While today may be the darkest day of the year, it also signals that brighter days are on the horizon.

This Sunday marks the shortest day of the year for those living north of the equator, as the solstice ushers in the beginning of astronomical winter. Conversely, in the Southern Hemisphere, it is the longest day of the year as summer commences.

The term “solstice” is derived from the Latin “sol,” meaning sun, and “stitium,” meaning “pause” or “stop.” The solstice represents the culmination of the sun’s journey higher or lower in the sky annually. During the winter solstice, the sun reaches its lowest and shortest arc. For those who cherish sunlight, there’s good news: the sun will begin to rise higher in the sky, and daylight will gradually increase until late June.

Throughout history, solstices have been celebrated with festivals and structures like Stonehenge, which aligns with the sun’s paths during solstices. But what exactly occurs in the cosmos? Here’s an insight into Earth’s orbit.

What is the solstice?

As Earth orbits the sun, it does so at an angle, causing the sun’s warmth and light to be distributed unevenly across the northern and southern hemispheres for most of the year.

The solstices mark the times when the Earth’s tilt toward or away from the sun is at its maximum. 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 winter solstice the upper half of the Earth is at its furthest lean away from the sun — leading to the shortest day and longest night of the year. The winter solstice falls can fall between Dec. 20 and 23 — this year it’s the 21st.

The opposite happens at a Northern Hemisphere summer solstice: The upper half of the Earth is leaning toward the sun, creating the longest day and shortest night of the year. This solstice falls between June 20 and 22.

What is the equinox?

During the equinox, the Earth’s axis and its orbit align so that both hemispheres get an equal amount of sunlight.

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.oo

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.

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.