The longest day of the year for people in the Northern Hemisphere — the summer solstice — is today, June 21. It’s easy to forget that the first day of summer 2018 is actually a cosmic event. Here’s what you need to know:

When is the 2018 summer solstice?

The summer solstice’s exact moment is when the sun sits directly over the Tropic of Cancer (the 23.5° north latitude mark). This will take place at 6:07 a.m. ET/3:07 a.m. PT. (You can look up the time for your location here.)

What is the summer solstice?

The Earth’s axis — think of it as an invisible pole that the Earth spins on — is tilted rather than perfectly upright. This tilt always points the same direction. As the Earth orbits the sun, different parts of the planet receive more sunlight — thus our seasons: winter, spring, summer and fall.