The average distance between the Earth and the Moon is 384 400 km (238 855 miles), however the answer to this question is not as straightforward as you might think as it depends on when you ask the question. The exact distance to the Moon is determined by the shape of its orbit and precisely where it is in its orbit.
An elliptical orbit
No orbit is perfectly circular. Some are very close, but they are all at least slightly elliptical in shape. Astronomers can measure how near to a perfect circle an orbit is by calculating its eccentricity.
This is expressed by a number that is between 0 and 1. The closer the eccentricity is to 0, the closer the orbit is to a circle. In fact, a circle can be thought of as a special kind of ellipse that has an eccentricity of 0.
Venus’ orbit is the least eccentric out of all of the planets in our Solar System and closest to a circle, with a value of 0.007 and Mercury’s is the most eccentric with a value of 0.2.
The eccentricity of the Moon’s orbit is 0.05. Furthermore, the Earth is not at the very centre of the Moon’s orbit either. It is located at one of the foci of the Moon’s elliptical orbit, so is closer to one edge of the orbit than the other.
Apogee, perigee and average distance
Astronomers tend to talk about three different numbers when talking about the Earth-Moon distance.
At its furthest point from the Earth, the Moon is about 405 696 km (252 088 miles) away and astronomers say that the Moon is at apogee (‘apo’ means ‘away’). On the other hand, when the Moon is at perigee (‘peri’ means ‘near’), the Moon is at its closest approach to the Earth. The distance between them is only 363 104 km (225 623 miles).
These two figures differ by 42 592 km (26 465 miles) - this is more than three times larger than the diameter of the Earth! The average distance between the Earth and the Moon is 384 400 km (238 855 miles).
The Moon’s elliptical orbit with the distances at apogee and perigee. Note that the Earth is not at the very centre of the orbit and that the eccentricity of the orbit has been exaggerated here! Credit: NASA/Luc Viator/Affelia Wibisono
The supermoon and micromoon
But do the two distances affect us in any way? Not really. The full moon will look slightly bigger if it occurs during perigee (sometimes called a supermoon) and slightly smaller at apogee (a micromoon). However, the difference won’t be easily noticeable to the naked eye and a side by side photographic comparison is the only way to really see it.
Comparison between a micromoon and a supermoon. Supermoons appear to be 14% larger and 30% brighter than a micromoon. Credit: Marcoaliaslama
Our tides are caused by the gravitational pull of the Moon and the Sun and the Earth’s rotation. High tides are highest and low tides are lowest when it is a full moon or a new moon because of the gravitational pull of our natural satellite and star add up. These tides are called spring tides, as the tides spring out and spring back, they do not have anything to do with the spring season. At perigee, the Moon’s gravitational pull will be slightly stronger than usual and so the difference between high and low tides would be larger – but only by about 5cm! Similarly, when the Moon is at apogee, the difference between high and low tides will only be about 5cm smaller than usual.
The biggest tides occur when the Sun, Moon and Earth align because the gravitational pull of the Sun and Moon combine. Neap tides are smaller and occur when the Sun and Moon are perpendicular to each other. Credit: NASA/Luc Viator/HalloweenNight/Affelia Wibisono
How long does it take to travel to the Moon?
On average it would take 3 days, but it depends on how fast you are travelling and the exact route you take. It also depends on what you want to do – passing by the Moon does not require you to slow down to enter its orbit, minimising the time it would take to get there.
8 hours and 35 minutes
The record for the shortest journey time to the Moon is currently held by the New Horizons spacecraft with a time of 8 hours and 35 minutes!
1 day 10 hours
The first spacecraft to attempt to reach the Moon was Luna 1 in 1959. Unfortunately, it did not slow down enough to complete its mission, but it did reach the vicinity of the Moon within 34 hours (1 day 10 hours).
SMART 1, a European Space Agency spacecraft powered by an ion engine was launched in 2003. It was very fuel efficient but did take 13.5 months to complete its journey!
The Apollo missions
Human space travel will usually take longer than robotic ones. On average, the nine crewed missions to the Moon, (including Apollo 8, Apollo 10, Apollo 13 and the six that landed on the surface) took just over 78 hours (3 days 6 hours) to enter lunar orbit. The quickest was Apollo 8 which took 2 days, 21 hours and 8 mins, while Apollo 17 took the longest with a time of 3 days, 14 hours and 41 mins (times include the time spent in Earth orbit).
Is the Moon moving away from the Earth?
Astronomers have discovered that the Moon is currently moving away from the Earth by 3.8 cm every year!
Astronauts from the Apollo 11, 14 and 15 missions and the two Soviet Union rovers, Lunokhod 1 and Lunokhod 2, left a total of five mirrors on the surface of the Moon. Astronomers on Earth can reflect laser beams off these mirrors and record the time it takes for the laser to return. We know how fast the laser beam is travelling (the speed of light) so we can easily calculate the distance that the laser beam has travelled. The Earth-Moon distance would then be half of this value.
The retroreflector mirror left by Neil Armstrong and Buzz Aldrin as part of the Lunar Laser Ranging Experiment. Credit: NASA
This would mean that in the far future, total solar eclipses would be a thing of the past as the Moon will appear to be smaller, therefore, its disc will not be large enough to completely obscure the Sun. It will eventually stop receding from the Earth in about 50 billion years’ time according to theory. However, the Sun would have already entered the next stage of its life long before that happens. As it expands into a red giant star in 5 billion years or so, it will push the Moon back towards the Earth causing it to disintegrate due to strong tidal forces.
This article has been written by an astronomer at the Royal Observatory, Greenwich
01/06/2018: Affelia Wibisono