What is a Solstice? - Definition & Facts

Instructor: Linda Fye
Learn about the summer and winter solstices and how they are related to the seasons. Understand the different reasons they occur and why they are important.

Solstice in History

Four to five thousand years ago, ancient people in Wiltshire, England, built a round monument with giant stones known as Stonehenge. It is believed that the stones were positioned to align with the sunrise and sunset on the days of the summer and winter solstices. To this day, thousands of people go to festivals at Stonehenge to celebrate the solstices.

Summer solstice sunrise over Stonehenge June 2005
Summer Solstice Sunrise over Stonehenge June 2005

What is so important about the solstices that people built a massive monument to them, and that people still go there to celebrate? Well, the solstices are special days, and they have to with Earth's relationship to the Sun. In order for you to appreciate that special relationship, first you need to understand some basic information about Earth.

Lines of Latitude and Longitude

Locations can be shown on Earth with a grid system. Lines of latitude, or parallels, run horizontally around Earth and describe north-south locations. The values range from 0 degrees at the equator to 90 degrees north at the North Pole in the Northern Hemisphere. The Southern Hemisphere is the opposite and values go from the equator to 90 degrees south at the South Pole. Lines of longitude, or meridians, run vertically around Earth, and describe east-west locations. Their values range from 0 degrees at the Prime Meridian to 180 degrees on the other side of the planet. These horizontal and vertical lines intersect at right angles and create a grid for Earth. Two lines of latitude are especially important to the solstices: the Tropic of Cancer at 23.5 degrees north and the Tropic of Capricorn at 23.5 degrees south.

Lines of latitude and longitude
Lines of latitude and longitude

Seasons

Unlike what many people think, seasons have nothing to do with how close Earth is to the Sun. Instead, there are four other things that cause Earth to have seasons.

1.) Earth rotates once every 24 hours, or one day. This is called rotation.

2.) It also revolves around the Sun every 365 ¼ days, or one year. This is revolution.

3.) Earth is tilted by an angle of 23.5 degrees, and this is inclination. If Earth was not tilted, all areas of the planet would get the same amount of sunlight all year long, and seasons would never change.

4.) The fourth is called polarity. The North Pole is always pointing in the same direction toward the North Star, or Polaris.

Rotation, revolution, inclination, and polarity all cause the change of seasons on Earth.

Earth revolves around the Sun. See its position during each season
Earth

Summer and Winter Solstices

A solstice is special because it marks the beginning of a new season. This happens because Earth is at a certain position relative to the Sun. The summer solstice occurs on June 21st or 22nd. On that day, the North Pole is pointing most directly at the Sun, and the vertical rays of the Sun are striking the Tropic of Cancer. That means that if you were standing on the Tropic of Cancer at 23.5 degrees north latitude, the Sun would be directly over your head. The summer solstice marks the first day of summer in the Northern Hemisphere, and it is the longest day of the year.

Summer solstice
Summer solstice

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