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Why doesn’t the earliest sunrise occur on the longest day of the year?

By: Jens Ramrath

We all know that the day with the longest daylight in the northern hemisphere is 21 June, during the Summer solstice, and the shortest day is 21 December, during Winter solstice. But depending on where you are, that is most likely not the day of the earliest sunrise or latest sunset. Why? Off to STK to create some reports and graphs.

Turns out that because of the Earth not being in a perfectly spherical orbit around the Sun, the time of solar noon (time of highest daily elevation of the Sun) shifts from one day to the next. This shifts the sunrise and sunset times with it. Using AGI HQ as an example (the default Facility location), you can compute the following solar noon times

  • 19 Jun 2019 17:03:44.798
  • 20 Jun 2019 17:03:57.682
  • 21 Jun 2019 17:04:10.569
  • 22 Jun 2019 17:04:23.437
  • 23 Jun 2019 17:04:36.264

At our latitude, the time of solar noon shifts back about 13 seconds/day around the summer solstice. At the same time, the length of daily sunlight increases by a few seconds per day until the solstice. Combining the shift in mid-day and change in overall length of day, you end up with the earliest sunrise and latest sunset being a few days apart.

I wrote a MATLAB script that plots all this. Here are the annual changes in sunrise, sunset, and solar noon for one year. The yellow days are those with the earliest/latest sunrise/sunset while the blue line indicates solar noon. Note that I removed daylight savings times to remove sudden discontinuities.

 

Zooming in around the summer solstice, we get...

Just to prove that this is really due to the shift in solar noon, I shifted all daily lighting times so that solar noon would fall at directly 12:00. This moved both the earliest sunrise and the latest sunset to the summer solstice on 21 June.

The duration between summer solstice and latest sunset is a function of latitude and varies from a few days at higher latitudes to about a month at the equator. So generate the “Lighting Times” report for a facility at your location to find the day with your latest sunset.

Posted: 6/17/2019 8:00:00 AM


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