Moon Phase Calculator

Check the Moon's phase and illumination for any date to plan whether a night favors deep-sky targets or the Moon and planets.

How it works

Pick any date and the calculator tells you how many days into the lunar cycle the Moon is, what phase that corresponds to, and roughly how much of its disc is lit. This matters for planning a session: a bright Moon washes out faint galaxies and nebulae, but it barely affects bright targets like Jupiter, Saturn, or double stars.

Worked example: enter 2024-04-23. The Moon was about 14.6 days into its cycle that date, landing squarely in the Full Moon window and close to 100% illuminated. That's a poor night for deep-sky targets and a great one for studying the Moon itself, since the higher Sun angle at full phase shows off the bright ray craters like Tycho and Copernicus. Enter 2024-04-08 instead and the calculator reports 29.1 days old, essentially a new moon, the best kind of night for chasing faint clusters and nebulae under a dark sky.

The math behind it is the mean synodic month, 29.53059 days, measured from a known reference new moon. That's an approximation of the real cycle, which speeds up and slows down slightly depending on the Moon's position in its orbit, so treat the result as accurate to about a day. It's plenty for deciding whether a weekend is worth setting up a telescope; it is not precise enough for predicting the exact minute of an eclipse.

FAQ

Why does the Moon matter for stargazing?

Moonlight scatters through the atmosphere and brightens the whole sky, the same way city light pollution does. Around full moon, faint fuzzy objects like most galaxies and nebulae disappear into the glow even from a dark site. Around new moon, with no moon in the sky at all, those same objects come back into view.

What counts as a "good" window around new moon?

Roughly three days on either side of new moon gives you dark evening skies with little or no moon interference, which is why the verdict above flags that range as best for deep-sky observing. Outside that window, check moonrise and moonset times too, since a thin crescent that sets early in the evening still leaves you a dark sky later on.

Can I still observe anything during a full moon?

Yes. The Moon itself is at its brightest and most detailed-looking near full phase (though oddly, the flat lighting actually hides crater relief better seen near the terminator), and the brightest planets and double stars barely notice the extra sky glow.

How accurate is this compared to an almanac?

Within about a day, which is fine for deciding "is this weekend dark or bright." A proper almanac or planetarium app accounts for the Moon's elliptical orbit and will be more precise if you need exact phase timing.

For more on planning around the sky, see planning a session around the Moon and weather, how the Moon's phases affect stargazing, and what to look for on the Moon itself.