In the Northern Hemisphere, the winter solstice occurs on either Dec. 21 or 22 every year. This year, it falls on Dec. 21.

Though temperatures have already started to drop in some parts of the country, winter doesn't officially begin until mid-December.

Here's what to know about the winter solstice, the astronomical start to the snowy season and the shortest day of the year.

The winter solstice marks the beginning of astronomical winter. It's different from the beginning of the meteorological winter, which is based on our 12-month calendar.

To answer this question more completely, let's go back to middle school science class for a minute. The Earth rotates in two primary ways: first, it moves in a big circle around the sun; second, it spins around an axis like a top, creating day and night. That axis is tilted on a slight angle, causing the Northern and Southern hemispheres to receive different amounts of sunlight at different times of the year.

So, though we tend to think about the winter solstice as a day-long event, the term actually refers to a precise moment at which one half of the Earth -- either the top or the bottom -- is tilted the furthest away from the sun, meaning that that half of the planet sees the fewest hours of sunlight it will see all year.

Though the winter solstice marks the shortest day of each year, it also signals an increase in the amount of sunlight each day for the next six months.

Similarly, the day when the Earth's tilt is closest to the sun is called the summer solstice. It's the longest day of the year and marks the astronomical beginning of summer.

In the Northern Hemisphere, the winter solstice occurs every year on Dec. 21 or 22. This year, it falls on Dec. 21.

The summer solstice is six months later around June 21.

For people who live below the equator, however, those days are flipped. Their winter solstice occurs in June, while their summer solstice happens in December.

Each year, there are two equinoxes -- one in the spring, and one in the fall.

During the equinoxes, the sun is directly above the Earth's equator, meaning there are equal amounts of sunlight and darkness on those two days.

The vernal equinox, which happens in March in the Northern Hemisphere, marks the astrological end of winter. In September, the autumnal equinox signals the astrological end of summer.

Below is a list of the astronomical season changes we can expect to see following the upcoming winter solstice next month:

Sort of. Each day, the sun traces a path across the sky, rising in the east and setting in the west. As the winter solstice approaches and the number of hours of sunlight in a day decreases, the sun's path sinks lower in the sky, though it follows the same general arc.

By the time the winter solstice arrives, the sun sits at the lowest position in the sky it can.

As a result, the shadow you cast becomes longer and longer as the winter solstice approaches. So, while solstices aren't nearly as visible as eclipses, if you go outside at noon on the winter solstice, your shadow will be the longest it will be all year.