The Moon

The Moon is Earth's only natural satellite, an average of 384,400 km away.^[1] Its phases set the rhythm of the night sky: a bright Moon washes out faint meteors, comets and aurora, while the days around new Moon give the darkest skies of the month.

Why the Moon has phases

The Moon makes no light of its own. The Sun always lights half of it, and the phases are how much of that lit half can be seen from Earth as the Moon travels around its orbit.^[2] The eight phases, in order, are new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, third quarter and waning crescent, and the cycle repeats every 29.5 days.^[2]

The Moon takes about 27 days to orbit Earth, but because Earth is moving around the Sun at the same time, the cycle of phases takes longer.^[1] The Moon also turns once on its axis in the same time it takes to orbit, so the same side always faces Earth.^[1] During a crescent phase, the dark part of the Moon often glows faintly; this is earthshine, sunlight reflected from Earth onto the Moon.^[2]

When each phase is in the sky

• New Moon rises and sets with the Sun and cannot be seen, because its unlit side faces Earth.^[2]
• First quarter rises around midday and sets around the middle of the night, high in the evening sky.^[2]
• Full Moon is opposite the Sun, rising around sunset and setting around sunrise, so it is up all night.^[2]
• Third quarter rises around the middle of the night and sets around midday.^[2]

Solar eclipses can happen only at new Moon and lunar eclipses only at full Moon; see Eclipses.^[3]

Why moonlight matters

Moonlight brightens the whole sky, and a brighter sky hides faint objects. NASA's skywatching guidance suggests comparing how many stars can be seen at full Moon and at new Moon to gauge the difference for yourself.^[4]

• Meteor showers. Light from a bright Moon can be as harmful to meteor watching as city lights.^[5] When a bright Moon is up during a peak, NASA advises watching in the hours before dawn, after the Moon has set.^[6]
• Aurora. A full Moon makes the aurora look fainter, although it does not change how bright it really is.^[7]
• The Milky Way. It needs a dark sky in which stars of about magnitude 5 can be seen, so the days around new Moon are best.^[4]

A lunar eclipse is the one time a full Moon darkens: as Earth's shadow dims it, constellations become easier to see than on an ordinary full-Moon night.^[8] Watching meteors themselves is covered in How to watch a meteor shower.