Happy first day of summer! Oh wait, that was yesterday.

Though June 21 often marks the official start of summer, occasionally it falls on the 20th, as happened this year.

But this is no arbitrary decision; it relates to the tilt of the Earth as it orbits the sun. The start of summer is determined by the solstice, the exact time when the northern half of the Earth is most tilted toward the sun, resulting in the longest day of the year in the Northern Hemisphere. After this solstice, the sun’s rays begin to slowly shift south and our days grow shorter.

“The solstice itself is only an instant,” said Stephen Reynolds, a physics professor at N.C. State University.

The official start of summer is the date on which this instant occurs. Last year, that instant was at 1:17 p.m. on June 21 in the eastern U.S., making that day the start of summer in the Northern Hemisphere. Since the Earth orbits the sun about every 365¼ days, it returned to this solstice position that many days later. That would make the solstice about six hours later this year.

But because this is a leap year, we added a day to the calendar back in February, so the solstice is seemingly one day behind, coming at 7:09 p.m. on June 20.

“It moves a little bit, for the same reason we have leap year,” says Reynolds.

Indeed, next year’s solstice is predicted to happen about 6 hours later, pushing the start of summer forward to June 21, where it is three out of every four years.

For the latest conditions and updated forecasts, go to our weather center.

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