these two meet is the prominent mountain Mons Hadley, named for British optician and instrument maker John Hadley (1682–1743). This is where Apollo 15 landed in July 1971. The lunar Alps, the Montes Alpes, sweep off to the northwest, enclosing the perfect oval crater Plato.

On the barren floor of the Mare Imbrium are two of the most impressive single mountain peaks on the surface of the moon, Mons Piton and Mons Pico.

The Mons Piton has a base 16 miles (25 km.) in diameter and towers 7,380 feet (2,250 m.) over the surrounding plain. The Mons Pico is even more impressive, with a base measuring 9 x 16 miles (15 x 25 km.) and a height of 7,870 feet (2,400 m.) Both these are named after mountains on the island of Tenerife in the Canary Islands.

Although these mountains look impressive under the low light of a rising sun, they really are quite gentle when compared to the mountains of Earth. If you look again in a day or two, they will be practically invisible except for their whiter color.

Enjoy your lunar mountain climbing expedition.

Ring of Fire Eclipse

On May 9 and 10 there will be an annular eclipse of the sun by the moon, visible only in some of the most remote parts of the world: western and northern Australia, Papua New Guinea, the Solomon Islands, and a few remote Pacific atolls.

The annular eclipse of the Sun by the Moon, as it will appear from Cooktown, Queensland, Australia on the morning of May 10 at 8:49 a.m. Credit: Starry Night software.

What is an annular eclipse?

The orbit of the Earth around the sun is an ellipse, not a circle. Similarly, the orbit of the moon around the Earth is also an ellipse. This means that sometimes the Earth is closer to the sun (called perihelion) than at others (called aphelion), and sometimes the moon is closer to the Earth (called perigee), sometimes farther away (called apogee).

We are fortunate to live in a time when the sun and the moon are very close to the same apparent size in our sky. This is an illusion of perspective: the moon is small (2,159 miles/3,475 km.) and close by (238,855 miles/ km.) while the sun is large (865,278 miles/1,392,530 km.) and far away (92,955,808 miles/149,597,872 km.)

Notice that the sun is about 400 times larger than the moon in diameter. It is also about 389 times farther away, pretty close to 400. This explains why the two appear to be almost the same size in the sky. But “almost” is not exact, which explains why there are different kinds of eclipses.

Distances in the sky are measured in angles, 360 degrees making up a full circle. Both the sun and the moon appear to be just slightly more than half a degree in diameter. Degrees are divided into 60 arc minutes, and the exact size of the sun varies from 33 arc minutes when it is closest to the Earth on January 2, to 31 arc minutes when it is farthest from the Earth on July 5. On May 10 it will be 32 arc minutes in diameter.

Over the course of a month, the moon’s size also varies. On April 27 it was at its closest to Earth and appeared to be 33 arc minutes in diameter. If an eclipse had occurred on that day, the moon would have covered the sun completely, and we would have had a total eclipse. On May 10 the moon will appear to be 30 arc minutes in diameter, since it is only a few days away from its farthest from the Earth on May 13. A 30 arc minute moon doesn’t quite cover a 32 arc minute sun, so the sun peeks out as a ring all around the moon. “Annular” is Latin for “ring,” so this is called an “annular eclipse.”

Astronomers tend not to get as excited over an annular eclipse than a total eclipse. Because the moon doesn’t cover the sun completely, you don’t see the prominences and corona which are the most exciting part of a total eclipse. Thus I was quite surprised by the annular eclipse I observed from Toronto exactly 19 years ago on 1994 May 10. Having observed a total eclipse in the past, I wasn’t expecting much from this annular eclipse, yet I found it to be a very powerful emotional experience. Even though 5 percent of the sun was still peeking around the moon, it had the same ominous feel as a total eclipse, much more so that the several partial eclipses I’ve witnessed. Seeing the “ring of fire” around the moon is far more impressive than seeing only part of the sun covered.

Where to see it?

Unfortunately, very few people will get to see this annular eclipse, as its path travels over some of the most remote and unpopulated parts of the Earth.

The eclipse begins at sunrise over the wilderness of Western Australia. It then sweeps over equally empty Northern Territory and on to northern Queensland, far to the north of Cairns where many people witnessed last years total eclipse. Only a few roads intersect the eclipse path. The eclipse path crosses the Coral Sea and touches the eastern end of Papua New Guinea, then crosses through the middle of the Solomon Islands. From there the path neatly avoids just about every island in the south Pacific except for Tarawa and Fanning Islands, both part of the Republic of Kiribati, formerly known as the Gilbert Islands.

Although few people will see the complete annular eclipse, a much larger number will see it as a partial eclipse. This includes all of Australia, Papua New Guinea and the Hawaiian Islands, much of Indonesia, the Philippines, and New Zealand. Unfortunately the partial eclipse just misses being visible in North America, except just at sunset at the southern tip of Baja California. In Honolulu, maximum eclipse will be at 3:48 p.m. on May 9, when 32 percent of the sun will be hidden by the moon.

How to observe it?

For most people who may see this eclipse, it will be a partial eclipse, which is the most dangerous kind of eclipse, because people will be tempted to take quick glimpses of it without proper protection. DON’T DO IT! Looking directly at the sun is always dangerous and can cause permanent damage to your eyes.

There are two safe ways to view a partial (or annular) eclipse. The first is with an approved solar filter. These can be purchased from telescope stores. The only safe equivalent is a #14 welder’s glass. This is denser that the #12 widely available, and usually can only be found in dealers specializing in welding supplies.

The other safe viewing method is to use a large cardboard box to make a pinhole camera. Make a pinhole in one end of the box to act as the lens, and a large hole in the bottom of the box to stick your head through to view the image of the sun. Natural pinhole cameras often are formed by chinks in window blinds or gaps between leaves of trees. So don’t look at the sun, but put your back to it and look instead at the ground in front of you.