Moon phases explained: 8 phases, waxing and waning, moon age

The 8 phases

The cycle is usually split into eight phases. Four of them (new, first quarter, full, last quarter) are exact moments and are called the major phases.

• New moon: the Moon is on the Sun's side of the sky and is essentially invisible.
• Waxing crescent: a thin crescent in the western sky after sunset.
• First quarter: the right half is lit (northern hemisphere).
• Waxing gibbous: more than half lit and growing.
• Full moon: the whole disk is lit.
• Waning gibbous: more than half lit and shrinking.
• Last (third) quarter: the left half is lit (northern hemisphere).
• Waning crescent: a thin crescent in the eastern sky before sunrise.

Waxing and waning

From new moon to full moon the lit part grows (waxing); from full moon back to new moon it shrinks (waning). In the northern hemisphere a waxing moon is lit on the right and a waning moon on the left.

What moon age means

Moon age is the number of days since the last new moon. Age 0 is new moon, about 7.4 days is first quarter, about 14.8 is full moon, about 22.1 is last quarter, and it resets near 29.5. Illumination (%) is the fraction of the disk that is lit; together with moon age it tells you exactly where in the cycle the Moon is.

Why the lit side flips in the southern hemisphere

Observers in Australia or New Zealand look north to see the Moon and are effectively upside down relative to northern observers, so the image is rotated: a waxing moon appears lit on the left. Near the equator the crescent looks like it is lying on its back. The phase itself (illumination and moon age) is the same everywhere on Earth.

How Lunadot does it

Lunadot shows tonight's phase name, illumination, moon age and Earth–Moon distance on one screen, draws each night's moon on a monthly calendar, and lets you share a moon card for any date.