How Long Is a Lunar Cycle?

As the Moon orbits our planet time and again, it passes through various lunar cycles. A lunar cycle, however, isn’t quite as clearly defined as it seems. In this article, we take a look at how the Moon travels around the Earth and just how long a lunar cycle might be.


The Moon and the Earth

The relationship between the Earth and the Moon is rather interesting, considering we always see the same face of our closest neighbor. Surprisingly, this wasn’t always the case.

When the Earth and the Moon first formed, the Moon was much closer and had its own rotation. The Moon quickly moved away from the Earth, settling in some 238,900 miles (384,500 kilometers) away.

However, the Earth’s pull on the Moon is so strong that it actually caused the Moon’s rotation to slow over time. Eventually, the Earth forced the Moon to stop spinning entirely, leaving us seeing the “Man on the Moon” every night.

Is the Moon Still Drifting Away?

The Earth and the Moon
NASA camera shows moon crossing face of earth. (Image credit: NASA)

The Moon is actually still moving away from the Earth, even now. In one calendar year, the Moon moves right around 1.5 inches (3.8 centimeters) further away from Earth. This minuscule difference isn’t even significant enough to affect the time it takes the Moon to make a trip around the Earth.

By all estimates, the Earth will never lose its Moon. If the Solar System survives long enough, the Moon will finally stop moving away from Earth in about 50 billion years.

The Lunar Cycle

The Lunar Cycle
The Lunar Phases. (Image credit: Thomas Splettstoesser on Wikimedia Commons CC BY-SA 3.0)

When we talk about a lunar cycle, there are actually a few different things we could be referring to. Most likely, a lunar cycle points to the length of a day or a year on the Moon since these events take roughly the same amount of time. Even so, what exactly constitutes a lunar day or year needs some further explanation.

Depending on exactly what you’re after, you could either be talking about a sidereal month or a synodic month. However, we need to consider whether we’re talking about a day in relation to the Earth or in relation to the Sun.

Sidereal Month

We mentioned earlier that the Moon rotates at the exact same rate that it revolves.

A sidereal month defines how long it takes the Moon to orbit the Earth one time without regard for any other celestial object. It reflects precisely the amount of time it takes the Moon to travel in a 360° circle around the Earth and reach the location it started.

It takes the Moon just shy of 27.5 Earth days to complete this journey. Since the Moon appears to revisit the same constellations once every 27.5 Earth days, many ancient Asian cultures used the sidereal month to define their months.

Sidereal and Synodic Month
Lunar Sidereal and Synodic Periods.

Synodic Month

The synodic month represents the Moon as it travels through each of its eight phases. For example, this would be the time it takes the Moon to go from a full Moon to the next full Moon. This process, in its entirety, takes an average of 29.5 Earth days.

The amount of time it takes the Moon to complete a synodic month and a sidereal month are clearly different. This is because the phases of the Moon are directly related not only to the Earth’s position but also to the position of the Sun.

As the Moon moves around the Earth, the Earth moves through space at an incredible 67,000 mph (107,000 km/h) as we travel around the Sun. So, when the Moon completes its sidereal orbit in 27.5 Earth days, the Earth and the Moon are now in a new position relative to the Sun.

As a result, the Moon has to travel an additional 30° around the Earth to complete its synodic month. This puts the Moon once again in the same location relative to the Sun. This extra 30° journey accounts for the fact that a synodic month is two days longer than a sidereal one.

Thinking about this in a different way, it takes 29.5 Earth days for the Sun to set, rise, and appear in the same spot in the lunar sky.

The Metonic Cycle

The lunar cycle can also be a reference to the Metonic cycle. The Metonic cycle is a period of 19 Earth years. At the end of this 19-year cycle, the phases of the Moon return to the same (or approximately the same) dates on our calendar year.

For instance, take a full Moon sitting between the horns of the constellation Taurus. The following year, on that same night, the Moon will not only be in a different phase, but it will also not be in exactly that same location.

However, if you look up at the Moon 19 years in the future to the day of your original observation, you will find the Moon once again full and nestled between Taurus’s horns.
A detailed explanation of the Metonic Cycle.

Once in a Blue Moon

Synodic months, linked to the phases of the Moon, occur once every 29.5 Earth Days. With February as a notable exception, this means that the Moon passes through an entire synodic cycle within one month’s time.

Although rare, there are months where it’s possible to see two full Moons within the confines of that month. This occurrence is given the name “Blue Moon,” even though the Moon will not be a blue color.

Blue Moons appear about once every two and a half years, for a total of 7 times during a Metonic cycle. The phrase “once in a blue Moon” comes from the rarity of this event.


People have long researched the Earth’s relationship to the Moon and how our satellite travels through space. Many ancient cultures have used the Moon as a way of keeping track of time for festivals and harvests. Even today, we mark the phases of the Moon on our calendars at home. Needless to say, the sky would not be the same without the Moon watching over us.

About Noah Zelvis

Noah is a content writer who has had a love of all things astronomy for as long as he can remember.
When not reaching for the stars, you’ll likely find Noah traveling or running.