Winter comes along and the days get shorter because the earth is in the part of its orbit whereby the hemisphere one lives in is tilted away from the sun. Fine. No rocket science there. But how come the process of the sun going down is faster? In the summer, the sundown seems to take forever. After the sun dips below the horizon, it still seems to stay light outside for quite a while. But in the winter, once the onset of sundown starts it seems to complete in a much shorter time. WTF?
Aw, crap…I meant “faster sundowns” in the OP subject line.
Picture yourself living in a three-dimensional bowl, with sky above and beneath you and the Earth cutting it in half. Twilight occurs when the sun is in the circular band extending from the Earth downward for 18 degrees.
In summer (from the mid-latitudes) the sun sets at a much shallower angle and spends a longer time within that band. North of 49 degrees, at the solstice, the Sun never leaves the band, as it passes from northwest to northeast below the northern horizon, and it never gets completely dark.
I think you actually meant faster.
I mean slower :smack:
To look at it in another way – at the poles, the sun takes 6 months to set. And six to rise. At the equator, it sets much faster, you can actually see it move (but look at it with eclipse glasses, please).
In winter the sun also has a shallower angle, the way it is at the poles, so it also actually sets more slowly. However, it is twilight so much longer that it feels like it sets sooner than it actually does. In short, it’s our perception, not reality. Personally, I think the night literally FALLS in winter, but the day just kind of melts and mellows into twilight in the summer.
No, it doesn’t. It takes about 30 hours for the disk of the sun to sink beneath the horizon at the poles. It takes less than two months after that for the sky to become completely dark.
No, it’s reality. The Sun sets at a shallower angle in summer, and both the disappearance of the disk below the horizon and the ensuing twilight last longer than in winter.
Hmmm…there appears to be a divergence of opinion here. The way I see it, the sun does indeed set at a shallower angle in the winter…that’s why it’s winter (shallow angle = less direct rays = colder days; also, shallow angle = shorter arc = shorter days). As I think about this, though, it appears to me that this should make the sundown process slower than the summer sundown process (by the “sundown process” I mean the time it takes the disc to disappear completely once it starts to go below the horizon). My initial reaction is that that’s not the case …or is it? When I think about it, I can’t really say for sure that the winter sundown process is faster than the summer sundown process; rather, I can say for sure that the whole process including the twilight experience is faster in the winter. So, I guess we could have a slower sundown process in the winter, but a faster “light to dark” experience in the winter as well. Okay…maybe. But again: if that’s the case, why is the twilight experience faster in the winter?
Crap. Cecil needs to iron this one out personally.
That is my experience. If you spend some time in the tropics, where it’s essentially always summer, it seems like twilight comes and goes quickly.
Imagine that you could see the ecliptic plane in the sky - the path that the Sun and planets follow (and the Moon, more or less, though the Moon’s orbit is inclined with respect to the ecliptic). Because the Earth is inclined at about 23 degrees to the ecliptic, the plane seems to flap up and down during the day. At midday on the winter solstice, the ecliptic is at its lowest (and hence the sun is at its lowest noonday point) and at midday on the summer soltice, it’s at its highest.
Okay so far? Now, the position of the ecliptic plane at midnight is the opposite of its position at midday - it’s at its highest at winter midnight, lowest at summer midnight. That’s why the moon gets seriously high in the winter.
Still okay? Now at midwinter nightfall the ecliptic plane is rising quickly (in the south) which means that it’s quickly becoming more steeply angled at the horizon. So as the sun slips down the ecliptic, it’s following a steep path at the horizon.
Similarly, in midsummer it’s doing the opposite - the plane is descending quickly from its midday high and so quickly becoming less steeply angled at the horizon, and so the sun is following a shallow path at the horizon and takes longer to disappear below the edge of the world.
Did I miss something in my rocket science class? December 21st is the first day of Winter and the shortest day of the year. That means that the days for the next six months after that will be getting longer. Or did I miss something?
This conversation would benefit from some hard data. Astronomical Twilight in New york City lasts…
92 minutes on March 21
127 minutes on June 21
92 minutes on September 21
99 minutes on December 21
Here’s where the info came from:
The United States Naval Observatory has a website (http://aa.usno.navy.mil/data/docs/RS_OneYear.html) which can be used for calculating sunrise, sunset, twilight, and other stuff for any date or location. I used “Form A” there to request a table for Astronomical Twilight for 2006 at New York, New York, and then I ran it again for Sunrise/Sunset. I got this info, which is NOT adjusted for daylight time:
lite rise twi set dark twi
Mar 21 4:26 5:58 92 6:09 7:41 92
Jun 21 2:18 4:25 127 7:30 9:37 127
Sep 21 4:10 5:42 92 5:55 7:27 92
Dec 21 5:37 7:16 99 4:31 6:10 99
So what that tells us is that twilight is fastest in spring and summer. (My guess is because the sun’s path to the horizon is a near-perpendicular drop, so it gets under the horizon fastest.)
It is slower in the winter, but lots slower in the summer. (My guesses: In the winter the angle is shallow, but it is a very short distance. In the summer, it is a shallow angle and a long distance – the worst of both.)
Hmm…got it, Malacandra. Visualizing this took some heavy lifting, in small thoughtfull bites. There’s got to be illustrations of this somewhere…
If the sun were moving latteraly in the sky it would never set. This happens at the poles in mid summer. If the sun were moving at 90[sup]o[/sup] to the horizon it would set in 2 minues. The earth rotates 15[sup]o[/sup]/hour and the sun is 1/2[sup]o[/sup]in diameter.
So the time for the horizon to cover the sun is 2 minutes divided by the cosine of the angle the sun’s path makes with the horizon.
Er, ah, um. Let’s make that divided by the sine of the angle, shall we?
The key here is recognizing that the arc the sun cuts across the sky is not a straight line, due to the angle of inclination, no? I might add it pays to visualize this as if the sun is going around the earth instead of the other way around.
No offense, Galileo. You’re still right.