The shortest days

Not many people know  this but the middle of September, has the shortest solar days of the year. I’ve decided to re-blog my post from 2015 on this interesting fact.

Explaining Science

Revised 10 September 2018

Most people are probably unaware of this but the length of a solar day, which is the natural day measured by the rising and setting of the Sun isn’t  always 24 hours. It varies slightly throughout the course of the year and that September 18 is in fact the shortest solar day in the year. This post discusses this curiosity, which is not widely known.

Background- the variation in the length of the day.

Although a day for practical timekeeping purposes is always 24 hours, the actual length of a solar day, which is the time difference between two successive occasions when the Sun is at its highest in the sky, varies throughout the year. As shown in the graph below, it is at its longest, 24 hours 30 seconds, around Christmas Day and is at its shortest, 23 hours 59 minutes 38 seconds, in mid-September.

Day length

How the length of a solar…

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12 thoughts on “The shortest days”

  1. I kind of figured there would be some variation in the length of a solar day, but I didn’t realize it varied by that much. That’s almost a full minute between the longest and shortest day of the year!


    1. Yes that’s right the term natural day and solar day mean the same thing.

      But I said in my previous comments the different in the length of solar day is negligible from one day to another in September,


  2. As I understand it September 18, as natural day, starts at 00:00 AM and ends at 12:00 PM, whereas a solar day starts at high noon and ends at the next high noon. So in this case, is it betwween high noon September 17 and high noon September 18 or is it between September 18 and high noon September 19 or is this a stupid question?

    Liked by 1 person

    1. There is a certain degree of ambiguity about what time a solar day starts. A solar day is most commonly taken to start at midnight, which is the approach I’ve adopted for this post. In reality, the length of the solar day changes so slowly in mid September that the difference in the length of the solar day from one day to another is negligible.


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