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How Many Moons Does Earth Have?

by Fraser Cain on June 12, 2008

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How many satellites does Earth have? Image of the Full Moon at perigee, taken from Tabuk, Kingdom of Saudi Arabia, on March 19th., 2011 at 20.05UT using a Canon 30D camera set at 1/800sec and 1000ASA. The camera was attached to an 80mm refractor of 500mm focal length and a x3 teleconverter giving an effective focal length of 1,500mm. Credit: Colin Henshaw.


With more the 60 moons orbiting Jupiter, and 2 going around Mars, you might think that we should have plenty of moons orbiting Earth. But no, Earth only has one moon… the Moon.

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Scientists think that the Moon was formed when a giant Mars-sized object crashed into the Earth, early on in its history. The enormous collision threw up a tremendous amount of debris. Most of it rained back down on the molten planet, but some of it stayed in orbit, and coalesced into the Moon we know today.

Although there are many different names for the Moon: blue moon, harvest moon, new moon, full moon, it’s all just describing the same object.

There are two other objects that some scientists have referred to as Earth’s other moons, but they aren’t really moons.

The first is called 3753 Cruithne, and it’s not a moon at all, but an asteroid that orbits the Sun. Since its orbit takes the same amount of time to orbit the Sun as the Earth, it sometimes looks like it’s following Earth in orbit around the Sun. Although the orbit of Cruithne comes very close to Earth, the two objects can never really collide because Cruithne’s orbit is tilted to that of the Earth’s by 19.8°. It’s a very interesting object, and has an orbit that interacts with Earth, 3753 Cruithne really doesn’t quality as a satellite of Earth.

Have you been keeping count? How many moons does Earth have? Still just 1.

The other object is called 2002 AA29, and it takes an even more bizarre orbit around the Sun. It spends most of its time in a horseshoe orbit, oscillating up and down near the Earth. Every 95 years or so, 2002 AA29 comes within 5.9 million km of Earth. Because it’s so close to Earth, scientists have suggested that it might make an ideal target for a space mission to retrieve a sample and bring it back to Earth.

Astronomers thought they might have discovered another moon in 2002, which they designated J002E3. But it turned out to be the third stage of the Apollo 12 Saturn V rocket.

Still keeping count? The Earth has only one moon.

We have written many articles about the Earth’s moon for Universe Today. Here are some interesting facts about the Moon, and here’s an article about the Earth’s Moon.

If you’d like more info on the Moon, check out NASA’s Solar System Exploration Guide on the Moon, and here’s a link to NASA’s Lunar and Planetary Science page.

We’ve also recorded an entire episode of Astronomy Cast all about the Moon. Listen here, Episode 113: The Moon, Part 1.

Reference:
NASA Solar System Exploration: Moon

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About

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

  • omega1

    New research shows we now have identified 5 moons orbiting the Earth. Witch means ‘the moon’ should only be called lunar

  • omega1

    Cruithne

    In 1986 D. Waldron in Australia discovered a 5 km diameter companion to Earth a co-orbital near Earth asteroid. He named it 3753 Cruithne, pronounced Crueenya, after a Celtic tribe also known as the Picts. It has a very strange orbit about the sun which is elliptical and sweeps in passed Venus almost to mercury and then out almost to mars. One orbit takes almost exactly a year. It is choreographed so that the orbit is stable and Cruithne will never strike the earth.
    It takes one year for Cruithne to orbit the sun, as does the Earth and the Moon.

    Just as the moons orbit is transformed from a sinusoidally wiggling ellipse about the sun into a nearly circular orbit of the earth when viewed in the earth frame of reference, so to does Cruithne’s orbit present an interesting pattern when viewed from a reference frame attached to the earth. (The transformation from the sun frame to the earth frame is well shown in a movie on Paul Wiegert’s site.) When viewed from a point above the north pole of the earth the asteroid Cruithne follows a type of complex horseshoe orbit.

    2002 AA29

    Another co-orbital asteroid in a horseshoe orbit, named 2002AA29, it is only 100 m in diameter. It comes near the earth every 95 years, most recently January 8, 2003. In another 600 years it will orbit the earth once a year, then after 50 years of orbits drift away again. It is a quasi satellite of earth at a distance of 0.2 AU. It is 100 m in radius. (It last orbited earth in 572 AD.) Viewed from the earth frame it sometimes orbits over the poles, toward and away from the sun. It thus can claim to be earth’s third moon, on a part time basis.

    The other moons have classifications like above

  • bea marion

    the planet that have plenty of moons?

  • Cindy

    i need to now what days and months for 2008 have a full moon and new moon

  • kayla

    and all i need to know is how many satellites dose earth have to

  • Island200 yay

    ew

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