Is there the same amout of day light 2 months before the longest day of the year as 2 months afterwards. For example if there are 16 hours of day light May 21st is there 16 hours of day light Aug 21st. For example if there is 12 hours of day light 4 months before June 21st is there the same amount of light 4 months after.
If there is then its easy, but if it is not linear is there a formula to perdict the amount a day light.
Roger Sachs rogersachs@comcast.net
Roughly, yes. Or should I say: no, but almost. It depends on how picky you want to be.
There will be small asymmetries due to the Earth’s orbit not being a perfect circle, and due to the fact that the solstices are really instants of time that can occur at any point during the “solstice day”. But generally speaking, pairs of days that are equidistant from a solstice will have equal amounts of daylight, to within a few minutes.
If the small differences matter to you, you can compute the length of daylight for your location. In olden times an advanced astronomy book would be necessary, but nowadays there’s at least one web page that’ll do it for you. (Well, actually that page just computes sunrise and sunset times, but the length of daylight follows easily from that.) For example, in Topeka, Kansas, some lengths of daylight around the summer solstice are:
Jun 21, 2004: 1009 minutes of daylight
May 22: 871 min; Jul 21: 870 min
Apr 22: 811 min; Aug 20: 809 min
Mar 23: 737 min; Sep 19: 736 min
Hope this helps.
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What is the name of that figure-8 shaped diagram that map’s the sun’s course across the sky?
The analemma.
It’s an analemma. There’s a time-lapse photograph of it here.
(Actually, the term “time-lapse” is probably inappropriate, since we’re talking about a photograph and not a movie. But you know what I mean.)
Is there a reference on the web where I can check analemmas for different locations? Like, say, the Anchorage analemma vs. the Springfield analemma vs. the Miami analemma, etc.
It’s the same shape and size regardless of where you are on Earth.
With one caveat: if you’re north of the Arctic Circle, or south of the Antarctic Circle, then some portion of the analemma will be hidden beneath the horizon. It will still be the same curve though.
(And to correct my earlier, bogus correction, I just noticed that Mr. Di Cicco calls his photo a “time lapse” photo — so never mind on that one. Sorry.)
But why is it that sunrise/sunset times before and after the summer and winter solstice does not match?
The sunset reaches its earliest (15:41) at December 7th and stays there for 4 days until the 11th, when it starts getting later again. However, the sunrise keep getting later even after the solstice, reaching its latest at December 27th (06:58) and remaining at that peak for 13 days before starting to get earlier again on January 9th.
Why the asymmetry? This is where I got the data… I think it’s similar for all latitudes.
I started to write a long, thorough explanation (really I did), but then found this page, which answers the question pretty well. It even links to the same Di Cicco photograph already mentioned.