Waxing or waning: telling the two halves of the cycle apart
The lit side tells you which way the Moon is going — west means growing, east means shrinking — and the rule inverts south of the equator.
Every moon question after “what phase is it” is really “is it growing or shrinking”. The answer is in the shape, and the rule takes one sentence:
The lit limb points at the Sun. If the bright side is toward where the Sun is — or where it set, or where it will rise — the Moon is waxing. If the lit limb points away from the Sun, it is waning.
That rule works everywhere on Earth, at every phase, and needs no mnemonic. It also explains why the northern-hemisphere shortcuts fail in the south.
The northern rule, and why it inverts
In the northern hemisphere, looking south at the Moon, west is on your right. So a waxing Moon — lit on the west, toward the setting Sun — appears lit on the right. That is where “right side bright, getting brighter” comes from.
Cross the equator and the geometry inverts. Standing in the southern hemisphere facing the Moon, west is now on your left, so the same waxing Moon appears lit on the left. Nothing about the Moon changed; you did. Every guide that omits this is wrong for roughly half the planet’s readers, which is why almost all of them do omit it.
The lit-limb rule survives the crossing because it does not depend on which way you are facing — only on where the Sun is.
Reading the terminator
The line between light and dark — the terminator — is a curve, not a straight edge, except at the quarters. Its two ends point away from the Sun. On a waxing Moon the terminator’s horns point east; on a waning Moon they point west.
A crescent is worth a second look. The illuminated part of the disc is the day side seen almost edge-on, so the crescent’s outer edge is the Moon’s limb and its inner edge is the terminator — a semicircle. That is why a crescent is asymmetric: one side is the sphere’s edge, the other is the day–night boundary. A shape that looks like a symmetric “C” or “D” is not how a crescent Moon is actually shaped.
Daytime is the clearest test
The fastest way to settle it is to look while the Sun is still up. A waxing Moon is visible in the afternoon and early evening, its lit edge toward the Sun in the west. A waning Moon is visible in the morning, lit toward the rising Sun in the east. The daylight visibility page covers when each phase is up during the day.
Two mnemonics that fail
“DOC” and “COD”. On the theory that a waxing Moon makes a “D” and a waning Moon a “C”. It is a fair description of the shape’s curvature in the northern hemisphere, and it is silent on latitude, which is the one thing that changes the answer. It is also easy to misremember which letter means which, which defeats the purpose of a mnemonic.
“The Moon is fuller on the left before a storm.” No. Weather has no relationship to lunar phase that survives measurement, and the belief is mostly an artefact of noticing the Moon when the sky is clear.
The 29.53-day bookkeeping
Waxing runs from new moon to full moon — elongation 0° to 180°, about 14.8 days. Waning runs from full back to new — 180° to 360°, another 14.8. The two halves are not quite symmetrical in length because the Moon’s orbital speed varies, which is why some lunations are a few hours longer than others.
To get the exact answer for a date rather than a shape, the phase tool computes the elongation directly, and if you want the eight names set out properly, the phases page explains what each one is and is not.
Astronomy only — no claim about what a phase means for anything other than the Moon’s illumination.
Frequently asked questions
- Why is the lit side on the right in the northern hemisphere?
- Because the Sun sets in the west and the Moon follows it across the sky. On a waxing Moon the illuminated hemisphere faces the Sun, which is behind it in the sky — so the bright limb is on the side where the Sun is, the west. From south of the equator you are looking at the same Moon from the other side of the Earth's axis, so the whole picture appears rotated 180°.
- Is a crescent moon ever caused by Earth's shadow?
- No. The crescent is the lit hemisphere seen at a steep angle — you are seeing a sliver of the day side, with the rest of the disc in its own night. Earth's shadow only ever touches the Moon during a lunar eclipse, and then it covers it gradually over hours, which looks nothing like a crescent.
- How can I tell before sunset?
- Look at the Moon when it is visible in daylight: the bright limb points at the Sun. That works at any phase and in any hemisphere, because it is the definition rather than a memory aid. A waxing Moon in the afternoon sky has its lit edge toward the Sun, which is still above the horizon; a waning Moon in the morning has its lit edge toward the rising Sun.
Sources
Each source is a primary or official reference, checked before publication. Links open the original.
- NASA Science — Moon: the illumination geometry and the synchronous rotation that fixes which hemisphere faces Earth (science.nasa.gov/moon/)
- U.S. Naval Observatory — lunar phase instants for 2026, against which this site's computed phases are checked (aa.usno.navy.mil)
- Meeus, Astronomical Algorithms, 2nd edition, chapter 47 — elongation as the difference in apparent ecliptic longitude, which is what waxing and waning formalize