The winter solstice is Saturday, the shortest day and longest night of the year in the Northern Hemisphere – ideal conditions for Christmas lights and warm blankets.

If you’d rather have more sunlight, you can try heading to the southern hemisphere where it’s summer. Or be patient: from Sunday onwards, the days in the northern hemisphere will be a little longer every day until the end of June.

These annual changes in sunlight as the Earth revolves around the Sun have been known to humans for centuries. Monuments such as Stonehenge in England and the Torreon at Machu Picchu in Peru were designed in part to align with the solstices.

Here’s what you should know about how Earth’s march around the sun divides the year.

What is the solstice?

As the Earth moves around the Sun, it does so at an angle. For most of the year, the Earth’s axis is tilted either toward or away from the sun. This means that heat and light from the Sun fall unevenly on the northern and southern halves of the planet.

The solstices mark the times of year when the Earth is tilted most toward or away from the sun. This means that the hemispheres receive very different amounts of sunlight – and days and nights are the most unequal.

During the winter solstice in the Northern Hemisphere, the upper half of the Earth tilts away from the sun, creating the shortest day and longest night of the year. The winter solstice falls between December 20th and 23rd.

Meanwhile, the Northern Hemisphere faces the sun on the summer solstice, resulting in the longest day and shortest night of the year. This solstice falls between June 20th and 22nd.

What is the Equinox?

During the spring and fall equinoxes, the Earth’s axis and orbit align so that both hemispheres receive equal amounts of sunlight.

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

The vernal or vernal equinox in the Northern Hemisphere can occur between March 19 and 21, depending on the year. The autumnal equinox can occur between September 21st and 24th.

What is the difference between meteorological and astronomical seasons?

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

Meteorological seasons are defined by the weather. They divide the year into three-month seasons based on annual temperature cycles. According to this calendar, spring begins on March 1st, summer on June 1st, autumn on September 1st and winter on December 1st.

Astronomical seasons depend on how the Earth moves around the Sun.

Solstices usher in summer and winter. The equinox marks the beginning of spring and autumn.

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The Associated Press Health and Science Department receives support from the Science and Educational Media Group of the Howard Hughes Medical Institute. The AP is solely responsible for all content.