How Many Minutes of Daylight Do We Gain or Lose Each Day?

How many minutes of daylight does the northern hemisphere gain every day in March? How about in June? What's the astronomical reason for these changes? Keep on reading to find out!

Jason Marshall, PhD,
Episode #191
Seasons diagram

Why Do Daylight Hours Change?

More specifically, for our purposes here the most important byproduct of Earth's tilted axis is the fact that the number of hours of daylight changes throughout the year—and exactly how it changes depends on the latitude at which you live. If you think about it (or take a look at that flashlight and ball model you played around with earlier), you'll see that the top half of the Earth is tilted towards the Sun for half the year and away from it for the other half.

Parts of the planet tilted towards the Sun receive more than 12 hours of sunshine per day, parts pointed away from it receive less. As the Earth travels around the Sun throughout the year, the degree to which a part of the planet is tilted towards or away from the Sun changes. And with that change comes a change in the number of daylight hours that part of the Earth receives. 

Parts of the planet tilted towards the Sun receive more than 12 hours of sunshine per day.

As a location goes from winter into summer, the rate of change in the number of daylight hours peaks at the spring equinox—which is why the number of daylight hours is increasing at a maximum rate right now in the northern hemisphere. After the spring equinox, the rate at which daylight increases tapers off until halting at the summer solstice. At that point the number of daylight hours gradually begins to decrease, picking up steam until reaching a peak at the autumnal equinox and then gradually decreasing until once again halting at the winter solstice.

At which point the cycle begins anew.

Daylight Hours, Sine, and Cosine - What's the Connection?

As this periodic nature might lead you to guess, the number of daylight hours and the rate at which that number of hours changes turns out to be closely related to the sine and cosine trigonometric functions we've been talking about lately. In fact, if you make a plot of the number of daylight hours throughout the year, you'll see that it looks almost exactly like a sine function.

Why is that? And what does the graph of a sine function actually look like in the first place? Unfortunately, we're all out of time for today. So the answer to those questions is going to have to wait until next time.

Wrap Up

Okay, that's all the math (and astronomy) we have time for today.

Please be sure to check out my book The Math Dude’s Quick and Dirty Guide to Algebra. And remember to become a fan of the Math Dude on Facebook where you’ll find lots of great math posted throughout the week. If you’re on Twitter, please follow me there, too.

Until next time, this is Jason Marshall with The Math Dude’s Quick and Dirty Tips to Make Math Easier. Thanks for reading, math fans!


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