When a calendar month has two full moons, the second is marked as a blue moon.
Why the moon has phases
The moon does not make its own light. Half of it is always lit by the sun, and as it travels around the Earth we see different amounts of that sunlit half. At new moon the lit side faces away from us. Over the next two weeks a growing crescent, the first quarter and a waxing gibbous moon appear in the evening sky until the whole face is lit at full moon. Then the cycle runs in reverse through the waning gibbous, last quarter and waning crescent phases before the next new moon. One full cycle, the synodic month, lasts about 29.5 days, which is why phases drift about a day earlier each calendar month. MoonPhaseKit calculates the exact moments of each principal phase with the standard formulas used by astronomers.
The eight phases
New moon: invisible, rises and sets with the sun. Waxing crescent: a thin sliver in the west after sunset. First quarter: half lit, high in the sky at sunset. Waxing gibbous: more than half lit, up most of the evening. Full moon: rises at sunset and sets at sunrise. Waning gibbous and last quarter: rise late at night and are visible in the morning. Waning crescent: a sliver in the east before dawn.
Full moon names and blue moons
Each month’s full moon has a traditional North American name: the Wolf Moon in January, the Snow Moon, Worm Moon, Pink Moon, Flower Moon and Strawberry Moon, then the Buck, Sturgeon and Corn moons, the Hunter’s Moon in October, the Beaver Moon and the Cold Moon in December. Because a lunar cycle is shorter than most months, roughly every two and a half years one month contains two full moons and the second is popularly called a blue moon. A supermoon is a full moon near the moon’s closest point to Earth, when it looks slightly bigger and brighter.
Using moon phases
Photographers and stargazers plan dark-sky trips around the new moon, when faint stars and the Milky Way are easiest to see. Anglers and hunters often watch the moon because tides and animal activity follow it. Gardeners who follow moon planting sow leafy crops while the moon waxes. Night hikers prefer the bright nights around full moon. Times are shown in your device’s time zone, and the phase is the same everywhere on Earth at the same moment, although the moon appears flipped in the southern hemisphere.
Frequently asked questions
What is a blue moon?
Popularly, the second full moon in one calendar month. It happens about every two to three years.
Where do the full moon names come from?
They come from North American folk traditions popularised by farmers’ almanacs.
What is the Harvest Moon?
The full moon closest to the September equinox, in September or early October.
Is MoonPhaseKit free?
Yes, with no sign-up or ads.
Does it need my location?
No. Moon phases are the same everywhere on Earth; times are shown in your device’s time zone.