Sunday is the longest day of the year for half the planet. A guide to the summer solstice

The word “solstice” comes from the Latin words “sol,” for 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. The bad news for sun lovers: It then starts retreating and days will get a little shorter every day until late December.

People have marked solstices for eons 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.

Here’s what to know about the Earth’s orbit.

What is 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. This means the hemispheres are getting very different amounts of sunlight, and days and nights are at their most unequal.

At the Northern Hemisphere’s summer solstice, the Earth’s upper half is leaning toward the sun, creating the longest day and shortest night of the year. The summer solstice falls between June 20 and 22. This year it’s 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. The winter solstice falls between Dec. 20 and 23.

What is the equinox?

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 sun rises almost exactly due east and it sets almost exactly due west.

The word equinox comes from two Latin words meaning equal and night. That’s because on the equinox, day and night last almost the same amount of time — though one may get a few extra minutes, depending on where you are on the planet.

The Northern Hemisphere’s fall — or autumnal — equinox can land between Sept. 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.

What’s the difference between meteorological and astronomical seasons?

These are just two different ways to carve up the year.

While astronomical seasons depend on how the Earth moves around the sun, meteorological seasons are defined by the weather. Meteorologists break down the year into three-month seasons based on annual temperature cycles. By that calendar, spring starts on March 1, summer on June 1, fall on Sept. 1 and winter on Dec. 1.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.