The Moon

The most studied and nearest celestial object is our moon. The Moon is the only natural satellite of Earth. The Moon goes through phases as it orbits Earth—new, first quarter, full, and third quarter. The Moon’s orbital period matches its rotational period keeping the same side pointed toward Earth. During phases other than new or full, we see part of the Moon in sunlight and part in darkness. The boundary between the two is called the terminator. The surface of the Moon consists of lighter highlands and darker maria. The Moon is heavily cratered. Craters formed when asteroids struck the Moon. Volcanism on the Moon billions of years ago created the maria as lava flowed from large surface fissures to fill the lower elevation regions. Unlike the similarly sized moons around Jupiter, Saturn, and Neptune, the Moon does not orbit in Earth’s equatorial plane. It orbits in a plane tilted five degrees to the ecliptic plane (the plane of the Earth’s orbit around the Sun). The Moon is composed of silicate rocks and does not have an iron core or magnetic field like the Earth.