Summer officially arrives on Wednesday, June 21st at 12:24 a.m EDT. For those of us in the Northern Hemisphere, this marks the longest day of the year, and the moment when the Sun reaches the Tropic of Cancer, its highest point. For those who live in the Southern Hemisphere, this is the shortest day of the year, and the arrival of winter. The solstice happens at the same moment for everyone, everywhere on Earth.
Though many people consider Memorial Day to be the first day of summer, the Solstice marks the official start of the season. In fact, you might forget that this ushering in of summer has nothing to do with weather, but rather, it’s an astronomical event.
The change of seasons is all about the Earth’s 23.44 degree tilt either toward or away from the Sun as we orbit around it. The tilt determines whether the Sun’s rays strike at a low angle or more directly. At the Summer Solstice, that tilt toward the Sun is at its maximum.
The term “solstice” comes from the Latin words sol (Sun) and sistere (to stand still). At the solstice, the angle between the Sun’s rays and the plane of the Earth’s equator (called declination) appears to stand still — its noontime position changes very little for several days before and after the summer solstice. This phenomenon is most noticeable at the Arctic Circle where the Sun hugs the horizon for a continuous 24 hours, thus the term “Land of the Midnight Sun.”
Fun Fact: Around the time of the solstice, take a look at your noontime shadow: it’s your shortest noontime shadow of 2017!
Ancient cultures observed changes in the Sun’s path across the sky, and made note that the length of daylight and the location of the sunrise and sunset all shifted throughout the year. Monuments such as Stonehenge were built in order to follow the Sun’s path.
During the solstice, many celebrations were — and continue to be — held around the world:
- In Ancient Egypt, the summer solstice coincided with the rising of the Nile River. As it was crucial to predict this annual flooding, the Egyptian New Year began at this important solstice.
- In centuries past, the Irish would cut hazel branches on Solstice eve to be used in searching for gold, water, and precious jewels.
- Many European cultures hold Midsummer celebrations at the solstice, which include gatherings at Stonehenge and the lighting of bonfires on hilltops.
While Wednesday marks our longest day of the year in terms of daylight (all locations north of the equator have days longer than 12 hours in length) the days begin getting shorter until we reach the Winter Solstice, December 21st at 11:28 a.m. EST, when we’ll have our shortest day and our longest night of the year.
Check out this animated video for an easy explanation of the Earth’s tilt and how it impacts the change of seasons.