Your daily cosmic companion

How the lunar cycle works

The moon takes about 29.5 days to go from one new moon to the next. That period is called a synodic month, and it is slightly longer than the 27.3 days the moon actually takes to orbit Earth — the gap exists because Earth is also moving around the sun, so the moon has to travel a little further each cycle to return to the same position relative to the two of them.

The moon does not produce light. What you see is sunlight reflecting off its surface, and the phase is simply how much of that lit half is facing you. At new moon the lit side points away from Earth and the moon is effectively invisible. At full moon Earth sits between the moon and the sun, so the entire lit face is turned towards us. The phases in between — crescent, quarter, gibbous — are the gradual transitions, and the percentage shown above is exactly how much of the disc is currently illuminated.

A useful thing to know: the moon is waxing when the lit portion is growing and the light appears on the right-hand side (in the northern hemisphere), and waning when it is shrinking and the light sits on the left. Southern hemisphere viewers see this reversed. If you are ever unsure which half of the cycle you are in, that is the quickest tell.

Why people track the phases

Long before it meant anything symbolic, the lunar cycle was a calendar. It is the most visible repeating unit of time available to the naked eye, which is why almost every early culture built a month around it, and why lunar and lunisolar calendars still set the dates of Ramadan, Passover, Diwali and Easter. Farming and fishing communities used it to time planting and tides.

The modern use is mostly different. Treating the cycle as a rhythm gives you a recurring, external prompt to check in with yourself — roughly every two weeks something changes in the sky, and that change is a reasonable excuse to sit down and take stock. New moons tend to get used for setting intentions, full moons for noticing what has come of them, and the waning half for letting things go. None of that is a claim about the moon causing anything. It is a scaffold for reflection, and a fortnightly one is easier to keep up than a daily one.

Whether the moon has measurable effects on human beings is a genuinely open question in a couple of narrow areas. The evidence on sleep is the most interesting of them, and we have written up what the studies actually found rather than what gets repeated about them.

What you can do here

The moon phase calculator works for any date, past or future, so you can look up the phase on the night you were born, on a wedding day, or on a date you are planning around. The sky page goes wider, showing where the planets currently sit and which aspects they are forming. The days alive counter converts a birthdate into a total number of days, breaks it down into years, months and days, and tells you which round-numbered milestone you are approaching next. The affirmations page holds the full set by category rather than just today's.

If you would rather read than click, the articles section covers the phases individually, the difference between a moon sign and a star sign, how to run a full moon ritual without buying anything, and what moon water is supposed to be.