This is the sun’s time to shine: Sunday is the longest day of the year in the Northern Hemisphere.

Sunday is the solstice, marking the start of astronomical summer north of the equator. It’s the opposite in the Southern Hemisphere, where it is the shortest day of the year and winter will start.

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:

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.

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.

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.