Let there be (more) light


Earliest sunset arrives this Saturday, latest sunrise arrives early next month. Summer constellations can still be seen (but not for much longer) And planets aplenty for skygazers this week.

December 2, 2013


[Dave Heller] Still weeks out from winter solstice though the earliest sunset comes this week. Let’s figure out why with Derrick Pitts, chief astronomer at the Franklin Institute. Derrick, help us solve this riddle.

[Derrick Pitts] This is something that comes around every year when we realize that the earliest sunset comes on December 7. How can that be if the winter solstice is actually on December 21? Well if we actually look at all the times and all the factors that are involved in this, what we come up with is a very, very interesting thing. Here’s what goes on: The earliest sunset comes December 7, early in December all the time, every year — nothing ever changes as far as that’s concerned. But the latest sunrise doesn’t come until early January. So that would make one think that things are somewhat out of balance. Well they aren’t, really.

What’s really going on is that it’s sort of a fight between the way in which we humans set up our calendar to be evenly set, and the way that we set up our day to be an even number of hours and minutes when it really is not. Our day is not exactly 24 hours, the year is not exactly 365 days, but we’ve set it that way so that things work evenly and smoothly for us. If we were to go by the Earth’s motion around the sun and the Earth’s axial tilt, what we’d find is that we are exactly where we’re supposed to be and that shortest number of hours of daylight really does come right around the 21st. And that’s the ideal day, there’s no question about it. It’s just that due to the elliptical orbit of our planet around the sun, the earliest sunset comes earlier in the month; the latest sunrise comes early the next month. But they do all eventually balance out.

So solstice sort of splits the difference?

In a way, you could say solstice splits the difference. Although it is correct, it’s exactly the right time, but the way I sort of see it is the mid-winter point. And I think of December 7 — the earliest sunset — as the mid-winter point because in just a few days after this, next week we’ll start to see that sunsets are beginning to come later.

Invoking mid-winter though brings up puzzle number two: How is it that we’re still seeing summer constellations in the night sky?

Let’s remember that the motion around the sun takes about 360 degrees to make us go all the way around. And there are 365 days in the year, so it’s about a degree per day. So it’s a very slow transition from constellation to constellation, from seasonal sky to seasonal sky. We’re right on the very end of that, so the bright constellation of the summer Cygnus, which we saw overhead in the summer now stands upright as a cross on the western horizon right after dark. But pretty soon, in another 30 days or so, we’ll no longer be able to see it at all, it’ll be gone from the sky and we’ll have the full layout of winter constellations.

The full layout this week includes a panoply of planets as well.

Oh, we have a great set up of planets. It’s amazing if you start off in the evening sky just after sunset, just as the sky gets dark we see Venus in the southwestern sky as bright as it ever gets. And I’m sure most of our listeners have seen it already because it’s the big, bright white object that stands motionless almost in the southwestern sky. This week if you’re willing to do just one early morning from 5:45 to 6:45 here’s what you get: High in the southwestern sky is the largest planet in the solar system, Jupiter. To the left of Jupiter, to the south, southeast is Mars. Down lower on the eastern horizon we find Saturn. Even closer to the horizon is bright Mercury. And then just to the left of those two is Comet ISON.

It’s an interesting race though, because if you do one day, you have to fight a number of things. And that is you’re fighting the advance of sunrise light, so the later you go out and start this, the more difficult it will be to see lower objects on the eastern horizon — Saturn, Mercury and Comet ISON. If you do start at 5:45, the sky is still dark enough. With a pair of binoculars you can identify those. Mars looks wonderfully rosy. And of course also with a pair of binoculars, you should be able to see the four Galilean moons orbiting Jupiter. So that’s a great sight to see.

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