Ask a rocket scientist - A problem of identification

What is that bright star near the horizon at sunset?

Dr. Jeff

I'll give you a hint: it was near the full Moon a couple of weeks ago.

This was actually a tricky problem one of my colleagues gave on a test in his college-level astronomy course a few years ago.  I was delighted when seeing the star in real life reminded me of the problem.

Many people know that Venus is generally the brightest "star" in the sky and is typically seen at sunset or sunrise.

WHICH PLANET IS DOMINATING THE SKY?

In fact, all summer long, we had a wonderful view of this planet at sunset.  Mars and Saturn were up too.

But what is this bright interloper to the east?  Just knowing that there is a bright planet visible at sunset isn't quite enough information to identify it.  Knowing the planet was visible next to the full Moon pretty much clinches it.

Still wondering?

Venus orbits inside the Earth.  That means that the angle between the Sun and Venus is always relatively small.  In other words, it appears in the glow of the sunset or sunrise.

This other bright planet appeared next to the full Moon.

The full Moon can appear at sunset, but on what side of the sky: east or west?  How sunlight striking the moon is viewed from Earth determines the phases of the Moon.  You can't see the full Moon unless the sunlight passes by the Earth and strikes the Moon "face on."  In other words, you have to be looking away from the Sun in order to see the full Moon.  At sunset, away from the Sun is toward the east.  If you think about the geometry of the solar system for a moment, you will realize that looking away from the Sun at sunset means that you are looking toward the outer solar system, where planets like Jupiter, Saturn, Uranus and Neptune orbit (Pluto has been re-categorized as the first Trans-Neptunian Object - more on that some other day).

Venus orbits inside the Earth, so you won't see it when you look away from the sunset or sunrise.  So that bright "star," which is gradually marching westward over the weeks, which makes it a planet (planet means "wanderer in Greek) is one of Jupiter, Saturn, Uranus or Neptune.

If you remember the order and sizes of your planets, and the fact that I already mentioned that Saturn was up over the summer, you probably have come to the conclusion that the bright planet rising in the east is Jupiter.

Whew.

It took a very long paragraph to answer that question, which is probably why very few of the college students in my colleague's class got the question right.