The moon completes an orbit in 27.3 days and takes 29.5 days to get back to the same phase. Both numbers are correct and they measure different things: one is the orbit against the stars, the other is the cycle we actually see.
The gap exists because the Earth is moving too. By the time the moon has gone once round, the Earth has travelled a month’s worth along its own orbit, so the moon needs another couple of days to line up with the sun again.
What are the two months?
The orbital one and the phase one, and only the second matters for a calendar.
| Period | Length | Measured against |
|---|---|---|
| Sidereal month | 27.3 days | the fixed stars |
| Synodic month | 29.5 days | the sun, so the phases |
The synodic figure is what every lunar calendar is built on, and it is why lunar months alternate between 29 and 30 days — no whole number of days fits, so they take turns.
It is also why a purely lunar year of twelve months runs about eleven days short of a solar year, which is why some calendars drift through the seasons and others insert an extra month to stop it.
Does everyone see the same phase?
Yes. The phase depends on the angle between the sun, the moon and the Earth, so at any given instant it is the same everywhere on the planet.
What differs is the orientation. A crescent that opens to the left in the northern hemisphere opens to the right in the southern, because observers there are effectively upside down relative to each other — and near the equator it can appear to sit like a bowl.
The local date can differ too, since a phase occurring at a particular instant falls on different calendar days either side of the date line. That is a calendar effect rather than an astronomical one.
Why is the same side always facing us?
Because the moon rotates once per orbit, an arrangement produced by tides over a very long time. The result is that one hemisphere permanently faces the Earth.
It is not quite exact. The moon’s orbit is elliptical and tilted, so it appears to nod and rock slightly over a cycle — and that wobble reveals a little more than half of the surface to a patient observer over time.
The far side is not the dark side. Every part of the moon gets sunlight over the course of a lunar month; the far side is simply the part we never see from here.
What makes a supermoon?
Proximity. The orbit is elliptical, so the moon is measurably closer at one end than the other, and a full moon coinciding with the near point appears somewhat larger and brighter than one at the far point.
The difference is real and modest — around a seventh larger in diameter — and it is nearly impossible to notice without a side-by-side comparison, because there is nothing in the sky to compare against.
The moon looking enormous near the horizon is a separate thing entirely, and it is an illusion. Photographs taken at the horizon and overhead show the same size, which is the easiest way to demonstrate it.
What is a blue moon?
Two different things, and the newer meaning came from a misreading. The older definition is the third full moon in a season containing four; the modern popular one is the second full moon in a calendar month.
The second definition spread from a magazine article in the 1940s that misinterpreted the first, and it has stuck because it is easier to explain. Neither has anything to do with colour.
Both are consequences of the 29.5-day cycle not dividing evenly into months or seasons. A month of 31 days can just fit two full moons, which happens every couple of years.
Why are there not eclipses every month?
Because the moon’s orbit is tilted by about five degrees against the plane the Earth orbits in, so at most new and full moons it passes above or below the line rather than through it.
Eclipses happen when a new or full moon coincides with the moon crossing that plane, which occurs in seasons a couple of times a year rather than monthly. That is the whole reason they are events instead of routine.
The tilt is also why the moon’s path across the sky shifts through the year rather than repeating exactly, and why a full moon rides high in winter and low in summer.
Questions people ask
Why does the moon rise later each day? By about fifty minutes, because it has moved along its orbit while the Earth turned.
Does the moon affect sleep or behaviour? The evidence is weak and inconsistent, despite the persistence of the belief.
Why can I see the moon during the day? Because it is above the horizon and bright enough. Only near the new moon is it too close to the sun to see.
Do the phases affect tides? Yes. The largest tides come when the sun and moon pull in line, at new and full moon.
Is the phase the same on my birthday each year? No. The lunar cycle and the calendar year do not divide evenly, so it drifts.
Work from the synodic cycle. The moon phase calculator gives the phase for any date, and the zodiac sign calculator and birthstone by month cover the other conventions tied to a date rather than to the sky.