MarkGeldmeier's Blog

You might not have been up at 5:36 this morning...but when you did get up the sun was higher in the sky than any other morning this year.  So how come the earliest sunrise occurs today and not next week on the Summer Solstice (June 21)...which is the day of the most daylight?  I think it's one of the strangest astronomical "mysteries"!

Go ahead and take a look at the Sunrise/Sunset charts for St. Louis.   You can find this on the web...or check out an old copy of "Dave Murray's Almanac"!  The earliest sunrise appears to be this week, while the latest sunrise is not until the last few days of June (a week after the Solstice).  It does work out that the longest "day" is on the 21rst...but why don' the sunrise and sunset matchup??

It's all about orbital mechanics.  The tilt of the earth (which is the same thing that gives us the seasons), the eccentricity of the Earth's orbit about the sun (it's not a circle), and the fact that the Earth goes faster in portions of the orbit around the sun.  It also depends on where on the Earth you're watching the sunrise/sunset.  Put them together and you get the position of the sun in the sky which is different almost everyday.  In fact, if you could take a picture of the sun in the sky at the same time everyday for an entire year...you would get a picture that looks like a lop-side "figure-8".  This figure is called the "analemma".  This figure used to be printed on old globes (it was useful for ancient navigation).  You can find more information on wikipedia and other sites...including pictures taken as described above.

Okay, what does this have to do with our original mystery?  This might be difficult to picture.  But the analemma is tilted depending on your latitude on Earth.  So, in St. Louis imagine a tilted "figure-8"...halfway between straight up and on it's side.  The exact top and the exact bottom are the longest and shortest days.  But because of the tilt, the portion of the figure that is closest (of furthest) to the horizon is slightly off from the exact top and bottom.....hence the 7 days where the earliest sunrise and latest sunset varies from the Solstice.  At the winter solstice there is a similar affect...but the difference is more like 14 days.  The Earth is actually closer to the Sun at that time and moving faster in it's orbit!

So, while you're enjoying the long days of summer -- consider how the complex movements of the Earth and Sun are giving us a subtle yet amazing show!