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The summer solstice arrives Wednesday, June 21 at 12:24 a.m. EDT, marking the first day of summer in the Northern Hemisphere.

(MORE: A Fascinating New Way to Define the Seasons)

Portions of the United States are already dealing with record or near-record heat as spring comes to an end, but astronomically speaking, summer does not officially begin until the solstice. Here's what you need to know about this event.

First of all, note that the Earth spins on an axis tilted 23.5 degrees from the Earth's orbit around the sun.

As a result, the most direct sunlight shifts between a band of latitudes of the Earth throughout the year, providing the change of seasons we know in the middle and higher latitudes.

At solar noon Wednesday, the sun will be directly over the Tropic of Cancer, a line of latitude located at 23.5 degrees north of the equator.

During the summer, the Northern Hemisphere receives the most direct sunlight because it is tilted toward the sun.

(MORE: Why Seasons Aren't the Same to Meteorologists As the Rest of Us)

Think of the summer solstice as the exact moment each year when the North Pole is most directly oriented toward the sun. According to NASA, the North Pole receives 30 percent more incoming solar radiation than the equator on the solstice.

The sun will be at its highest point in the sky at solar noon (1 p.m. EDT) on June 21 for areas north of the Tropic of Cancer. These locations will experience their greatest amount of daylight of any day of the year.

For example, in Seattle (latitude of 47.5 degrees), the sun will rise at 5:12 a.m. local time Wednesday and not set until 9:10 p.m. That's a total of roughly 16 hours of daylight.

In Key West, Florida (latitude of 24.5 degrees), the sun will rise at 6:39 a.m. local time and set at 8:19 p.m. – a total of 13 hours and 40 minutes of daylight.

(MORE: 2017 Summer Temperature Outlook)

Alaska-based climatologist Dr. Brian Brettschneider (blog) constructed a handy map on his Alaska Climate Info Facebook page showing how much daylight you'll see in the U.S. on the summer solstice.

Along the Arctic Circle, at roughly 66.6 degrees north latitude, the sun doesn't set on the summer solstice. Instead, the sun takes a circular path as the Earth rotates.

(MORE: Barrow, Alaska, Won't See the Sun Set Again Until Early August)

Soon after the summer solstice, the length of daylight in the Northern Hemisphere will gradually grow shorter each day until the winter solstice in December, when the shortest amount of daylight occurs.

MORE ON WEATHER.COM: Summer Solstice Around the World