It’s starting to look a lot like winter; Solstice is here

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

For those who would rather have more sunlight, try to get to the southern hemisphere where it is summer. Or be patient: From Sunday, the days get a little longer in the Northern Hemisphere every single day until the end of June.

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

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

What is Solstice?

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

Solstices mark the times during the year when the Earth is at its most extreme tilt towards or away from the Sun. This means that the hemispheres get very different amounts of sunlight – and days and nights are at their most unequal.

During the winter solstice in the northern hemisphere, the top half of the Earth is tilted away from the sun, creating the shortest day and longest night of the year. The winter solstice falls between December 20 and 23.

Meanwhile, at the summer solstice, the northern hemisphere faces the sun, leading to the longest day and shortest night of the year. This solstice falls between June 20 and 22.

What is an equinox?

During the vernal and autumnal equinoxes, the Earth’s axis and its orbit are aligned, so both hemispheres receive the same 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 – although you can get a few extra minutes depending on where you are on the planet.

The Northern Hemisphere’s spring or vernal equinox can land between March 19 and 21, depending on the year. Its fall or autumnal equinox can land between September 21 and 24.

What is the difference between meteorological and astronomical seasons?

These are just two different ways to wrap 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 that calendar, spring starts 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.

Solstice starts in summer and winter. The equinox marks the start of spring and autumn.

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