The longest day of the year for people in the Northern Hemisphere -- the summer solstice -- is Thursday, 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.

Following the winter solstice, the Northern Hemisphere begins to get more sunlight; the daytime and nighttime hours are nearly equal at the spring equinox, which takes place in March.

During the summer solstice, “the sun shines directly on the Northern Hemisphere and indirectly on the Southern Hemisphere,” NASA explains. Thus, anywhere north of the equator gets a peak amount of daylight (or a full day of sunlight if you’re at the Arctic Circle). And contrary to what some may think, the Earth is actually furthest from the sun in its orbit during the Northern Hemisphere’s summer season.

What about the Southern Hemisphere?

June 21 marks the 2018 winter solstice for people south of the equator. Residents of places like Australia will experience the year’s shortest day (and the start of winter).