Earliest Sunset, Latest Sunrise

Why does the Earliest Sunset come before the Winter Solstice and the Latest Sunrise after?


Why does the Earliest Sunrise come before the Summer Solstice and the Latest Sunset after?

Ever wonder? I have. And aside from some hand-wavy explanations, I’ve never been able to explain this very well. Here’s the best explanation I have seen yet, provided in the December 2007 issue of Sky & Telescope, p. 55:

You’d think the earliest sunset would come on the shortest day (or longest night) of the year, at the winter solstice. But in fact, the day-night cycle shifts back and forth a little with the seasons, due to the tilt of Earth’s axis and the ellipticity of Earth’s orbit. At the beginning of December, sunrise, midday, and sunset all happen a little earlier than they “should”, and in January they run a little late. So the earliest sunset ends up being two or three weeks before the solstice, and the latest sunrise is two or three weeks afterward. The exact dates depend on your latitude.

Continuing along that same line of thought…

At the beginning of June, sunrise, midday, and sunset all happen a little later than they “should” and in July they run a little earlier. So the earliest sunrise ends up being about a week before the solstice, and the latest sunset is about a week afterwards. The exact dates depend on your latitude.

I know, I know. You still have a question. “Why are the dates of earliest sunrise and latest sunset closer to the summer solstice than the dates of earliest sunset and latest sunrise to the winter solstice?” Good question. I think it has everything to do with the fact that the Earth is near aphelion at the time of the summer solstice, and thus moving most slowly in its orbit around the Sun (the Earth’s orbit is slightly elliptical and not circular). That means that the Sun is moving slowest against the background stars and thus the accumulated difference between the sidereal day and solar day is the smallest at that time of year. That means the spread of days between earliest sunrise and latest sunset is less. Conversely, at the winter solstice, Earth is near perihelion, and therefore it is moving most quickly in its orbit around the Sun. That means that the Sun is moving fastest against the background stars and thus the accumulated difference between the sidereal day and solar day is largest at that time of year. That means the spread of days between earliest sunset and latest sunrise is more.

Here in Dodgeville, Wisconsin, where the latitude is just shy of 43˚ N and the longitude is just a tad over 90˚ W, the earliest sunset this year is today, Tuesday, December 8, 2020, at 4:25:49 p.m.

Latest sunrise in 2021 will be on both Saturday, January 2 and Sunday, January 3 at 7:31:51 a.m.

Pause to consider that if we were on year-round daylight saving time, latest sunrise wouldn’t be until 8:31:51 a.m.

My preference would be to stay on standard time year-round, as Arizona does.

Year-Round Daylight Saving Time?

I’ve never been a fan of daylight saving time. During the warmest months for stargazing and other astronomy activities, daylight saving time (DST) puts the end of twilight (and every other astronomical event) an hour later: near, at, or past bedtime for children and early-rising adults.

The last time we tinkered with DST in the U.S. was to extend it in 2007 to begin the second Sunday in March and end the first Sunday in November (previously it was the first Sunday in April to the last Sunday in October). We currently observe daylight saving time 65.4% of the year (almost ⅔) and standard time the remaining 34.6% of the year (a little over ⅓).

DST is a zero-sum game. Getting that extra hour the first weekend in November sure is nice, but we pay for it when we lose an hour the second weekend in March. For a few days in November, we feel like we’re sleeping in an extra hour, but for a few days in March, we feel like we’re getting up an hour earlier than usual.

While I would much prefer to stay on standard time all year long nationwide, there doesn’t appear to be much public support for that. On the other hand, there is a groundswell of support for going to year-round DST. Even this would be preferable to our current system, in my opinion.

We have toyed with the idea of year-round DST once before: from January 6, 1974 to October 27, 1974. During the winter months in early 1974, there was a lot of public outcry about schoolchildren going to school in the dark, and I’m sure the pre-sunrise cold was a factor, too. So, the year-round DST experiment was terminated early (it was supposed to last until April 27, 1975). Would it be any different this time around?

Northern states (where the winter nights are longest) would be most affected by year-round DST, as would areas in the far-western reaches of each of the time zones. Here in Wisconsin, we would see something like the following:

Some Highlights of Year-Round Daylight Saving Time in Wisconsin (times are for Dodgeville, WI)
  • Earliest End of Evening Twilight: 7:08 p.m. (around December 6)
  • Earliest Sunset: 5:26 p.m. (around December 9)
  • Latest Sunrise: 8:32 a.m. (around January 3)
  • Latest Onset of Morning Twilight: 6:50 a.m. (around January 6)
DateSunriseSunset
November 17:35 a.m.5:53 p.m.
November 157:53 a.m.5:37 p.m.
December 18:12 a.m.5:27 p.m.
December 158:25 a.m.5:27 p.m.
January 18:32 a.m.5:36 p.m.
January 158:29 a.m.5:51 p.m.
February 18:16 a.m.6:13 p.m.
February 157:59 a.m.6:31 p.m.
March 17:36 a.m.6:51 p.m.
March 157:12 a.m.7:08 p.m.

I have an idea. If we extend DST to year-round, why not also start the school day an hour later? There are studies that show that most students would benefit from a later start of the school day. Of course, that would also mean that many parents would probably want to start their work day an hour later, too. But if we do that, then what’s the point in going to year-round DST in the first place?

Many states are currently considering and some have even passed legislation extending DST to year-round, but federal law will have to change to allow any of these states to do this. Right now, states only have the right to opt out of DST altogether, as Arizona and Hawaii currently do.