Growing darkness: How it really works

(Photo by Jenny Downing)

Of course you’ve noticed that we have less daylight than we did in July. Maybe you listen to public radio and hear the meteorologist recite the number of hours and minutes of sunshine today. Then he adds, “It’s a decrease of two minutes from yesterday.” Yet the next day he might say that there’s only a one-minute decrease, or a three-minute decrease. It seems to jump around. Let’s get to the bottom of this.

This weekend, the Sun rises at around 6 a.m. and sets around 8 p.m. We get roughly 14 hours of daily sunshine. That’s a loss of only about one-and-a-half hours from our maximum, the third week of June. So far, things haven’t changed too much. Unfortunately for Sun-lovers, that’s about to change dramatically.

In the next month we will lose two hours of daily sunshine. When days and nights are equal on September 26 (not on the Autumnal Equinox, September 23), the Sun will rise at 7 and set at 7: 12 hours of sunshine. During the next nine weeks, our decrease will average about three minutes per day.

But the WAMC meteorologist consults daily sunrise and sunset tables that round off to the nearest minute. Sometimes by rounding off it seems that only one minute has changed, while other times it can be four minutes. But in reality it’s about three each day, and this is quite steady during this period that surrounds the Equinox.

At the same time, the midday Sun is becoming less high. It is one solar width lower every two days. This is rapidly affecting the intensity of sunlight, and we can feel this change on our skin. The year’s greatest daylight decrease and sun intensity diminution both unfold between now and late October: a profound alteration packed into just nine weeks or so.

That’s our story. It’s very different for those who live significantly north or south of us. People at the Equator experience no change at all. For Quito residents it’s always 12 hours of day and 12 of night, year-round. Those who live at the North Pole (namely, no one) also experience no change. For them right now, there is unrelenting sunshine 24/7: no night at all. And this will be true next week, and the one after that. Suddenly, around September 25, over the course of just two days, the situation dramatically switches to zero sun. It’s all or nothing.

But most folks on Earth do experience a quantifiable sunlight change during the next two months. In Fairbanks, Alaska, they are now losing seven minutes of sunshine each day: a loss of three-and-a-half hours of daily sunshine during the next month alone. Even up in Montreal, not far from here, they lose nearly an hour more than we do during the next month. Thus their decrease is almost four minutes daily. The daylight reality very much depends on one’s latitude.

But any way you slice it, these are our final few weeks of long days.

Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at

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