Earth and the Moon move through space together like dancers linked by gravity. The Moon is about one quarter of Earth’s diameter and orbits at an average distance of roughly 384,000 km. It rotates once on its axis in the same time it takes to orbit Earth, so we see the same lunar face most nights. This locked spin–orbit motion is called synchronous rotation and it keeps the familiar near side always turned toward us.
Comprehension Check
Why do we always see the same side of the Moon from Earth?
Which statement best describes the Earth–Moon relationship?
Phases and Eclipses
Lunar phases occur because we observe different portions of the Moon’s sunlit half as it orbits Earth. From new Moon to full Moon and back, the cycle repeats about every 29.5 days. Eclipses are special alignments: a solar eclipse happens when the Moon moves between Earth and the Sun, while a lunar eclipse occurs when Earth’s shadow falls on the Moon. Because the Moon’s orbit is tilted about 5°, perfect alignment is rare, so eclipses do not happen every month.
Comprehension Check
What causes the Moon’s phases?
Why don’t we experience an eclipse every month?
Tides and Orbits
The Moon’s gravity pulls on Earth’s oceans, creating two high tides and two low tides most days. When the Sun, Earth, and Moon line up at new or full Moon, the tidal forces add together and produce higher spring tides. At first and third quarter phases, the forces partly cancel, giving lower neap tides. These predictable patterns help sailors, fishers, and coastal communities plan safe activities along the shore.
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