High School Earth Science: Earth, Moon, and Sun - The Sun and the Earth-Moon System — Flashcards

Just like the Earth revolves around the Sun, the Moon revolves around the Earth. The Moon revolves around Earth once every 27.3 days. We call this length of time a lunar month. The time it takes for the Moon to rotate once on its axis is about 27 Earth days.

Earth’s Tides    
Earth’s tides are the regular rising and falling of Earth’s surface waters in response to the gravitational attraction of the moon and sun. The moon’s gravity pulls most strongly on Earth’s waters on the side of Earth facing the moon.    
This causes the water to bulge out from the surface in the direction of the moon. At the same time, another high tide occurs on the opposite side of Earth. This high tide occurs because the moon pulls Earth slightly away from the overlying surface water. The overlying water bulges out from the surface because it is pulled by the moon less strongly. The places in between the two high tides experience low tides because some of the water is drawn away to the high tide locations. There are two high tides and two low tides each day in a given location. Because Earth is rotating on its axis, the tidal cycle moves around the globe in a 24-hour period.    
The gravity of the sun also pulls Earth’s surface waters toward it, but the sun’s pull is weaker than the moon’s pull because the sun is so much farther away. When the sun and moon are in a straight line with respect to Earth, which occurs during the new and full moon phases, their gravities combine to create a spring tide.

During a spring tide, high tides are at their highest and low tides are at their lowest, creating the greatest tidal range during each two-week period. When the sun and moon are at right angles with respect to Earth, which occurs during the first and third quarter moon phases, their gravities pull in different directions to create a neap tide. During a neap tide, high tides are at their lowest and low tides are at their highest, creating the least tidal range during each two-week period.