Today is the longest day of the year for half the planet, marking the start of astronomical summer in the Northern Hemisphere. The summer solstice is the point at which the Earth’s upper half is leaning toward the sun, creating the longest day and shortest night of the year.

Understanding the Solstice

The word ‘solstice’ comes from the Latin words ‘sol,’ meaning sun, and ‘stitium,’ which can mean ‘pause’ or ‘stop.’ The summer solstice is the end of the sun’s annual march higher in the sky, when it makes its longest, highest arc. After this point, the sun will begin retreating, and days will get a little shorter every day until late December.

People have marked solstices for centuries with festivals and monuments, including Sweden’s midsummer eve celebrations and Stonehenge, which was designed to align with the sun’s paths at the solstices.

Earth’s Orbit and the Solstice

As the Earth travels around the sun, it does so at an angle, making the sun’s warmth and light fall unequally on the northern and southern halves of the planet for most of the year. The solstices mark the times when the Earth is tipped most extremely either toward or away from the sun, resulting in very different amounts of sunlight and unequal days and nights.

The summer solstice falls between June 20 and 22, and this year it’s on June 21. The opposite happens at the Northern Hemisphere winter solstice: the Earth’s upper half leans the furthest away from the sun, leading to the shortest day and longest night of the year.

Equinoxes and Seasons

During the equinox, the Earth’s tilt is neither toward the sun nor away from the sun, so both the northern and southern hemispheres get an equal amount of sunlight. The word equinox comes from two Latin words meaning equal and night, as day and night last almost the same amount of time on the equinox.

The Northern Hemisphere’s fall or autumnal equinox can land between September 21 and 24, depending on the year. Its spring or vernal equinox can land between March 19 and 21. The exact time of the equinox is the moment the sun is directly overhead at the equator.

Meteorological seasons, on the other hand, are defined by the weather. Meteorologists break down the year into three-month seasons based on annual temperature cycles. By this calendar, spring starts on March 1, summer on June 1, fall on September 1, and winter on December 1.

Original reporting: 40/29 / KHBS (NW Arkansas) — read the source article.