Home > Concepts, Ephemeris Program > 12/02/2014 – Ephemeris – The unequal dates of latest sunrise and earliest sunset

12/02/2014 – Ephemeris – The unequal dates of latest sunrise and earliest sunset

December 2, 2014

Ephemeris for Tuesday, December 2nd.  The sun will rise at 8:00.  It’ll be up for 9 hours and 3 minutes, setting at 5:03.   The moon, 3 days past first quarter, will set at 4:41 tomorrow morning.

This evening’s sunset is just a minute from the earliest sunset of the year.  The Earliest sunset will actually be on the 9th.  However the latest sunrise won’t occur until January 2nd.  The reason combines the effects of the tilt of the earth’s axis and the fact that the Earth is only a month from perihelion, its closest to the Sun.  Both these effects cause the sun to appear to move faster eastward than average, so the Earth has to rotate a bit farther each day to catch up with the Sun.  This makes the sunrise and setting events later than one would expect, so they don’t occur together on the shortest day of the year, the 21st this year.  Our sunrise this morning is still 19 minutes earlier than the latest sunrise on January 2nd, 2015.

Times are for the Traverse City/Interlochen area of Michigan.  They may be different for your location.



This figure 8 is called an analemma. One can find it on old globes in the Pacific Ocean. Explanation below. Created using my LookingUp program for Traverse City, MI near +45° latitude.

The analemma is a graphical representation of a daily value called the Equation of Time.  It’s best known use is in corrections to sundial time.  The vertical axis is the sun’s declination or north-south position.  It is highest at summer solstice and lowest at winter solstice.  It is the result of two effects:  the tilt of the Earth’s axis to the plane of the Earth’s orbit around the Sun, and the change in the Earth’s velocity around the Sun as the Earth moves from perihelion, its closest to the Sun in early January to aphelion, its most distant in July.

If the Earth’s orbit were circular, and it orbited the Sun at the same speed.  The analemma would be skinner and the north and south lobes would be of equal size.  Since we’re closer to the Sun in the winter, we move faster than average around the Sun, so it appears to move faster eastward.  That combines with the faster appearing movement of the sun crossing the closer hour lines at higher and lower declinations.  In the diagram above note that the vertical hour lines are slightly closer together at the bottom and the top, so the Sun, moving eastward each day crosses them quicker.  Near the winter solstice the two effects work together making sunrise and sunset trending to be later than normal.  For the summer solstice the eastward speed of the sun is slower than normal, because we’re farther from the Sun.  This works against the effect of the earth’s tilt but cannot completely negate it, making the top of the loop smaller than the one at the bottom.  The arrows show the speed and direction of the Sun at the solstices.

To see real analemmas search for analemma images on the Internet.  It takes a year to photograph one.

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