The winter solstice is on Monday. A planetary scientist's simple animation explains what solstices are and how they work.

• The winter solstice happened Monday at 5:02 a.m. ET.
• Earth's northern hemisphere sees the shortest day of the year during the December solstice, while the southern hemisphere sees its longest day.
• That's because of the way Earth is angled toward the sun.
• A planetary scientist created an animation that shows how Earth's angle and orbit creates equinoxes and solstices.

The winter solstice came on Monday at 5:02 a.m. ET.

The shortest day of the year heralds the arrival of winter; after this, days finally start getting longer in the northern hemisphere, which houses about 90% of Earth's population. For the southern hemisphere, it signifies the opposite: shortening days and the dawning of summer.

To illustrate what's going on with the solstice, Dr. James O'Donoghue, a planetary scientist at the Japan Aerospace Exploration Agency, created an animation that neatly summarizes how these events work, along with their relationship to the equinoxes.

Solstices and equinoxes are the products of Earth's axial tilt: the degree to which the planet is tilted relative to the sun. The axis around which the Earth spins isn't straight up and down - it's about 23.5 degrees off. Because of that, different parts of the Earth get exposed to more or less sunlight as the planet rotates around the sun. That's why we have seasons.

It's also why the northern and southern hemispheres experience seasons at opposite times: During winter in the northern hemisphere, the southern hemisphere is tilted more towards the sun, and vice versa.

Meanwhile, Earth is also constantly rotating, which keeps its heating even - kind of like a planet-sized rotisserie chicken twisting over a spit.

The axial tilt's most dramatic effect comes during the solstices, since those are the two days when one side of the planet is tilted the farthest away from the sun and the other is the closest. On Monday, the northern hemisphere will receive less than nine hours of daylight, while the southern hemisphere is getting more than 15.

As a result, anyone in the northern hemisphere who stands outside at noon on Monday will cast their longest shadow of the year.

During the summer solstice, O'Donoghue explained on Twitter, "sunlight is most intense as it only has to pass through a short column of atmosphere." That's why it gets hot during summertime in general.

The two times of the year when Earth's axis isn't tilted towards or away from the sun - leading sunlight to hit the northern and southern hemispheres equally - are the equinoxes. On those days, both halves of the planet experience an equal 12 hours of sunlight and darkness.

So if you were to stand directly on the equator at the exact time of an equinox, your shadow would be at its absolute minimum. The sun would also appear almost directly overhead.

But the shadowless moment would be fleeting, since Earth moves around the sun at about 66,600 mph.