The Moon is Earth's only natural satellite. It is the
fifth largest satellite in the Solar System and the largest and most
massive relative to its parent planet, with a diameter about one-quarter
that of Earth (comparable to the width of
Australia). The Moon is a planetary-mass object with a differentiated
rocky body, making it a satellite planet under the geophysical
definitions of the term. It lacks any significant atmosphere,
hydrosphere, or magnetic field. Its surface gravity is about one-sixth
of Earth's at 0.1654 g, with Jupiter's moon Io being the only
satellite in the Solar System known to have a higher surface gravity and
density.
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The Moon is nearly back to the point in its orbit where its dayside
directly faces the Sun, and all that we see from our perspective is a
thin curve.
Learn more about moon phases.