Sunrise, Sunset, and the Winter Solstice
Published on Dec 17, 2006 at 5:07 am.
14 Comments.
Filed under Earth, skywatching, time.
In a few days, it will the the Winter Solstice here in the northern hemisphere. The Sun will be the farthest south that it will get all year. That means that it will be the shortest day of the year for us. Now, for our friends in the southern hemisphere, it is the other way around. For them, it will be the longest day of the year.
The seasons are caused by the tilt of the Earth as it orbits the Sun. Our rotational axis is tilted about 23.5 degrees with respect to the axis of our orbit. So, for part of the year, the northern hemisphere is tilted towards the Sun (we have summer, and the southern hemisphere has winter), and for part of the year the southern hemisphere is tilted towards the Sun (they have summer, and we have winter). We can determine exactly when the tilt of the Earth brings the northern hemisphere pointed most away from the Sun. That will occur this Thursday evening. Technically, it will be at 0:22 UT on December 22. So, the solstice wil be on Friday in Europe. But here in Texas, that will be 6:22pm on December 21. Ever since June 21, when the Sun was highest in the sky, it has been moving south, and the days have been getting shorter. That will end December 21, and the Sun will again start to move northward (as seen from Earth), and the days will be getting longer again. In fact, the word Solstice means that the Sun stops its motion and begins to move back the other way. For us here in Texas, this time of year the Sun rises in the southeast, at noon is no more than about 35 degrees above the southern horizon, and sets in the southwest.
Now, with the days getting longer and shorter, you would naturally assume that the longest day of the year will have the earliest sunrise an the latest sunset, and that likewise the shortest day of the year will have the latest sunrise and latest sunset. Well, it doesn’t work that way!
You see, the Earth rotates at a fairly regular rate. But, it also moves. So, when the Earth has completed one rotation, it has also moved a little around the Sun. To see what effect this might be, imagine the Sun being highest in the sky one day — high noon, as it were. Well, you might imagine that after one complete rotation of the Earth, the Sun would again be highest in the sky. But, you’d be wrong! Since the Earth had moved a little in its orbit, one complete rotation doesn’t quite bring the Sun back to its highest position in the sky. That takes about four minutes more. So, a complete rotation is about 23 hours 56 minutes. But, a solar day is going to be a bit longer. OK, that is easy to understand, but what does that have to do with sunrise and sunset? Well, the problem comes in that Earth’s orbit is elliptical. That means that it speeds up and slows down as it orbits the Sun. When the Earth is moving slower, as it does near June and July, then the Earth needs to turn less extra beyond a complete rotation to make a day. But, in December and January, Earth is moving fastest in its orbit, so there needs to be a little more rotation to make up a day. But, the clocks all run at a constant rate. That means that in the summer the clocks are running too slow, and the Sun is gaining time. In the winter, it is the other way around, and the Sun is losing time (that’s summer and winter for the northern hemisphere). The effect of this is that the Sun rises and sets earlier and earlier when it is gaining time, and it rises and sets later and later when it is losing time. Now, this would not be a problem if we just adjusted the clocks every day so that noon was when the Sun is highest in the sky, but that would be inconvenient. So, we run the clocks at a constant average rate and just ignore the fact that the Sun is highest earlier than noon or later than noon. Sunrise and sunset are pretty symmetric with respect to the time that the Sun is highest in the sky, so that skews the whole sunrise and sunset times.
Now, exactly when your earliest and latest sunrise and sunsets occur will depend upon your exact latitude. The end result, combining the effects of Earth’s orbital eccentricity (elliptical nature) and axial inclination is that for most of the US, and nations with similar latitude, the latest sunrise actually occurs a week to ten days into January, and the earliest sunset is about a week or so after the start of December.
-Astroprof
Ed Minchau on December 17, 2006 at 5:17 pm: 1
Shouldn’t the axial tilt value be 23.5 degrees rather than 25.5 degrees?
Astroprof on December 18, 2006 at 9:09 am: 2
Yes. Oops. Typo. I must have been thinking of the upcoming digit.
Thanks for noticing that. I’ll correct it in the original post.
Ed Minchau on December 18, 2006 at 6:41 pm: 3
That’s the good thing about blogs - lots of proofreaders.
robert run on December 20, 2006 at 1:14 pm: 4
Hi..
want to post a coupole of nice links on my blog about the solsice? or whatever you want. I like your page. there is on my blog, a link on the right, to the realtime status of the aurora borealis.. i’m up northerly like, so i kind of dig keeping an eye out for that stuff.
please visit and leave me whatever you’d like..
i’ll have a spring, summer, and autumn blog too.. so post away.
http://thefrozennorth.wordpress.com/
A Ler…-- Rastos de Luz on December 21, 2006 at 1:32 pm: 5
[…] “Sunrise, Sunset, and the Winter Solstice“, no Astroprof’s Page. […]
Dean on December 23, 2006 at 10:17 am: 6
Very helpful info… btw, the sentence right before “It doesnt work that way!” I think you mean to say “latest sunrise and EARLIEST sunset”
Thanks for posting this… I’m working on a sermon where I’m tying Christmas to the winter solstice.
sunrise sunset calculator on April 23, 2008 at 11:48 pm: 7
Very helpful writeup and it has helped me understand a lot more than I did before.
Richard Moyer on December 15, 2008 at 7:24 pm: 8
Thanks for the information on the earliest sunset and the latest sunrise NOT being on the solstice. I have told many people this as they looked on with disbelief. Do you know the astrological name for this phenomenon? I saw it a couple of years back, but have forgotten it.
Thanks again for the interesting explanation.
Richard
Richard Moyer on December 15, 2008 at 7:28 pm: 9
Oops. I meant “astronomical” and not astrological in the last post!
Richard
Astroprof’s Page » Apollo 8, 40 years ago on December 21, 2008 at 4:04 pm: 10
[…] south that it will appear in the sky. That would be a topic worth blogging about. However, I wrote a bit about the solstice several years ago, and there is nothing particularly different about this one. What is unique about […]
Pegasus on January 17, 2009 at 8:31 pm: 11
Ever notice that the approximate date of the earliest sunset has been “memorialized” by the Christian church with St. Nickolas night?
Travis on December 25, 2009 at 9:19 am: 12
Is it true that Orions belt and Sirius align in the sky and point to the sunrise on the winter solstice?
John McLaughlin on January 7, 2010 at 11:57 am: 13
But why does the sunrise time remain unchanged for maybe 2 weeks in January while the sunset time gains 10-15 minutes? Why is this so lopsided? I do understand the solar vs. clock time, but I’d expect some movement at both ends of the day.
Douglas Lehman on February 5, 2010 at 12:30 pm: 14
I think that the best explanation for the movement on sunset but not on sunrise in January is that there is very, very slow movement around the day of the latest sunrise and/or earliest sunset. So if the latest sunrise happens in January, then you’ll see almost no movement in the sunrise times during January. For sunset, however, its earliest occurs in early December, so by January, we’re already 20 - 50 days out from the date of the earliest sunset.