It's been cold for many weeks far north of the equator, but astronomically speaking, the season of winter begins only when the solstice occurs. That happens this Saturday, when the Northern Hemisphere has its shortest day and longest night of the year, and the Southern Hemisphere has its longest day and shortest night of the year.

Here's everything you need to know about this week's solstice -- and why it matters.

Winter solstice in the Northern Hemisphere (and summer solstice in the Southern Hemisphere) occurs on Saturday, Dec. 21, at 4:19 a.m. EST. At that exact time, the sun shines above the Tropic of Capricorn. It appears at its most southerly as seen from the Northern Hemisphere.

"The word solstice comes from the Latin words sol sun and sistere (to stand still)," said Shyam Balaji, a researcher in astroparticle physics and cosmology at King's College London, in an email. "It refers to the point where the sun's apparent movement north or south pauses before reversing direction."

"The December solstice occurs because the Earth is tilted on its axis by about 23.5 degrees relative to its orbit around the sun," said Balaji. "This tilt causes different parts of the Earth to receive varying amounts of sunlight throughout the year. During the December solstice, the Northern Hemisphere is tilted the farthest away from the sun, resulting in the shortest day and longest night of the year for that hemisphere."

The sun is also lower in the sky and takes a shorter path through the sky. Cue cold temperatures in the Northern Hemisphere.

The seasons are related to how Earth is titled and have nothing to do with how close Earth is to the sun. Earth reaches the perihelion -- the closest it gets to the sun on its slightly elliptical orbit -- on Jan. 3, 2025, just two weeks after the solstice.

Instead, the winter solstice marks the moment when the sun is above the Tropic of Capricorn, after which the days begin to get longer. So, winter solstice is generally celebrated as being the rebirth of the sun and as the beginning of the solar year.

The length of the shortest day varies depending on your location on the planet. Here are some examples from the Northern Hemisphere:

"The exact duration will differ based on how far north or south you are from the equator," said Balaji. "After the December solstice, the days start to get longer in the Northern Hemisphere."