11.05.09

How long is a day?

Posted in astronomy, math at 5:39 pm by danvk

As we approach the winter solstice, the days get shorter and shorter. There’s a common misconception about how quickly this change happens. Most people know that:

1. The summer solstice (June 21) is the longest day of the year.
2. The winter solstice (December 21) is the shortest day of the year.
3. The days get shorter between Summer and Winter.
4. The days get longer between Winter and Summer.

Many people take these four pieces of information and assume that the day length changes like this over the course of the year:

(The x-axis is the date. The y-axis is length of the day in hours.)

This is consistent with the four pieces of information, but is incorrect! There aren’t many sharp edges like that in Physics. Reality is much smoother:

The length of the day slowly increases as we approach the summer solstice, then slowly decreases as we leave it. This is great — it means that there are lots of long days in the summer. As we get to the autumnal equinox, the rate of change hits a maximum. The same thing happens around the winter solstice, only in reverse.

The summer solstice is the longest day of the year, but not by much! Here’s some day lengths for San Francisco:

Date	Day Length	Difference
Jun 18, 2009	14h 46m 45s	+ 09s
Jun 19, 2009	14h 46m 51s	+ 06s
Jun 20, 2009	14h 46m 54s	+ 02s
Jun 21, 2009	14h 46m 54s	< 1s
Jun 22, 2009	14h 46m 50s	− 03s
Jun 23, 2009	14h 46m 43s	− 06s
Jun 24, 2009	14h 46m 33s	− 10s

The lengths of the days around the solstice differ by only a few seconds! On the other hand, here are some day lengths around the autumnal equinox (September 22):

Date	Day Length	Difference
Sep 19, 2009	12h 15m 35s	− 2m 24s
Sep 20, 2009	12h 13m 10s	− 2m 24s
Sep 21, 2009	12h 10m 46s	− 2m 24s
Sep 22, 2009	12h 08m 21s	− 2m 24s
Sep 23, 2009	12h 05m 56s	− 2m 24s
Sep 24, 2009	12h 03m 32s	− 2m 24s
Sep 25, 2009	12h 01m 07s	− 2m 24s

The length of each day changes by several minutes in September. Over a single week the day gets a whole 15 minutes shorter!

note: the interactive graphs are dygraphs, a JS library I created. Check it out!

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