Moon Phase Calculator

Created by Kenneth Alambra
Reviewed by Wojciech Sas, PhD candidate and Jack Bowater
Last updated: Mar 10, 2022

This Moon phase calculator will help you determine the approximate Moon phase on any date for almost any nearby year. If you've ever wonder what the Moon phase is on a particular date, then you're in the right place.

In this calculator, you will learn the different factors that explain why we have Moon phases. You will also learn how to calculate the Moon phase for any date that you want, as well as find when the next full moon is. Keep on reading to start learning.

Why do we have Moon phases?

An image of the waxing gibbous moon.

The Moon is the only natural satellite the planet Earth has, and it has been orbiting the earth for billions of years already. As we look up at the sky and search for the Moon, we see that the Moon seemingly changes its shape every day.

This change in the Moon's shape is because our Moon is a spherical object illuminated by the Sun. Because of the geometry between the Sun, the Earth, and the Moon, we see the Moon from different angles throughout the year. Sometimes, the illuminated side of the Moon is fully round, and sometimes it's completely dark. This geometry is why we have Moon phases.

The Earth moves through the Solar System at a consistent pace, completing a revolution around the Sun once every 365.25 days and rotating once on its own axis every 24 Earth hours. On the other hand, the Moon completes a full revolution around the Earth, relative to a singular point on Earth, approximately every 27.32 days, known as a sidereal month.

However, since the Earth is also moving, the faces of the Moon don't repeat every sidereal month. If we were to draw a line between the Earth and the Sun, and time the duration the Moon takes to complete this cycle, we'll see that it takes roughly 29 days 12 hours 44 minutes 3 seconds or 29.53058770576 days. This is when the Moon is in the same position relative to the Sun, and so has the same lunar phase. This measure is of the Moon's orbit is known as a synodic month, and we'll be using this for the Moon phase calculation.

How to use our Moon phase calculator?

Though it's nice to know how to calculate the Moon phase on your own, what if you need to check multiple dates quickly? This is where our Moon phase calculator comes in handy! As our Moon phase calculator is very straightforward to use, you simply have to select the desired date from the calendar or input the date you want (in the format shown in the calculator field). You will instantly see the results below the date field 🌝.

You can also use our calculator to find out when the next new or full moon is from your selected date. This is great if you plan to stargaze or watch an upcoming meteor shower and want to know if there will be any moonshine obstructing the view of the stars or meteors. This feature can also help you properly time a telescope viewing of a full moon.

As an added feature, you can also see the calculated number of new moons elapsed since 6th Jan 2000, as well as how many lunar days into the cycle your date is (starting from each new moon). Simply go into the Advanced mode of our Moon phase calculator to see!

Now, how about we learn how to calculate the Moon phase ourselves πŸ™‚?

How to calculate the Moon phase?

Because we can track the almost constant movement of the Sun, Earth, and the Moon, we are able to approximate how many days the Moon is along its cycle. However, we need some starting point, both physical and temporal, to start counting from. Astronomers have decided to use the new moon as this point, and we can always calculate the phase of the Moon as long as we know when the new moon was during that lunar day.

However, not all new moons are created equal. For example, the Moon was new on 13th Jan 2021, at 1:02 PM, and we can use that date. However, on January 6, 2000 (at around 12:24 PM), the new moon started closer to 12 noon. We want to use it for more precise calculations.

Let's say we want to know the Moon phase at a particular date. We count how many days have already passed after January 6, 2000, up to that date. Dividing that number of days by synodic month, we determine how many new moons had already occurred, as shown in this equation:

new moons elapsed = days from a known date with new moon / 29.53058770576

By performing a modulo operation to obtain only the remainder of this division, we obtain the fraction of the synodic month that has already elapsed. We then multiply this fraction by the length of the synodic month in mean solar days to evaluate how many lunar days have already passed since the last new moon before our desired date. If our desired date turns out to have a new moon, then we would get less than 1 for this operation. In equation form, we have it as:

lunar day = ((days from a known date with new moon) mod 29.53058770576) * 29.53058770576

We can now use our calculations above to find the Moon phase on our desired date using the table below:

Moon phase
Lunar day
β €πŸŒ‘ New Moon
0 < lunar day <= 1
β €πŸŒ’ Waxing Crescent
1 < lunar day <= 6.382647
β €πŸŒ“ First Quarter
6.382647 < lunar day <= 8.382647
β €πŸŒ” Waxing Gibbous
8.382647 < lunar day <= 13.765294
β €πŸŒ• Full Moon
13.765294 < lunar day <= 15.765294
β €πŸŒ– Waning Gibbous
15.765294 < lunar day <= 21.147941
β €πŸŒ— Last Quarter
21.147941 < lunar day <= 23.147941
β €πŸŒ˜ Waning Crescent
23.147941 < lunar day <= 28.530588
β €πŸŒ‘ New Moon
28.530588 < lunar day <= 29.530588

Sample Moon phase calculation

Let's say we want to know the Moon phase on Vincent Van Gogh's birthday (March 30) in 2033. To quickly know how many days have passed since January 6, 2000, we can use our days between dates calculator. We can see that there will be 12,137 days between the two dates. Using our first formula, we can calculate how many new moons will occur between those dates:

new moons elapsed = days from a known date with new moon / 29.53058770576

new moons elapsed = 12,137 / 29.53058770576

new moons elapsed = 410.9957718782 new moons

That means that there will be 410 new moons and approximately 99.5771 % of a 29.530588-day lunar cycle between those two dates. Multiplying 29.53058770576 days by 0.997771, we get approximately 29.46 days after the last new moon. Checking our table for this value, we see that it falls under a lunar day with a new moon as its phase πŸŒ‘. And when is the next full moon after that? Using our Moon phase calculator, you can find that it'll be on 13th April, 2033.

FAQ

What are the different Moon phases?

The different Moon phases in increasing amount of illumination are

  • New moon;
  • Waxing crescent;
  • First quarter;
  • Waxing gibbous;
  • Full moon;
  • Waning gibbous;
  • Last quarter; and
  • Waning crescent.

We use the term "waxing" to denote an increase in the visible illuminated side of the Moon and "waning" when we start to see the unilluminated side of the Moon.

What causes the Moon phases?

The positioning of the Moon with respect to the Earth and the Sun determines the Moon phase we see. Another factor that affects the Moon phase is the fact that the Moon is spherical in shape. As the Moon orbits the earth, we see differing portions of the illuminated and unilluminated side of the Moon in a cycle that completes every 29.53 earth days.

How long does a Moon cycle take?

A Moon cycle takes around 29 days, 12 hours 44 minutes 3 seconds or 29.53058770576 days. This duration, also called the synodic month, and is the duration between two consecutive new moons. This is different from the sidereal month, which is the duration it takes the Moon to orbit the earth, which is around 27.321661 days.

What phase of the Moon is a solar eclipse?

New moon. Solar eclipses only happen when the Moon gets in between the Sun and the Earth. During that time, the Moon's illuminated side is facing the sun, and therefore viewers from the Earth can only see the unilluminated side of the Moon. When the Moon's shadow is cast across the Earth, that's when a solar eclipse occurs. During such time, we can also see the Moon cover the Sun either partially or totally.

Kenneth Alambra
Use this calculator to find the Moon phase on any day of any year. You'll also see what the approximate Moon phase on that day.
Date
The Moon phase on the selected date
will be a waxing gibbous.
πŸŒ”
I want to know the next...
new moon
Next new moon πŸŒ‘ from the selected date: 24 Oct 2022
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