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Why Don't Sunrise and Sunset Times Change Symmetrically?

  1. Sep 5, 2010 #1
    I've wondered this for a few years now. I have a list of local sunrise and sunset times, and I notice that, as the seasons change, sunrise and sunset times don't change symmetrically. I thought the listed times might just be wrong, but I do observe times close to these each year.

    Here is an example. For my location, here are some sunrise/sunset times in early winter:
    December 1 7:26AM 4:45PM
    December 15 7:39AM 4:41PM
    January 1 7:46AM 4:51PM
    January 15 7:43AM 5:06PM
    February 1 7:30AM 5:28PM

    Look at the date range December 1 to January 1 for example. During that time period, 20 minutes of daylight is lost in the morning, but 6 minutes is actually gained in the evening. Shouldn't the shortening and lengthening of days with seasons be symmetrical about local noon?
  2. jcsd
  3. Sep 5, 2010 #2

    Filip Larsen

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    If the Sun were to culminate every day at 12 you would be correct to assume that the times for sunrise and sunset should be symmetrical around noon. However, since the Earth is not moving around the Sun in a perfect circle the angular velocity over the sky as seen from a fixed observer on Earth varies through the year giving rise to what is called Equation of Time, meaning that the sun will not culminate at the same time of day through the year (our clock is based on what is called Mean Solar time which fits the average yearly angular speed of the Sun).

    During fall and early winter the sun will, due to equation of time, culminate later for each day totaling a difference of around 30 minutes from fall to early winter. In this period, the rate of increase in time for culmination almost cancels the rate of decrease in time for sunset, whereas it almost doubles the rate of increase in time for sunrise.
  4. Sep 5, 2010 #3


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    With only 4 dates and times it makes it a bit hard to know exactly where the extremes of the sunset and sunrise times are. Though you have made a key observation that they do not occur on the same day. So while the shortest day length falls near 20 Dec, the earliest sunset occurs in early Dec while the latest sunrise occurs in early Jan. This means that on the solstice the sunrise time is changing in a manner to make the day shorter, while the sunset time is changing to increase the day length.

    Now to why, I believe that it is all in how we define sunrise and sunset. Sunrise is when the first limb of the sun crosses the horizon, sunset is when the last limb of the sun crosses the horizon. So this results in the day length being made longer by twice the time time it takes the sun to cross the horizon. If we were to define sunrise and sunset as the time when the midpoint of the sun crosses the horizon then we would have the condition you want.
    You can observe this same asymmetry around the Summer solstice, though due to the shallower angle at which the sun crosses the horizon the date spread of the extremes is even greater.

    I hope Janus chimes in on this as if his answer is different, it will trump mine.
  5. Sep 5, 2010 #4
    Okay, let me see if I understand. You are saying the fact that it early winter is immaterial, except for the fact that at that time of the year the earth happens to be revolving around the sun at its slowest rate. Even if the earth was not tilted at all, we would still see the same variation in the time of "noon" (average time between sunrise and sunset). Correct?
  6. Sep 5, 2010 #5


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    Flip is correct in that it is due to the difference between the time we keep with our clocks(mean Solar time) and the position of the Sun in the sky(True Solar time). The Sun doesn't reach the midpoint of its path across the sky at noon according to our clocks on most days of the year. If you have a properly calibrated Sun dial, you can see this for yourself. The Sun dial keeps True Solar time, and will drift from what your clock reads over the course of a year.
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