How fast is daytime growing? Guy Ottewell explains

Originally published on March 11, 2023, at Guy Ottewell’s blog. Reprinted with permission.

How fast is daytime growing?

Clocks in the U.S. and Canada (but not Mexico) were twisted forward an hour on Sunday, March 12. From now until November 5, some North American clocks will have to grin and bear it, showing “daylight saving time” in which natural 12 noon, when the sun is highest, is called “1 p.m.” Of course, time isn’t really shifting or being saved. But – with or without daylight saving time – the sun at this time of year is gradually rising earlier and setting later.

Last chance to get a moon phase calendar! Only a few left. On sale now.

But by how much? I’ve written a program showing the rate of change, for latitude 40 degrees N. The table below shows the times of sunrise from January 1 to July 31.

In the table below, you can see that latest sunrise was on January 5. And then the sunrises started coming earlier, at first slowly, and then faster. The sunrises will be coming earlier, fastest – by more than 1.6 minutes a day – around the equinox on March 20.

June solstice

And time of sunrise will be changing most slowly around June solstice on June 21.

Here are a few more things to notice. The earliest sunrise isn’t on the day of the solstice itself (June 21). Instead, sunrise will be earliest on June 14. And then – for us in the Northern Hemisphere – the sunrises will start gradually getting later.

In this table – for 40 degrees N. latitude – The last column is the difference in minutes from the day before, negative meaning earlier.

2023 Jan 1 7:21.8 0.00
2023 Jan 11 7:21.3 -0.22
2023 Jan 21 7:17.2 -0.57
2023 Jan 31 7: 9.7 -0.88
2023 Feb 1 7: 8.8 -0.91
2023 Feb 11 6:58.2 -1.17
2023 Feb 21 6:45.4 -1.37
2023 Mar 1 6:33.9 -1.48
2023 Mar 11 6:18.5 -1.58
2023 Mar 21 6: 2.4 -1.62
2023 Mar 31 5:46.2 -1.61
2023 Apr 1 5:44.6 -1.61
2023 Apr 11 5:28.9 -1.54
2023 Apr 21 5:14.0 -1.43
2023 May 1 5: 0.6 -1.26
2023 May 11 4:49.1 -1.05
2023 May 21 4:40.0 -0.78
2023 May 31 4:33.9 -0.48
2023 Jun 1 4:33.4 -0.45
2023 Jun 11 4:30.7 -0.12
2023 Jun 21 4:31.2 0.19
2023 Jul 1 4:34.7 0.46
2023 Jul 11 4:40.6 0.68
2023 Jul 21 4:48.3 0.83
2023 Jul 31 4:57.1 0.91

Other times, other latitudes

I could run this table for other spans of time and other latitudes; and should widen it to show figures for sunset, and for the length of daylight. Several-dimensions reprogramming, and I’m tired of it for now and want to get on with my poster of the five escaping spacecraft.

Whatever the clock says, days get shorter as I get older.

Bottom line: How fast is daytime growing? A table in this post shows differences in the time of sunrise as we approach the March equinox, in contrast to the June solstice.

Read more from Guy Ottewell: Shifting Clock Times