During the first half of November the planets Mercury and Venus will appear close to each other in the evening sky. The challenge is seeing them. The difficulty arises from the geometric relationship between Earth’s axis tilt and its orbit at this time of year. This particular geometry results in a shallow viewing angle between the horizon and the orbital planes of planets in the evening sky (in the morning sky the angle is much larger). This means Venus and Mercury will appear low in the sky after sunset.

Finding them in the twilight so close to the horizon can be a challenge. Look for the pair in the southwest right after sunset. Binoculars can be a great help in searching for them. Venus is the brighter of the two. Mercury will appear about 2 degrees below Venus (an index finger held at arm’s length has an apparent width of about 2 degrees).

Looking southwest from Salt Lake City on November 4 at 6:10 p.m.

Other astronomical happenings in November:

We will experience a 25 hour day on Sunday, November 6 as Daylight Saving Time ends.

The Leonid meteor shower peaks on the evening of November 17, but light from a last quarter moon will interfere with the view for those observing after midnight.

Blog readers in Antarctica will have the opportunity to observe a partial solar eclipse on November 25.

Mercury should be visible in the night sky through most of March. It will be at greatest elongation east of the Sun on March 22. Details on observing Mercury and the MESSENGER mission can be found here.

Go outside and see Mercury at this historic time!

What is greatest elongation? Greatest elongation occurs when the angle between the Sun and either Mercury or Venus is largest as seen from Earth. The time around greatest elongation is a good time to view these planets as they are near their highest points above the horizon.

Mercury is so close to the Sun that even the largest angle is still small, (between 18 and 28 degrees). This is why it is only visible during the evening or morning twilight. On January 9, Mercury will be 23 degrees west of the Sun. In contrast, Venus will be 47 degrees west of the Sun on January 8 (about twice the angle of Mercury). Because they are both west of the Sun, they rise before the Sun and are visible in the early morning sky. Look for them in the southeast before sunrise. Venus will appear as the brightest object in the sky. Mercury will be very faint, much closer to the horizon.

Looking southeast from Salt Lake City on January 8 at 7:30 a.m.

Mercury moves quickly, so it will disappear from the morning sky by the end of January.  It will reappear in the evening sky by mid-March and will be at greatest elongation east of the Sun on March 22, 2011. This will be the best opportunity for northern hemisphere observers to view this planet in the evening this year. Look for it almost due west about 30 minutes after sunset.

Venus remains in the morning sky until early July. It will return to the evening sky in late September.

Of the five planets that can be seen with the unaided eye, Mercury is the most difficult to observe. This is because it is only visible during evening or morning twilight. At certain times each year the viewing geometry does make it easier to find. That occurs this year in late March and early April, or now.

Looking west from Salt Lake City on April 2 at 8:23 p.m.

There is an additional help this year, the planet Venus. Mercury lies within 4 degrees of Venus during the first ten days of April. Look for both planets a little north of west about 30 minutes after sunset. Binoculars can be a great help in searching for them in the evening twilight. Venus is the brighter of the two. Mercury will appear a little below and north of Venus.

How small is Mercury? It would take eighteen Mercury’s to make one Earth. Up close, Mercury resembles the Moon. Below is a color view of Mercury taken by the MESSENGER spacecraft during its third and final flyby of the planet on September 29, 2009. MESSENGER goes into orbit around Mercury in March of 2011. For more images, check out the MESSENGER website.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington