Third quarter moon

How it works

How it works is geometry, then a clock. The Moon is always half-lit by the Sun. From Earth we see different slices of that lit half. At third quarter we see half of the disk as bright and half as dark. That is a quarter of the whole Moon’s surface, which is why the name is “quarter” and not “half,” even though the picture looks like a half pie.

Third quarter and last quarter are the same primary phase. It is the last of the four primaries: new, first quarter, full, third quarter. After full moon the lit part wanes through gibbous, then hits this half, then thins to a crescent, then new. The phase is an instant on the clock, not a 24-hour festival. The night around that instant is when the half pie is the honest picture.

The civil calendar will print the instant in a time zone. Neighbouring dates can look almost the same to the eye. The named phase is still the instant, not “sometime this week when it looked pretty.” A lunar cycle from new to new is about 29.5 days, so a third quarter comes around once in that cycle, about a week after full moon and about a week before new moon.

New: disk dark, near the Sun in the sky. First quarter: half lit, light growing, rises around midday. Full: disk lit, rises around sunset. Third quarter: half lit, light shrinking, rises around midnight.

In the Northern Hemisphere the bright half at third quarter sits on the left as you face the Moon. In the Southern Hemisphere it sits on the right. The Moon did not flip its geology. You did, relative to the sky.

A phase that refuses dinner hour

First quarter is the evening half moon. Third quarter is the morning half moon. It rises in the middle of the night, stands highest around dawn, and sets around the middle of the day. If your only sky time is after supper, you will keep missing it and then wonder whether the month skipped a half moon. It did not. You were in bed.

Because it is up in daylight, you can see it in a blue morning sky. That is ordinary, not an omen. The Moon is bright enough. People simply do not look for a half moon over the office at 10:00. The calendar date of the phase does not change that habit. The clock of rise and set does.

Do not confuse this with a calendar quarter. A fiscal third quarter is a three-month block. A third quarter moon is a slice of a month-long orbit. Calling July “third quarter moon season” is a category error. July will contain one third quarter, plus other phases, like every other month on average.

The third quarter is half a pie and a whole night’s wait. If you only look up at dinner, you will keep missing your own half moon.

Putting it on a month grid

A month of 30 or 31 days is a civil box. The Moon does not finish a cycle in a civil month exactly, because 29.5 does not divide those boxes cleanly. Some civil months contain the third-quarter instant once. The date walks forward on the civil calendar from month to month. You read the time for your time zone. A table in another zone can sit on the neighbouring civil date when the instant is close to midnight.

After third quarter comes waning crescent, then new. If you are planning a dark-sky night, new is the dark one. Third quarter still lights half a disk and will wash a sky in the hours it is up. If you are planning a walk before breakfast, third quarter is the phase that may actually be overhead while you lock the door.

The eight named phases in order are new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent. Third quarter is not a rare extra. It is the regular seventh name in that list, once per cycle. Miss it and the next one is about 29.5 days later, not “next April.”