When Is the Full Moon December 2021? Cold Moon Meaning Explained

Tomorrow evening, a full "Cold Moon" will grace the night sky. But what time does the moon turn full and where does this name come from?

A full moon occurs when the moon appears opposite the sun in Earth-based longitude. Or in other words, when our planet is located directly in between the sun and our natural satellite.

When this happens, the face of the moon that we can see from the Earth is fully illuminated, appearing like a perfect circle.

Technically, the moon turns full at a specific point in time. On December 18, this moment occurs at 11:36 p.m. ET.

But to most observers, the moon will appear full for three days around this time—from Friday evening through Monday morning.

The names given to full moons originate from a number of places and historical periods, including Native American, colonial American and European sources.

The full moon in December is often called the "Cold Moon"—a reference to the cold temperatures that the month is characterized by. In a similar vein, other commonly used names are the "Frost Moon" or the "Winter Moon."

The December full moon is also sometimes referred to as the "Long Night Moon," given that it falls close to the winter solstice in the Northern Hemisphere.

The term "winter solstice" is often used to refer to an entire day, but technically it only refers to a specific moment in time—the point in the year when the Earth's North Pole is pointing furthest away from the sun.

The solstice marks the shortest day of the year—when the sun appears lowest in the sky—and the beginning of astronomical winter in the Northern Hemisphere.

The day on which the December solstice occurs in the Northern Hemisphere—which in 2021 will be December 21—has the fewest daylight hours and the longest night. From this date onwards, the days start to become longer again.

When the sun appears lowest in the sky for the year, the path of the full moon opposite the sun appears highest in the sky, hence the name "Long Night Moon."

For observers in the Washington D.C. area, for example, the moon will be visible in the sky for a total of 15 hours and 33 minutes—making this the longest full moon night of the year.

The December cold moon is also sometimes called the Moon before Yule, which refers to a festival celebrated around the time of the winter solstice by Germanic peoples in pre-Christian Europe, according to the Old Farmer's Almanac.

A full cold moon
A full Cold Moon rises behind the San Gabriel Mountains and the Los Angeles downtown skyline at sunset on December 29, 2020 in California. PATRICK T. FALLON/AFP via Getty Images