Get Out and Enjoy Autumns’ Gradually Changing Night Skies
Opinion Advocates for ideas and draws conclusions based on the author/producer’s interpretation of facts and data.
By Scott Levine
The last time we were together, we stepped out and tracked down the Summer Triangle. As the leaves gather at our feet and ghosts and goblins fill our streets, I thought it’d be fun to check back in with those stars.
In September, we found Vega almost straight up overhead at around eight o’clock. The Triangle’s other two stars, Altair and Deneb, stretched to the other side of the zenith. Together, they straddled the entire top part of the sky’s dome.
But where are they now? Sure, we could just look, but wouldn’t it be more fun to figure out what’s going on?
We usually talk about a day being 24 hours long. That’s what our clocks say, and what astronomers call a solar day. It’s the amount of time it takes for the sun to return to the same place in the sky. Since we’re always moving in our orbit, Earth needs to turn a little more than one full turn to catch back up with the sun each solar day.
We don’t need that extra push when we look at the night’s stars, though. They return to the same place in the sky about every 23 hours and 56 minutes, which we call a sidereal day. The difference between these two measurements means stars rise four minutes earlier each night, relative to our clocks. So, every star will arrive at the same place in the sky tonight four minutes earlier than it did the previous night.
Four minutes might not sound like much, but they add up fast. Over the course of a month, that’s 120 minutes. Two hours! Given enough time, as our clocks fall more and more out of sync with the sky, our view on the galaxy gradually changes.
Tonight, let’s head out and look for the Summer Triangle again. Rather than being up near the top of the sky, it’s pointing downward and looks almost like it’s diving toward the western horizon. All three stars are now firmly setting in the sky’s western half. If we keep watching as fall becomes winter, we’ll see them keep sinking into the dusk.
Meanwhile, those daily four minutes gradually bring new objects into the night. Rising in the east by mid-evening, let’s see if we can spot the tiny, dipper-shaped Pleiades. It’s a dusty cluster of young stars, one of the closest star clusters to Earth. Night by night, four minutes at a time, it’ll cross the sky until next spring, when it, too, disappears into the west.
These days, the Pleiades appear to be a short distance to the left of a very bright, orange-red light. That’s the giant planet Jupiter. It’s been particularly stunning lately, calling through the chilly evenings from 500 million miles away.
About the same distance to Jupiter’s right is muted-yellow Saturn, our solar system’s second biggest planet. Saturn is about 80 percent the size of Jupiter, but it’s around a billion miles away, about twice as far as Jupiter. It’s always interesting to see the two near each other and compare how different they look.
Autumn’s long nights give us plenty to see. I hope you’ll head out and look up this month. Happy Halloween, and clear skies!
Scott Levine (astroscott@yahoo.com) is an astronomy writer and speaker from Croton-on-Hudson. He is also a member of Westchester Amateur Astronomers, a group dedicated to astronomy outreach in our area. For information about the club including membership, newsletters, upcoming meetings and lectures at Pace University and star parties at Ward Pound Ridge Reservation, visit www.westchesterastronomers.org.
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