The Moon is a little over a quarter as wide as Earth. Its mean diameter is about 3,474 km (2,159 mi), against Earth's 12,742 km (7,918 mi), or 27 percent. Its surface area is about 37.9 million km², roughly 7.4 percent of Earth's. It has about 2 percent of Earth's volume, so around 49 Moons would fit inside our planet, and only 1.2 percent of its mass, about 1/81. Measured face to face, the Moon is about as wide as the contiguous United States. It is the fifth-largest moon in the Solar System and the largest of all compared with the planet it orbits.
Pole to Pole: 3,474 Kilometres Against Earth’s 12,742
Like Earth, the Moon is not a perfect sphere, though it comes close. Its equatorial radius is 1,738.1 km and its polar radius 1,736.0 km, a flattening of only 0.0012. Earth's two radii, 6,378.1 km and 6,356.8 km, differ by about 21 km, ten times the Moon's 2.1 km. Set the two side by side and about 3.7 Moons would span Earth's diameter. That ratio barely changes whichever direction you measure: the Moon's equatorial radius is 0.2725 of Earth's and its polar radius 0.2731. On a map of Earth, the Moon's disc would cover about the width of mainland Australia, of Europe, or of the contiguous United States from coast to coast. Each of those is a long journey by road, but each is also only a small patch of Earth's globe.
The gap between them is much larger than either world. The Moon orbits at an average of 384,399 km, ranging from about 356,400 km at its closest to about 406,700 km at its farthest over its elliptical path. That is roughly 30 Earth diameters of empty space. Most textbook drawings put the two bodies almost touching. A true-scale picture with Earth the size of a basketball would put a tennis-ball-sized Moon about 7 m away.
West to East Across the Near Side: Oceanus Procellarum to the Sea of Tranquillity
Lunar maps use latitude and longitude just as Earth's do. Because the Moon's equatorial circumference is 10,921 km, each degree of latitude on the Moon is about 30 km long, compared with about 111 km on Earth. A traveller crossing the Earth-facing hemisphere from its western edge to its eastern edge would cover roughly 180 degrees of longitude, or about 5,460 km along the equator.
Starting in the west, that traveller would cross Oceanus Procellarum, the Ocean of Storms. It is the largest lunar plain, stretching more than 2,500 km from north to south and covering about 4 million km², or 10.5 percent of the whole lunar surface. That is more than half the area of Australia. Its dark basalt formed in volcanic floods, some perhaps as young as one billion years. In its eastern part sits Copernicus, a crater about 93 km wide and 3.8 km deep, with a terraced inner wall and a central mountain.
North of Copernicus lies Mare Imbrium, the Sea of Showers, about 1,145 km across and second only to Procellarum among the maria. It formed when a protoplanet struck the Moon about 3.92 billion years ago. In November 1970 the Soviet Luna 17 lander set down in western Mare Imbrium and released Lunokhod 1, the first rover to drive on another world. Further east, past Mare Serenitatis, the route reaches Mare Tranquillitatis, the Sea of Tranquillity. That is where Apollo 11 landed Neil Armstrong and Buzz Aldrin in July 1969. Altogether, the dark maria cover only about 16 percent of the Moon, mostly on this Earth-facing side.
North to South: From Mare Frigoris to Shackleton Crater
A journey from pole to pole along a line of longitude is also about 5,460 km, half the circumference. In the far north lies Mare Frigoris, the Sea of Cold, a long, narrow band of dark lava that runs from north of Mare Imbrium east to north of Mare Serenitatis, just beyond the dark-floored crater Plato. Past it, the ground climbs into the cratered highlands around the north pole. Close to the pole, NASA's Lunar Reconnaissance Orbiter measured 26 K (−247 °C) inside the crater Hermite, the coldest temperature any spacecraft has recorded in the Solar System.
Heading south across the equator and the bright southern highlands, the route passes Tycho. It is a crater about 85 km wide and 4.7 km deep, only about 108 million years old, and its pale rays of ejecta reach out as far as 1,500 km, bright enough to stand out at full Moon. Further south, at about 69°S, India's Chandrayaan-3 lander touched down on 23 August 2023, the first soft landing near the lunar south pole.
The trip ends at Shackleton, a crater 21 km wide and 4.2 km deep whose rim the south pole itself crosses. Peaks on that rim get almost constant sunlight. The crater floor never sees the Sun at all, and it may trap water ice. Here the Moon's small size shows: from equator to pole is only about 2,730 km, a quarter of Earth's 10,000 km.
The Far Side and the South Pole–Aitken Basin
Because the Moon is tidally locked, the same hemisphere always faces Earth. Small wobbles called librations let observers see about 59 percent of the surface over time. The rest, the far side, is a different landscape: heavily cratered, with few maria, and on average about 1.9 km higher than the near side.
Its main feature is the South Pole–Aitken basin, the largest, oldest and deepest impact basin known on the Moon. Estimates of its diameter range from about 2,240 km to about 2,500 km, between two-thirds and three-quarters of the Moon's own diameter. Zircons thought to come from the basin suggest it formed about 4.34 billion years ago. Inside it is the lowest point on the Moon, about 9.18 km below the reference level in a crater within the crater Antoniadi. Just to the northeast is the highest, the Selenean summit, at about 10.63 km. That gives a total relief of about 19.8 km, almost exactly the drop from the summit of Everest (8,849 m) to the bottom of the Challenger Deep (about 10,935 m), on a body a quarter of Earth's width. On 3 January 2019, China's Chang'e 4 made the first soft landing on the far side, in the 186 km-wide crater Von Kármán inside this basin.
10,921 Kilometres Around: Rovers, Roads and Continents
The Moon's equator is 10,921 km long, compared with Earth's 40,075 km, so a lap of the Moon is about 27 percent of a lap of Earth. Driven non-stop at 100 km/h, that loop would take about 109 hours, around four and a half days. Nobody has come close to doing it. Apollo 17's Lunar Roving Vehicle covered about 35.7 km in total around the Taurus–Littrow valley in December 1972. Lunokhod 1 covered 10.54 km over 321 Earth days.
In area, the Moon's 37.9 million km² is a little less than North and South America combined (about 42.5 million km²). Each lunar hemisphere, about 19 million km², is slightly larger than South America (17.8 million km²). It is about 1.25 times the size of Africa (about 30.3 million km²) and a little over twice the size of Russia (17.1 million km²). Against Earth, though, it is small. Earth's total surface is 510 million km², and even its dry land alone, about 149 million km², is almost four times the whole Moon. How the Moon compares with Earth's biggest country is set out in is Russia bigger than the Moon?
No Time Zones Yet: Keeping Time Through a 29.5-Day Lunar Day
Earth spans 24 standard time zones because it turns once a day. The Moon turns once per orbit, so a full day-and-night cycle at any spot lasts a synodic month of about 29.53 Earth days. A lander like Chandrayaan-3's Vikram gets about two weeks of sunlight and then about two weeks of night. Surface temperatures swing from about 120 °C in sunlight to −171 °C in darkness.
No official lunar time zone exists yet. Missions run on the clock of their control centre. The Apollo crews kept Central Time because Mission Control was in Houston, and China's lunar missions run on China Standard Time. In April 2024 the White House asked NASA to work with other agencies on a "Coordinated Lunar Time" standard, an idea first put forward by the European Space Agency in 2023. One reason is that the Moon's weaker gravity makes clocks there run about 58.7 microseconds a day faster than on Earth.
Why 49 Moons Would Fit Inside Earth
The comparisons shrink as you go from length to area to volume. A body 27 percent as wide has about 7.4 percent of the surface (0.27 squared) and about 2 percent of the volume (0.27 cubed). Earth's volume of 1.083 trillion km³ divided by the Moon's 21.96 billion km³ gives about 49. Mass falls further, to 1.23 percent, because the Moon is less dense: 3.34 g/cm³ against Earth's 5.51 g/cm³.
Surface gravity is 1.622 m/s², about one-sixth of Earth's 9.807 m/s². Someone who weighs 60 kg on a bathroom scale here would read about 10 kg there. Escape velocity is 2.38 km/s against Earth's 11.19 km/s. Even so, the Moon is no lightweight among moons. Only Io is denser, and only Io has stronger surface gravity. The Moon is bigger than the largest dwarf planet, Pluto, though smaller than Mercury. To see the Moon from the other direction, as one bright disc next to the whole blue planet, our guide to Earth seen from space shows how the two look from orbit.