March 20 is the spring equinox. That’s when every place on Earth gets 12 hours of day and 12 of night, right?
Well, actually, no. What really happens is quite interesting, and leads to additional thoughts about day lengths and sun angles.
First, let’s look at some sunrise and sunset times and day lengths for March 20, this year’s spring equinox. In Washington, D.C., sunrise is at 7:12 a.m. and sunset at 7:19 p.m. for a day length of 12 hours and 8 minutes. In Fairbanks, Alaska, the numbers are 7:51 a.m., 8:06 p.m., and 12 hours and 15 minutes.
The farther north you go, the longer the day length gets. What about the North Pole? There, the sun is just barely above the horizon all day and the day length is 24 hours.
Meanwhile, in Quito, Ecuador, almost exactly on the equator, the sun rises at 6:18 a.m. and sets at 6:24 p.m., and the day length, like every other day of the year, is 12 hours and 6 minutes.
What’s going on?
Two things explain this departure from expectation.
First, sunrise is defined by the appearance of the top of the sun above the horizon, while sunset is the disappearance of the top of the sun below the horizon. If the sun were a point, then it would make no difference, but the sun isn’t a point, it appears as a disk. So the time taken by the sun to get all the way above the horizon isn’t added to the time of sunrise, but the time the sun takes to get all the way below the horizon is added to sunset time. If sunrise and sunset were both defined by the time the midpoint of the sun’s disk crossed the horizon, then this effect of adding to day length would disappear. But that wouldn’t make sense in terms of what we humans experience.
But there’s another effect, and that’s refraction. Because the Earth’s atmosphere refracts (bends) light, we see the sun rise before the sun is actually above the horizon, and we can still see the sun at sunset after it’s actually below the horizon. So not only is the equinox not a time when every place on Earth gets exactly 12 hours of day and 12 hours of night, there is no such time at all, ever. But as a rough approximation, it does give you an idea what most places are experiencing.
A friend in Fairbanks loves to joke about the people from the Lower 48 who ask him how long the day is at the equinox. He always tells them that it’s the same as where they are (and he’s almost right).
Thinking about these things makes me think of other aspects of sunrise and sunset. Of course, it’s the Earth that’s rotating, and making it seem as though the sun is moving. But that rotation causes the biggest environmental change we routinely see. At night, it gets dark and cool; during the day, it’s light and warmer. Of course, weather can change the temperature range, but in general, the difference between night and day, to coin a phrase, is absolutely dramatic and it changes the way we and other organisms live our lives. Just spend a few minutes thinking about this in terms of your own experience.
Another thing the change in seasons does for us — for every single one of us, everywhere on this planet — is change the sun’s angle above the horizon. It’s high at noon in summer and low at noon in winter. Even in Quito, on the equator, the solstices (June and December) are times of (relatively) low sun, while at the equinoxes, the sun is directly overhead at noon.
In Washington, in December and January the sun’s angle is low enough (a little less than 28 degrees at its highest at the winter solstice) that you won’t get much of a sunburn if you’re out in it and the sun melts snow only weakly. By the March equinox, the sun gets so high (about 51 degrees above the horizon at its highest) that snow doesn’t stand much of a chance and even if it falls, it doesn’t stay for long. But in Fairbanks, it’s not until April that the sun is strong enough to really begin to melt snow, and that’s in part because it stays up for so long by April.
These differences in sun angle and day lengths have a great deal to do with the overall differences between the tropics, the temperate zones and the high latitudes, and noticing them adds an important dimension to one’s travel experiences. As a bonus, it seems unlikely that people can affect them, short of a catastrophic interference so severe that nobody would even be around to notice sun angles and day lengths. We can affect climate, and are doing so, but it’s nice to know that your grandparents and grandchildren experienced and will experience the same variations in day lengths and sun angles that we do.
David Policansky is a retired scientist who worked in the Division on Earth and Life Studies at the National Academy of Sciences.