Every month Earth's moon goes through its phases, waning and waxing in its constant transformation from new moon to full moon and back again.
This lunar cycle happens in part because the moon does not produce its own light; the silvery glow we see comes from sunlight reflecting off the moon's monochrome surface. In addition, our view of the moon is governed by a gravitational quirk called tidal locking. In essence, it takes roughly the same amount of time for the moon to spin once on its axis as it takes for our celestial companion to complete an orbit around Earth. That means the same side of the moon always faces Earth, although both sides get illuminated as the moon orbits, so there is no perpetual dark side of the moon.
As the moon, Earth, and the sun go through their orbital dance, the part of the moon that's illuminated by sunlight moves in and out of our view, creating a predictable series of lunar phases. In any given month we see eight distinct phases of the moon, defined by how much of the lunar disk is illuminated from our perspective and whether the moon is heading toward or away from being full.
During this phase the moon is between Earth and the sun, which means none of the lunar half we see is illuminated, and the moon becomes nearly invisible in the night sky. We can see the moon in this phase only during a solar eclipse.
As the moon's illuminated surface increases, it's in a stage known as waxing, and it's a crescent as long as it's less than half full.
This is the phase when half of the moon is illuminated and the percentage of the lit surface is still increasing.
When the moon is more than half full and still increasing its illuminated surface, it's called waxing gibbous. The word “gibbous” comes from the Latin for “hump” and has been used for centuries to describe rounded or convex shapes, like swollen eyes or the back of a camel.
In this phase, the moon is behind Earth with respect to the sun, and its face is fully illuminated. This is when we can sometimes see lunar eclipses.
This is the stage when the moon is more than half lit but the illuminated surface we can see is decreasing.
During this phase, the moon is once again half illuminated, but the lit area that's visible is on the decline.
As the next new moon nears, the moon shrinks back to a crescent that's less than half full.
A full moon by any other name
One of the most dramatic sights in the night sky—and inspiration for poets, artists, and lovers for millennia—full moons captivate us like nothing else. Full moons occur every 29.5 days or so, as the moon moves to the side of Earth directly opposite the sun.
For millennia, humans have used the movement of the moon to keep track of the passing year and set schedules for hunting, planting, and harvesting. Ancient cultures the world over have given these full moons names based on the behavior of the plants, animals, or weather during that month.
January: Wolf moon
Native Americans and medieval Europeans supposedly named January's full moon after the howling of hungry wolves lamenting the midwinter paucity of food. Other names for this month's full moon include old moon and ice moon.
February: Snow moon
The typically cold, snowy weather of February in North America earned this full moon its name. Other common names include storm moon and hunger moon.
March: Worm moon
This last full moon of winter was named the worm moon after the worm trails that would appear in the newly thawed ground. Other names include chaste moon, death moon, crust moon (a reference to snow that would become crusty as it thawed during the day and froze at night), and sap moon, after the tapping of the maple trees.
April: Pink moon
Northern Native Americans call April's full moon the pink moon after a species of early blooming wildflower. In other cultures, this moon is called the sprouting grass moon, the egg moon, and the fish moon.
May: Flower moon
May's abundant blooms give its full moon the name flower moon in many cultures. Other names include the hare moon, the corn planting moon, and the milk moon.
June: Strawberry moon
In North America, the harvesting of strawberries in June gives that month's full moon its name. Europeans have dubbed it the rose moon, while other cultures named it the hot moon for the beginning of summer heat.
July: Buck moon
Male deer, which shed their antlers every year, begin to regrow them in July, hence the Native American name for July's full moon. Other names include thunder moon, for the month's many summer storms, and hay moon, after the July hay harvest.
August: Sturgeon moon
North American fishing tribes called August's full moon the sturgeon moon, since the species was traditionally abundant during this month. It's also been called the green corn moon, the grain moon, and the red moon for the reddish hue it often takes on in the summer haze.
September: Harvest moon
The most familiar named moon, September's harvest moon refers to the time of year after the northern autumn equinox when crops are gathered. Other names include the corn moon and the barley moon.
October: Hunter's moon
The first moon after the harvest moon is the hunter's moon, so named as the preferred month to hunt summer-fattened deer and fox unable to hide in now bare fields. The hunter's moon is also particularly bright and long-lasting in the sky, giving hunters the opportunity to stalk prey at night. Other names include the travel moon and the dying grass moon.
November: Beaver moon
There is disagreement over the origin of November's beaver moon name. Some say it comes from Native Americans setting beaver traps during this month, while others say the name comes from the heavy activity of beavers building their winter dams. Another name is the frost moon.
December: Cold moon
The coming of winter earned December's full moon the name cold moon. Other names include the long night moon and the oak moon.
Blue moons are not actually blue, and despite the common turn of phrase, they are not especially rare. While the precise definition has changed over the years, the term “blue moon” is commonly used today to describe a second full moon appearing in a calendar month, which happens every two and a half years, on average.
The moon's orbit around Earth is not a perfect circle, and its distance to our planet changes over the course of an orbital cycle. When a full moon coincides with an especially close approach, or perigee, the lunar orb is slightly bigger and brighter than average, delivering what's come to be known as a supermoon.