This year, the northern summer solstice falls on June 21 at 11:54 a.m. ET. South of the Equator, this same moment marks the unofficial beginning of winter. Solstices occur at the same time around the world, but their local times vary with time zones.

Solstices occur because Earth’s axis of rotation is tilted about 23.4 degrees relative to Earth’s orbit around the sun. This tilt is what drives our planet’s seasons, as the Northern and Southern Hemispheres get unequal amounts of sunlight over the course of a year. From March to September, the Northern Hemisphere is tilted more toward the sun, driving its spring and summer. From September to March, the Northern Hemisphere is tilted away, so it feels autumn and winter. The Southern Hemisphere’s seasons are reversed.

On two moments each year—what we call solstices—Earth’s axis is tilted most closely toward the sun. The hemisphere tilted most toward our home star sees its longest day, while the hemisphere tilted away from the sun sees its longest night. During the Northern Hemisphere’s summer solstice—which always falls around June 21—the Southern Hemisphere gets its winter solstice. Likewise, during the Northern Hemisphere’s winter solstice—which falls around December 22—the Southern Hemisphere gets its summer solstice.

You can also think about solstices in terms of where on Earth the sun appears. When it’s a summer solstice in the Northern Hemisphere, the sun appears directly over the Tropic of Cancer, the latitude line at 23.5 degrees North. (That’s as far north as you can go and still see the sun directly overhead.) During the Northern Hemisphere’s winter solstice, the sun appears directly over the Tropic of Capricorn, the Tropic of Cancer’s southern mirror image.

Earth is not the only planet with solstices and equinoxes; any planet with a tilted rotational axis would see them, too. In fact, planetary scientists use solstices and equinoxes to define “seasons” for other planets in our solar system. READ MORE: https://www.nationalgeographic.com/science/space/what-is-summer-winter-solstice-answer-might-surprise-you/