The Moon
Our Moon is a satellite that orbits the Earth.
Most planets have companion satellites, but among the unique properties of our Moon is its relative size compared to the Earth.
At nearly 1/4 of the size of Earth, by percentage, the Moon is the largest moon in the solar system.
In absolute terms, our Moon is the fifth largest moon in the solar system, behind
Titan, Ganymede, Callisto and Io. Below is a NASA image of many of the solar
system bodies that are smaller than Earth.
It is easy to underestimate the relative distance from the Earth to the Moon.
The Moon orbits about 240,000 miles from the Earth and its radius is just 1000 miles.
By contrast, the Earth's radius if 8000 miles, or 1/30th of the distance to the Moon.
The following image demonstrates the Moons true distance.
The image is accurate in its depiction of the relative size and distance between the Earth and Moon.
As the Moon orbits the Earth, we observe the illuminated portion pass through "phases." For a desciption of this process, view our Moon Phases page."
The Moon orbits the Earth every 27 days.
The Moon's orbit is inclined to the Earth's orbit by about 5°, so half the time the Moon is above the Earth and half the time below. The illustration shown below exagerages the inclination for better illustrate the
idea.
The points where the Moon passes through the plane of Earth's orbit are called nodes and are shown as red points. When the nodes are aligned with the line between the Earth and the Sun
(as shown), eclipses can occur. Both solar and lunar eclipses occur approximately 6 months apart when the nodes are aligned
with the Sun. A solar eclipse occurs when the Moon passes through the sun-ward
node and the shadow of the Moon is cast on the Earth. A Lunar eclipse occurs
when the Moon passes through the node away from the sun and through the Earth's
shadow.
The nodes' orientation slowly precess in 223 months time, a period of time called a "saros."
This period of nearly 19 years separates two eclipses that have very similar characteristics.
