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Name: Wes
Status: Educator
Grade: K-3
Location: GA
Country: United States
Date: February 2008


Question:
Our second grade class has been charting sunrise and sunset times for our area. We have noticed that we are now gaining approximately one minute per day on the sunset side of our chart. The sunrise side of our chart indicates that we only gain a minute in the mornings every three or four days. Our class wants to know why we gain a minute of daylight length as the sun sets each day but not when the sun rises. (I have asked everyone I know and no one can explain this to us.)



Replies:
Wes,

Theoretically, if the Earth revolved about the Sun at the same distance all year and maintained the same direction of tilt with respect to the Sun, you would gain a little more time at sunset than at sunrise (simply because at the end of the day more time has passed since the winter solstice). Then, again theoretically, the difference between sunset and sunrise gain would be fairly small (10 seconds or so at this time of year).

The situation is a bit more complex and the changes in sunrise and sunset times are affected by changes in the tilt of the Earth with respect to the Sun and your position on the Earth's surface (particularly where you live in reference to the center of your true time zone).

You live in Georgia, so it is a little different for you, being further south than me in northern Illinois. However, here, at this time of year, sunrise is shifting backward about 1 minute (we are gaining one minute at sunrise) and sunset is shifting forward one minute (we are gaining one minute at sunset).

Further south, where you are, sunrise is less than 1 minute earlier each day and sunrise is less than 1 minute later each day. You can confirm this by looking at the sunrise/sunset calculator at

http://aa.usno.navy.mil/data/docs/RS_OneDay.php

The inconsistency in sunrise gain where you are may occur if there is some topography, trees, building, etc. that prevents an unobstructed view of the horizon, as opposed to the unobstructed view of the horizon during sunset that I assume you have. Otherwise, the time differential change in sunrise and sunset should be essentially the same.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


The answer is complicated but has to do with the motion of the earth in orbit around the sun. See

http://answers.yahoo.com/question/index?qid=20071220155948AA6DbYy

and

http://curious.astro.cornell.edu/question.php?number=208

David Kupperman



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