Today’s the first day of summer. But no, it’s not the longest day of the year.

Astronomical summer hits at 6:34 p.m. tonight.

Children cool off in the fountain at the Christian Science Plaza.
–Timothy Tai / The Boston Globe, File

Meteorological summer—as in, warmest 90-day period on the calendar—began three weeks ago. Today, at 6:34 p.m., marks the start of astronomical summer—basically, just a moment in time telling us that the sun’s angle has reached its maximum for the year in the northern hemisphere.

As the Earth, on its tilted axis, travels around the sun in the elliptical orbit, it causes the sun to appear in different positions and heights each day as it crosses our sky. It’s not the sun that is actually moving; rather, it’s an illusion that’s created because our spinning Earth whirls around in space at 1,040 miles per hour.

The tilt of the Earth on its axis results in changes to the incoming angle of the Sun's rays
The tilt of the Earth on its axis results in changes to the incoming angle of the sun’s rays. —NASA

Today also marks what many say is the “longest day of the year.” This is incorrect.

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While today does mark the 24-hour period with the most amount of daylight, it’s not the date on which the sunrise is at its earliest or the sunset is at its latest. What would be more accurate would be to say that there is more daylight today than on any other day of the year in the northern hemisphere. The opposite is true south of the equator.

As the Earth travels around the Sun different amounts of light reach the surface of our planet
As the Earth travels around the sun, different amounts of light reach the surface of our planet. —NASA

In this part of the world, our earliest sunrise occurred back on June 14 or 15 (depending on your exact location), and the latest sunset will be on about June 26, although it’s not until early July that it will flip from 8:25 to 8:24 p.m., because it’s only moving by seconds a day this time of year. In December, the earliest sunset comes about two weeks before the December solstice, and the latest sunrise happens about two weeks after that.

The reason for all the discrepancies is because, although we humans have our clocks set at exactly 24 hours a day, the reality is that the solar day—the time between the highest position of the sun from one day to the next—isn’t exactly 24 hours. As a matter of fact, true solar days are longer than 24 hours several weeks before and after the solstices and longest around the winter solstice—otherwise they’re 23 hours and 56 minutes. The sun’s position at and around the solstices is very far north and south of the equator and, as a result, Earth must rotate farther on its axis for the sun to return to its daily noontime position each day. The elliptical shape of the orbit makes December the period during which a solar day is technically at the longest: around 24 hours and 30 seconds.

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So when folks start saying today is the longest day of the year, you can correct them and say, “Well, technically the longest day of the year is in December.” They’ll look at you funny, but you’ll be right. Happy summer!

 

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