Why we’re losing daylight so quickly, and what to know about the autumnal equinox

It’s been obvious lately that we are losing daylight. The gap between sunrise and sunset started decreasing immediately after the summer solstice. However, because of the way the Earth is tilted on its axis and the revolution of the planet around the sun, the speed at which we lose or gain daylight is not consistent or linear throughout the year. Instead, it is sinusoidal, peaking twice, along with two minimums. Right now, the loss of light is as fast as it gets all year.

Think of all this like a swinging pendulum. When we are close to the spring and autumn equinoxes, we are at either end of the pendulum and the rate of daylight change is rapid. Right now in Boston, we are losing nearly three minutes of daylight per 24-hour period. The autumnal equinox is Monday, Sept. 23, at 3:50 a.m.

The shrinking daylight is even faster the further north you go. The bottom of the pendulum would be when the rate of change is quite small — the summer and winter solstices. If the path of the earth around the sun were not elliptical and the earth’s axis were 0 degrees, then the length of day wouldn’t change.

In June, the change of daylight is relatively small both in how much light we gain at the beginning of the month and how much light we lose in those final 10 days. In December, the amount of light we lose and gain is also not significant.

In December, we start by losing just under 90 seconds of daylight, but it slows to a crawl before Christmas.TimeandDate.com

Another way to illustrate the changes in the sun’s position each day is to take a picture of it at the same time every day or every few days for a year. The picture this creates is called an analemma. The one below was created by taking a photo at 4:45 each day for a year. You can also see the path of the sun throughout the year by noting where it sets along the horizon.

Sunset positions are arranged along the bottom of the image with the equinoxes in the middle.Marcella Giulia Pace

Since we’re coming up on the autumnal equinox, it’s worth noting that that is the time of year where the entire planet has almost exactly 12 hours of night and 12 hours a day, but not on the equinoxes as you might suspect. This is because of the way sunrise and sunset are calculated.

Remember, of course, the sun is a disc, and it takes a while for it to fully go above and below the horizon. Humans have decided that sunrise is that point where you see the top of the sun above the horizon, but for sunset, we have to wait for the top to go below the horizon.

If the sun were some tiny little point, none of this would matter, but because it’s a big disc, we don’t add the time it takes for the sun to get above the horizon. We do add the time it takes it to go below at sunset.

If that variable wasn’t enough to mess things up, there’s this other phenomenon called refraction that lets us see the sun before it’s truly above the horizon and still see it when it’s actually below the horizon. Those move the date when there is exactly 12 hours of daylight to Sept. 26.

While this may all be somewhat confusing, it’s a reminder that we are on an earth ride, spinning around the sun at 67,000 miles per hour.

Follow Dave Epstein on Twitter @growingwisdom.