Summer Solstice 2018: When, What Is the Longest Day of the Year, First Day of Summer?

Thursday marks the astronomical start of summer for everyone in the United States. The summer solstice happens sometime between June 20 and June 22 every year for those in the Northern Hemisphere, and this year it's set to happen Thursday for people in the United States.

The summer solstice marks when the sun reaches the most northern point in the sky all year in the Northern Hemisphere. In the Southern Hemisphere, it reaches its southernmost point on this day. The exact time when the solstice occurs varies depending on location. On Thursday, it was set to happen at 6:07 a.m. EDT, which would be 3:07 a.m. PDT.

The day the sun reaches this northern point coincides with the longest day of sunlight for the year. This happens because, on the day of the summer solstice, the Northern Hemisphere is as tilted in the direction of the sun as it gets, so it receives the most sunlight on that day.

During the solstices, Earth reaches a point where its tilt is at the greatest angle to the plane of its orbit, causing one hemisphere to receive more daylight than the other. NASA's Goddard Space Flight Center/Genna Duberstein

In New York City, there will be a little more than 15 hours of daylight on Thursday. To the south, in Miami, there will be closer to 13 hours and 44 minutes of sunlight throughout the day because it is closer to the equator.

Conversely, the winter solstice in the Northern Hemisphere is the day of the year with the shortest amount of daylight there. During the winter solstice, there are only about nine hours of sunlight in New York City. The exact date of the solstice varies each year due to the wobble of the Earth on its axis, the gravitational pull from the moon and the planets, and also because of leap year. The date skips back and compensates for the Gregorian calendar, cutting the year short by about a quarter of a day each year.

The word "solstice" itself means "sun stands still" in Latin and the celebrations of the solstices date back thousands of years. Some think the fact that the iconic historic site Stonehenge lines up with the sun on the solstice indicates that it was built to celebrate the sun or the changing of the seasons. Each year, a celebration is held at the site, as it has been for thousands of years.

The ancient Romans and the Greeks also celebrated the solstice and the solstice would mark one month prior to the Olympic games.

summer sun
People enjoy an afternoon along the boardwalk at Coney Island on a hot day on June 14 in New York City. Spencer Platt/Getty Images