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Derivatives and Daylight
Posted by Tom Klemens on June 14, 2010 at 10:43 AM.

Sunset in Door County, Wis., August 2009

Because an engineering view of the world includes a combination of respect for, and curiosity about, physical phenomena, we offer this timely (although not directly steel-related) item for your consideration.


When is the longest day of the year? For those in the northern hemisphere, it is the summer solstice, coming up one week from today on June 21. But did you know that this week is when we experience the earliest sunrise of the year? For us in Chicago, on the eastern edge of the Central time zone, sunrise has been holding steady* at 5:15 a.m. since June 10. (Incidentally, without daylight savings it would be an hour earlier. What a thought!) On June 20 sunrise will slip to 5:16 for the next four days including the summer solstice.


Meanwhile, the sun continues to set later. It bottoms out after the summer solstice, between June 23 and July 1, when sunset officially is at 8:30 p.m. (It’s actually far more official to say 1930 Central Standard Time.)


The U.S. Naval Observatory is charged with keeping track of such things and offers a bounty of interesting astronomical information on its website,   


For example, you can generate a printable table of sun and moon rise/set times for your area by going to


While you’re there, click through to “The Sky This Week” for June 8-15. It includes a more complete explanation of the “solstice season” that we have now entered.


Be sure to also visit the Astronomical Information Center portion of the USNO site, which you can reach by going to There you can learn things like why the longest day of the year is longer as you get further away from the equator. (In Chicago, daylight on June 21 is an hour and 20 minutes longer than it is in Miami, for example.)


Welcome to summer!


* For those who want a formulaic approach, this week the second derivative with respect to the time of sunrise is zero. If you prefer a simile, it’s like those last few moments as the roller coaster goes over the top of the lift hill.

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