The Moon’s Orbit, Distance, and Day Length

Earth’s only natural satellite, the Moon, orbits Earth at an average distance of 1.28 light-seconds

It also shows only one side to Earth at all times, a trait called tidal locking. This occurs because the Moon has the same rotational and orbital periods

Earth and the Moon captured by the Galileo spacecraft
Earth and the Moon illuminated by the Sun (Source)

How does a day look like on the tidally locked Moon? How far or near does it get to Earth, and what happens then? 

Let’s find out.

How Many Times Does the Moon Orbit Earth in a Year?

Quick Answer: The Moon orbits the Earth just over 13 times per year.

The word month is derived from the time the Moon takes to complete one set of its phases from new moon, to full, and back to new again. 

This time also reflects the Moon’s complete orbit around Earth, which is exactly 27 days 7 hours, and 43 minutes. At this rate (365/27.3), the moon makes a little over 13.3 orbits around Earth in 365 days.

However, the moon actually takes 29.5 days to come back to the same point as a new moon. This means that from our perspective, the Moon makes 12.4 circuits around Earth in a year. 

The difference of 2.2 days between the two measures is because there are two ways of measuring the Moon’s orbit around Earth.

What’s the Difference Between Sidereal and Synodic Months?

Quick Answer: The sidereal month, calculated relative to stars, is 27.3 days. The synodic month, calculated with respect to Earth, is 29.5 days.

To make imagining this simpler, let’s begin with a New Moon.

The Moon completes a full circle around Earth – one complete orbit, or a sidereal month – in 27.3 days. However, the Moon has not returned to its new phase at the end of the sidereal month.

During the time it takes the Moon to complete its orbit of Earth, the Earth has moved further in its orbit around the Sun. This means that when the Moon returns to its starting position, the Sun is shining on it from a different angle, so it is not a new moon after 27.5 days.

To return to its new moon phase, the Moon has to travel an extra 2.2 days. When it does this, it’ll be at the same angle to the Sun as it was at the start of the journey, i.e. a new moon.

This is called the synodic month because the Moon is trying to be in sync with Earth. 

So from our perspective, the time between two new moons is on average 29.5 days, which is just over four weeks… or a month! The synodic month varies slightly over the course of a year due to orbital speed changes along the orbital path. At its shortest, the synodic month (new moon to new moon) lasts 29.2 days, at its longest the period is 29.9 days.

How Long is a Day on the Moon?

Quick Answer: A day on the moon is 29.5 Earth days. 

The length of a day is the period the Sun takes to reach a certain position in the sky and return to that exact same position. 

But because we have both synodic and sidereal months, we will have two answers. 

The Moon takes 27.3 days to complete a rotation on its axis, which is its sidereal day. 

However, if you were to stand on the Moon’s surface, you’d notice that an extra 2.2 Earth days are needed for the Sun to catch up to the same position in the lunar sky.

Since the length of a day is the time the Sun takes to reach the same position in the sky, the synodic day is 29.5 days.

How Far Away Does the Moon Get?

Quick Answer: At its furthest, the Moon is 252,088 miles or 405,696 km away from Earth.

Our satellite orbits Earth in an elliptical orbit with an eccentricity of 0.0549 (it is not perfectly circular), which causes it to move farther and closer to Earth. 

At its closest, called perigee, the Moon is only 225,623 miles (363,104 km) from Earth. When the Moon is closest, it appears 10% bigger and 30% brighter when compared to its apogee. 

The Moon’s distance varies by 13% over its orbit and, at its furthest (apogee), the Moon is 252,088 miles (405,696 km) from Earth.

The average lunar distance from Earth is 238,855 miles (384,400 km), which is about 30 Earth diameters.

The changing distance of the Moon leads to phenomena called micromoons and supermoons.

What is a Micromoon?

Quick Answer: A micromoon is a new or full moon that occurs when the moon is at its apogee.

A new moon or a full moon can happen at any point in the Moon’s orbit. But when it occurs when the moon is at its apogee (the furthest from Earth), it is called a micromoon.

There is no scientific basis for a micromoon but the threshold is generally accepted to be when the distance between the Moon’s and Earth’s centers is greater than 251,655 miles (405,000 km).

The table below shows micromoon dates for 2021.

DateType of micromoon
May 11, 2021New micromoon
November 19, 2021Full micromoon
December 18, 2021Full micromoon

What is a Supermoon?

Quick Answer: A supermoon is a full moon that occurs when the moon is at its perigee.

A supermoon occurs when the full moon coincides with the Moon’s closest approach to Earth (perigee).

A full moon occurs once every 29.5 days, but not every full moon is a supermoon.

Again, it is unscientific but it is accepted that the centers of the Moon and Earth need to be closer than 223,694 miles (360,000 kilometers) for the event to be a supermoon. This happens around 3 or 4 times every year.

Upcoming supermoons: 

  • April 27, 2021
  • May 26, 2021
  • June 14, 2022
  • July 15, 2022

A supermoon is 30% brighter and 7% bigger than a micromoon.

Comparison of micromoon and supermoon
Micromoon and supermoon (Source)

How Long Does It Take to Travel to the Moon?

Quick Answer: Three astronauts traveled in the fastest spacecraft launched to orbit the Moon, Apollo 8, which took 2 days 20 hours to get there. Light, on the other hand, travels to the Moon from Earth in just 1.28 seconds

To answer the question in more detail, we’ll first look at the time light takes to reach the moon from Earth. Next, we’ll board an imaginary spaceship and see how long it’ll take us to travel to our neighbor.

How Long Does Light Take to Travel from the Moon to Earth?

Light travels the fastest of all known objects because it has no mass. Its speed is astounding at 186,282 miles per second.

At this rate, light takes 499 seconds to reach Earth from the Sun. This distance is called 1 Astronomical Unit.

Let’s take a look at the shortest, longest, and the average time for light to travel between the Earth and the moon. 

Shortest = 225,623 miles (363,104 km) = 0.0024 AU = 0.0024*499 = 1.1976 seconds.

Average = 238,855 miles (384,400 km) = 0.0025 AU = 0.0025*499 = 1.2475 seconds.

Furthest = 252,088 miles (405,696 km) = 0.0027 AU = 0.0027*499 = 1.3473 seconds.

At its average distance of 238,855 miles (384,400 km), light takes 1.25 seconds to reach the Moon from Earth. As the moon has an elliptical orbit, this time changes between 1.20 seconds to 1.35 seconds.

This also means that when you look at the moon, you are looking at it as it looked at least 1.20 seconds ago!

How Long Does it Take a Spaceship to Travel to the Moon? 

There have been over 40 missions to the moon since the 1950s. The fastest spacecraft took 2 days 20 hours to get there.

However, a variety of factors are at play when traveling to the Moon, which means there have been journeys that took four or five days to complete.

Summary

Interestingly, there are two answers to how long a day is on the Moon, and how long it takes to orbit Earth. 

Moon, in its elliptical orbit, moves farther away and closer to Earth and in turn, becomes larger and smaller in our skies, leading to micromoons and supermoons. 

The Moon is in fact 1.28 light-seconds away but the fastest spacecraft took almost three days to reach it.

Improve your moon observing skills, with our Guide to The Lunar 100

Next time you look up at the Moon, guess if you’re seeing a micromoon, a supermoon, or just the regular ol’ Moon.