Moon Phase Calculator

How to Use the Moon Phase Calculator

1. 1. Set the moment. Choose a date, a time and a time zone. The calculator opens on the current moment in your own zone; Set to now brings you back to it. Any year works, but dates within a few centuries of today are the most accurate.
2. 2. Choose your hemisphere. The phase is the same everywhere, but the picture is not: from the southern hemisphere the Moon looks turned upside down, so a waxing Moon is lit on the left. The disc follows your choice.
3. 3. Read the phase. You get the phase name, the percentage of the disc lit, the Moon's age in days since the last new moon, how far through the lunar cycle that is, its distance from Earth and its angle from the Sun.
4. 4. Plan ahead. The table lists the next four main phases with their dates and times in your zone, and the month calendar below shows the Moon at noon on every day, with the days of the new, quarter and full moons marked.

What a Moon Phase Is

The Moon is always half lit by the Sun. What changes is how much of that lit half faces Earth, and that depends on the angle between the Moon and the Sun in the sky, the elongation. At 0° the Moon is between us and the Sun and shows its dark side: new moon. At 90° half of the visible disc is lit: first quarter. At 180° the Earth is between them and the whole disc is lit: full moon. Then it shrinks back to last quarter at 270° and new moon again at 360°. The fraction lit is (1 + cos i) ÷ 2, where i is the phase angle at the Moon, which is almost exactly 180° minus the elongation.

The 6° window is about half a day of the Moon's average motion (12.2° a day), so a quarter or full moon keeps its name for roughly a day around the exact moment. Sources disagree on exactly where a crescent becomes a quarter, which is why the calendar also marks the day on which the exact phase falls.

Exact Phase Times and Your Time Zone

Phase times come from the series in chapter 49 of Jean Meeus's Astronomical Algorithms, a mean-phase formula plus 25 periodic corrections for the pull of the Sun and the Moon's elliptical orbit, converted to Universal Time with the Espenak and Meeus ΔT polynomial. Against the US Naval Observatory's published tables they agree to under a minute. The illuminated fraction comes from the full 60-term lunar longitude and distance series of chapter 47, which reproduces Meeus's own worked examples to the printed digits.

A phase is one instant, not a day, so the date depends on where you are. The full moon of October 2026 falls at 04:12 UTC on the 26th, which is the 26th in London and Tokyo but 9:12 PM on the 25th in Los Angeles. If the date matters, check the time zone before the day.

Worked Examples

On 16 October 2026 the Moon is 5.8 days past a new moon (10 October, 3:50 PM UTC) and the next four phases are the first quarter on 18 October, the full moon on 26 October, the last quarter on 1 November and the new moon on 9 November at 07:02 UTC. On 8 April 2024 at 18:00 UTC, about twenty minutes before the new moon that produced the total solar eclipse of that day, the illuminated fraction rounds to 0.0%.

Why a Lunar Month Isn't Exactly 29.5 Days

The average gap between new moons, the synodic month, is 29.530589 days. Any single month can be shorter or longer, because the Moon's orbit is an ellipse and the Sun's pull changes its speed: between the new moons of 2026 the gaps run from 29.28 to 29.74 days. That is why the Moon's age at full moon is not always 14.77 days, and why two full moons can fall in one calendar month: the second is called a blue moon, and 2026 has one on 31 May.

Accuracy and Limits

• Geocentric. Illumination is computed as seen from Earth's centre. Seen from the surface the Moon is shifted by up to about a degree, which changes the lit fraction by less than one percentage point, most near the quarters.
• ΔT. The offset between uniform and Earth-rotation time is predicted for the future. By the 2030s the uncertainty is a few seconds, and it grows slowly beyond that.
• Not a visibility forecast. Whether you can see a thin crescent depends on the sky, the horizon and the Moon's height at twilight, none of which are modelled.
• Eclipses. The calculator does not say whether a new or full moon is an eclipse. That happens only when the Moon is close to the plane of Earth's orbit at that moment.

Frequently Asked Questions

What is the moon phase today?

Open the calculator and it starts on the current date and time in your time zone, showing the phase name, the percentage illuminated and the Moon's age. If your device clock is wrong, set the date and time yourself, or press Set to now after correcting it.

How do you calculate the percentage of the Moon that is lit?

Compute the angle between the Moon and the Sun as seen from Earth, from their positions along the ecliptic, then take the phase angle i at the Moon and use (1 + cos i) ÷ 2. At 90° from the Sun that gives about 50%, at 180° about 100%. The calculator takes both positions from Meeus's series, including the Moon's distance and latitude.

How many days are there between full moons?

On average 29.53 days, but individual gaps usually run from about 29.3 to 29.8 days. In 2026 the full moons fall on 3 January, 1 February, 3 March, 2 April, 1 May, 31 May, 29 June, 29 July, 28 August, 26 September, 26 October, 24 November and 24 December, all in UTC.

What is the age of the Moon?

It is the time since the most recent new moon, in days. A new moon is age 0, first quarter is around 7.4, full moon around 14.8 and last quarter around 22.1, though each real value differs by up to about a day because the Moon's speed varies.

Why does the Moon look different in the southern hemisphere?

Standing on the other side of the equator, you see the Moon's disc turned half a turn compared with the northern view. A waxing Moon is lit on the right in the northern hemisphere and on the left in the southern. The phase and the percentage lit are identical everywhere.

Is this calculator accurate?

For the main phases it agrees with the US Naval Observatory to under a minute for the dates we checked, from 2000 to 2030. Illumination follows Meeus's full series and reproduces his published example to four digits. It does not model the Moon's visibility or libration.