What Is a Full Moon?
A full moon is the phase when the Moon’s Earth-facing side is fully lit by the Sun. It happens when the Moon sits roughly opposite the Sun in the sky, so from Earth we see its entire daylit half as a bright, complete disc.
Why a Full Moon Happens
The Moon produces no light of its own. We only ever see the half that is lit by the Sun, and the phase is simply how much of that lit half is turned towards us.
At full moon, the Sun, Earth, and Moon fall into a near-straight line with Earth in the middle. The Sun shines past Earth and lights the whole face of the Moon that we can see. A full moon therefore rises around sunset, sits highest near midnight, and sets around sunrise, the opposite schedule to the Sun.
The position in the sky shifts through the year and depends on your hemisphere. In the Northern Hemisphere, a summer full moon follows a lower arc and a winter full moon climbs higher. South of the equator, the same relationship applies to the local season, but summer and winter occur at opposite points in the calendar, so the calendar pattern reverses. Near the equator, the seasonal contrast is smaller. The same tilt of Earth’s axis that shapes sunrise and sunset times also affects how high the full moon appears.
The Lunar Cycle
The Moon takes about 27.3 days to complete one orbit of Earth, but a single cycle of phases takes longer: roughly 29.5 days from one full moon to the next. This period is called the synodic month (averaging 29.53 days).
The two figures differ because Earth is also moving around the Sun. By the time the Moon has finished one orbit, Earth has travelled along its own path, so the Moon needs a little extra time to catch up and return to the same Sun-Earth-Moon alignment. That extra time is what stretches the cycle from 27.3 days to about 29.5.
Over a full synodic month the Moon moves through its familiar sequence: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent, and back to new. You can see the current stage on the moon phases page.
Why We Always See the Same Face
The Moon keeps broadly the same hemisphere turned towards Earth because it is tidally locked. It rotates once on its axis during each orbit around Earth. Its elliptical, tilted orbit still changes our viewing angle slightly, an effect called libration.
Libration lets us glimpse narrow areas around different parts of the Moon’s edge. Over time, observers can see about 59% of the lunar surface, although the far side never turns to face Earth as a whole. Calling it the “dark side” is misleading because it receives sunlight just like the near side.
Why a Full Moon Isn’t an Eclipse Every Month
If the Sun, Earth, and Moon line up at every full moon, you might expect Earth’s shadow to fall on the Moon each time, producing a lunar eclipse. It doesn’t, because the Moon’s orbit is tilted by about 5 degrees relative to Earth’s orbit around the Sun.
Most months the Moon passes a little above or below Earth’s shadow, so the alignment is close but not exact. Only when a full moon happens near one of the two points where the orbits cross does the Moon move through Earth’s shadow, giving a lunar eclipse. You can read more on the eclipses page.
Supermoons
A supermoon is a full moon that happens when the Moon is near perigee, the closest point in its slightly oval orbit. The Moon’s distance from Earth varies, averaging about 384,400 km (238,855 miles), and at perigee it sits noticeably nearer than at its farthest point, apogee.
A full moon at perigee looks around 14% larger in diameter and up to 30% brighter than a full moon at apogee. The difference is real but subtle to the naked eye, since there is no nearby reference to compare against in the sky. “Supermoon” is a popular term rather than a strict astronomical one, and is usually applied when a full moon falls within roughly 90% of its closest approach.
Blue Moons and Named Moons
A blue moon has nothing to do with colour. Because the 29.5-day cycle is shorter than most calendar months, occasionally two full moons fall within a single month; the second is the popular blue moon. By an older definition, a blue moon is the third full moon in a season that contains four. Either way, it is a quirk of the calendar, not an astronomical event.
Many full moons also carry traditional names tied to the time of year. The harvest moon, for example, is the full moon nearest the autumnal equinox in Northern Hemisphere traditions, while wolf moon is a midwinter name. These labels come from particular cultures and almanacs, so they are not universal and do not change how the Moon looks.
How the Date Is Predicted
The timing of a full moon can be calculated far in advance because the motions of the Sun, Earth, and Moon follow well-understood physics. Astronomers define the exact moment of full moon as the instant the Moon reaches the point opposite the Sun, which is a single point in time rather than a whole night.
That instant is given in UTC, so the calendar date can differ by a day depending on your time zone. A full moon listed for just after midnight UTC may still fall on the previous evening in the Americas. The next full moon falls on
Effects of the Full Moon
The full moon’s clearest real-world effect is on tides. When the Sun and Moon align at full moon (and again at new moon), their gravitational pulls combine to produce larger spring tides, with higher highs and lower lows. Around the quarter moons, the pulls partly cancel, giving smaller neap tides. A supermoon can make spring tides slightly more pronounced.
For stargazers, a full moon is a mixed blessing. Its brightness floods the night sky and washes out fainter stars, the Milky Way, and most meteor showers, so the darkest skies come around new moon. The full moon is, however, bright enough to light a landscape and cast shadows, which makes it striking to watch on its own.
Tips for Watching the Full Moon
- Catch it at moonrise. Just after the Moon clears the horizon it can look larger and warmer in colour, an optical effect known as the moon illusion.
- Check your local moonrise time. A full moon rises close to sunset, so an early evening view needs a clear eastern horizon.
- Look a night either side. The Moon appears essentially full for about a day before and after the exact moment, so you have a wide window.
- Use cloud cover in the forecast. Clear skies make all the difference; thin high cloud can still let the Moon shine through.
- For stars and meteor showers, wait for new moon. A bright full moon overwhelms faint objects, so plan deep-sky viewing around the darker part of the cycle.
How Airpult Shows the Moon
Airpult tracks the lunar cycle alongside your weather. The moon phases page shows the current phase and the dates of upcoming full and new moons, all calculated from live astronomy data. To plan a clear night, pair it with the sunrise and sunset times and cloud cover forecast for your location. Use the explore page to find any location and check its forecast.
