BYTETOOLS

Moon Phase Calculator

Find the moon phase for any date with illumination percentage, moon age in days, a drawn moon disc and the dates of the next new and full moons.

What is the Moon Phase Calculator?

The ByteTools Moon Phase Calculator tells you what the moon looked like — or will look like — on any date.

  • Phase name, illumination percent and moon age in days
  • Canvas-drawn moon disc showing the lit limb
  • Next new moon, first quarter, full moon and last quarter with times
  • Meeus periodic terms, not a flat 29.53-day average
  • Works for any date, past or future
  • Runs entirely offline in your browser

How to use the Moon Phase Calculator

  1. 1

    Pick the date you want to check, or press Today.

  2. 2

    Read the phase name, illumination percentage and moon age in days.

  3. 3

    Look at the drawn disc to see the shape and which limb is lit.

  4. 4

    Scroll down for the dates and times of the next four principal phases.

  5. 5

    Change the date to step through a month and watch the phase progress.

About the Moon Phase Calculator

The ByteTools Moon Phase Calculator tells you what the moon looked like — or will look like — on any date. It reports the phase name, the illuminated percentage of the disc, the moon's age in days since the last new moon, and draws the lit portion so you can see the shape at a glance.

It also lists the next new moon, first quarter, full moon and last quarter, computed from the periodic terms in Jean Meeus's Astronomical Algorithms rather than a flat 29.5-day average. That matters because real lunations vary by up to half a day either side of the mean, so simple average-based calculators drift.

Use it for photography planning, fishing and gardening calendars, night-sky sessions, or just to settle an argument about whether last Tuesday was a full moon. Everything is computed in your browser from the date you enter — no lookup table is downloaded and nothing is uploaded.

Frequently asked questions

How is the moon phase calculated?

The tool finds the exact moment of the new moon before your date using the periodic series from Meeus's Astronomical Algorithms, measures how far through that lunation your date sits, and computes the illuminated fraction from the moon's phase angle. Phase times land within a few minutes of published almanac values.

What is the moon's age in days?

Moon age is the time elapsed since the last new moon. A new moon is age 0, first quarter is about 7.4 days, full moon about 14.8 days and last quarter about 22 days, with the next new moon arriving at roughly 29.5 days.

Is the moon phase the same everywhere in the world?

The illuminated fraction is essentially identical worldwide at any given instant, but the orientation is not. Northern hemisphere observers see a waxing crescent lit on the right, southern hemisphere observers see it lit on the left, and near the equator it can look like a bowl.

Why does the calendar date of a full moon differ between sources?

Phase moments are single instants. A full moon at 01:30 UTC falls on the previous evening in the Americas, so calendars in different time zones legitimately print different dates for the same event. The times shown here are in your own local zone.

What is a blue moon?

The common modern definition is the second full moon in a single calendar month, which happens roughly every two to three years because the 29.5-day lunation is shorter than most months. The older almanac definition is the third full moon in a season that has four.

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