The first day of summer: What happens during the summer solstice?

Thursday marked the first day of summer, with the summer solstice happening at 4:51 p.m.

That makes Thursday and Friday the longest days of the year in the Northern Hemisphere. While the Earth rotates once every 24 hours, the key is that the Earth revolves around the sun and is tilted 23.5°.

On the first day of summer, it is the Northern Hemisphere that is tilted toward the sun, which is why we begin summer, while the Southern Hemisphere begins winter.

Six months from now, it is the Southern Hemisphere that is tilted toward the sun which is why they begin summer while we begin winter.

Right in the middle are the autumnal equinox and the vernal equinox, when the Earth is neither tilted toward nor away from the sun. These are the two days where there is direct sunlight over the equator and exactly 12 hours of day and 12 hours of night over the equator.