What Is the Orbital Plane of the Moon to the Earth?

Creation

• No one knows for sure how the moon came to be, according to the astronomy department at Case Western Reserve University. The most commonly accepted theory of the moon's creation is that it was born from a cataclysmic impact between a young, moonless Earth and a planet the size of Mars. This collision ejected into space huge fragments of planetary mass, which eventually coalesced into the moon we know today.

Orbital Plane

• Had the moon been created concurrently with Earth, that is if they coalesced from nebula dust at the same time, then it would lie on the same orbital plane as Earth, according to Case Western. The moon's orbital plane is tilted roughly 5 degrees from that of the Earth, the sun and the rest of the planets. Indeed, it was a similar difference in orbital plane that helped lead astronomers to declassify Pluto as a planet in 2006 -- though its orbital plane is tilted at a much more radical 20 degrees.

Eclipses

• Lunar eclipses occur when the Earth passes between the sun and the moon, casting its shadow over the moon and blocking its reflection of the sun's rays. Solar eclipses occur when the moon passes between the sun and Earth; this obscures the sun and leaves the affected portions of Earth in a brief darkness. Because of the moon's 5-degree difference in orbital plane relative to Earth and the sun, eclipses do not occur as often as they would if the moon were on the same orbital plane as those two larger bodies. Were that the case, an eclipse of some sort would occur every new moon and every full moon.

Orbital and Rotational Period

• The moon takes slightly more than 27 days to orbit the Earth and its rotational period is exactly the same. This means that the same side of the moon always faces Earth. Because of its orbital tilt, the moon crosses the Earth's orbital plane only twice in each orbit; these points of planar intersection are called nodes. If the moon is new or full when it crosses these nodes, then an eclipse will occur. These nodes regress along the Earth's orbital plane, making a complete rotation every 18.6 years, according to Arthur Few, professor emeritus at Rice University's Department of Physics and Astronomy.