Inferiority Complex

Seth Jarvis

There is a lot happening in the sky in 2012!

There’s the annular eclipse of the Sun on May 20th.

There’s the not-again-visible -in-America-until-the-year-2125 Transit of Venus on June 5th.

I’ll post entries in the Clark Planetarium blog about those and other night sky events as we get closer to them.

For now, I want to call your attention to something worth observing that will soon happen in the evening sky – the beautiful grouping of the planets Mercury, Venus and Jupiter that will appear in the western sky soon after sunset in late February and early March.

As I write this in early February, Venus is hugely bright in the west as evening falls, and Jupiter is almost as bright, high in the south.

Now let’s look ahead at the week of March 5th.  Low in the western horizon as evening twilight fades to night you can see the speed-demon of our solar system.  Below Venus just above the horizon and looking like a bright but otherwise ordinary star is the planet Mercury.

Mercury, the smallest and innermost of the planets, orbits the Sun very quickly, taking just 88 days for Mercury to make one trip around the Sun.

Because it is so close to the Sun and orbits so quickly it is only briefly visible in the night skies of Earth. When it is visible it changes its nightly position against the background stars faster than any other planet.

To the ancient Greeks, this was the god Hermes, the messenger who took communications from the Olympian Gods to humans. Hermes was traditionally depicted as having wings on his feet and a winged hat to aid his speedy movement.  To the ancient Romans, this was Mercury, also sporting wings on his feet.

Mercury the planet completes an orbit of the Sun in less than three months.

Mercury the ancient god was famous for his speed.

See the connection?

Mercury and Venus are referred to as “inferior” planets.  That’s not a judgment of their planetary quality, but rather an astronomical way of saying that these planets’ orbits around the Sun are smaller than Earth’s own orbit of the Sun.

Care to guess what we call planets whose orbits are larger than ours?  That’s right; they’re “superior.”

From where we are in our solar system, Mars, Jupiter, Saturn, etc., are “superior” planets, because they’re farther from the Sun than us.

To a resident of Mars, Earth would be an “inferior” planet.  To a resident of Mercury, all planets would be “superior.”  To a resident of Neptune, all planets are “inferior.”

Refer back to the first image at the top of this post: Here you are looking at the western sky on March 5, 2012 as twilight gives way to night.

There high in the west is Venus, brilliant as always, stealing the show. No planet is brighter in the sky – ever.  Above Venus and slightly to the left is an almost equally bright Jupiter, the largest planet in our solar system. It only appears dimmer than Venus because it is much farther from us.

Hold your arm out towards the western horizon and make a fist with your outstretched hand. Place the bottom of your fist on the horizon.  The top of your fist is then 10 degrees above the horizon, which is about the height at which you’ll find Mercury.

Mercury, Venus and Jupiter, two inferior planets and one superior planet, nicely grouped for an evening family portrait.  What’s going on that makes this view possible?

Now let’s imagine we have flown a few hundred million miles above Earth and are looking down at our inner solar system:

Notice that the angle between Mercury and the Sun is just about as great as it can possibly be. That’s a pretty shallow angle, and it means that Mercury is only going to be visible in the night sky in the general direction of the Sun even when it’s at its maximum angular separation from the Sun (known as elongation).

Now let’s go back to Earth and look into that western sky after sunset, but this time let’s subtract our horizon from the view and trace the orbits of Mercury and Venus into the picture. This is the exact same view as the first image at the top of this post, but tweaked slightly for your edification:

It starts to make sense now, doesn’t it? You can see that Mercury is at its maximum elongation, and Venus is very nearly at its maximum elongation. This is your best chance to see these two “inferior” planets together in the evening sky this year.

Now let’s look at the same western horizon thirty minutes or so after sunset, but a week later on March 12:

Mercury is just barely above the horizon, and becoming difficult to see in the glare of twilight.  By the time the sky is good and dark Mercury has set.

But shining brilliantly much higher in the western sky are the solar system’s two brightest planets, Jupiter and Venus, side by side.

I can guarantee you right now that the staff at Clark Planetarium who answer the phones are going to get a lot of questions from folks wondering what a pair of car headlights is doing so high in the sky.

If Jupiter is a superior planet, and Venus is an inferior planet, then how can we observe them together like that?

Fly about a billion miles above the Sun, look down on the solar system, and it all becomes clear.

Isn’t it great what a change in perspective can teach you? Inferior planets and superior planets can sometimes arrange themselves in memorable alignments.

Jupiter will continue to sink lower against the western horizon each night until by late April it’s setting so soon after sunset that it is pretty much long longer part of the evening stargazing experience.

Venus, however, remains visible in the west after sunset until late May.

Observing Mercury, Venus and Jupiter as they move through the western sky after sunset in the coming several weeks is a good way to get a feeling for the ever-changing positions of Earth and the other planets of our solar system.

Don’t worry about losing sight of these planets this spring.  As Mercury, Venus and Jupiter exit the scene to the west, we’ll have great views of Mars and Saturn, and I’ll cover observing these two fascinating planets in a future post.

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