Wednesday, September 12, 2007

We've been passed by!

Early this morning, I was taking my sister to the airport, and I noticed the planet Venus up pretty high in the pre-dawn sky. I was a little surprised by this, since the planet Venus was high in the evening sky just a couple months ago.

I should not have been surprised. First, all the stars appear to move from the evening sky to the morning sky, though takes place on longer timescales (a few months). And, while I'm often up and peering at the evening sky, I rarely am awake enough to see the dawn sky (unless I'm at the telescope).

But Venus's rapid appearance in the morning sky is also due to Venus's orbit in the Solar System. Venus is closer to the sun than the Earth, so it moves faster than the Earth due to a stronger pull of gravity. It also has a smaller orbit, and so can get around that much faster.

When we see in the evening sky, it is getting ready to pass the Earth. As it catches up to the Earth, Venus seems to hang lazily in the evening sky, but really it is hurtling closer to us, preparing to pass on the inside curve. When it gets close to Earth, it appears to zoom past, quickly moving from the evening to the morning, and then it hangs lazily in the morning sky as it races away from us. It is much like watching a faster car come up on you on the interstate. In your rearview mirror (which is our evening sky), you see the car coming -- you can see it getting bigger (as you could with Venus if you used a telescope), but the car appears to remain in about the same place in your mirror until it gets close. Then the car zips by, appears out your front windshield, and pulls away, all the while appearing smaller, but not changing position much. Likewise with Venus -- once it is in the morning sky, if you watch it with a telescope, it will rapidly decrease in size, although it appears to hang in about the same spot in the sky. So, as the nights are getting longer, if you happen to get up before sunrise, look toward the east. That bright "star" you will see there for the rest of the year is the planet Venus, leaving the Earth yet another lap behind in our race around the sun.

But don't worry about getting lapped by our sister planet. In December, we'll pass inside of Mars, and any Martians will just have to watch us pass them by.

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