For centuries, people have drawn a connection between the orbit of the moon and ocean tides. As the moon completes its monthly orbit around the Earth, it rises about 50 minutes later each day. Not coincidentally, the times of high and low tide also shift by a correspondingly predictable interval from one day to the next. At most locations, high and low tides occur 6 hours apart.
Earth’s satellite, the moon, is our nearest neighbor in space, at a distance of approximately 247,000 miles or 411,000 km. The moon is approximately one-quarter the diameter of Earth and has a mass estimated at 7.35 X 10^22 kg. Although the Earth is over 80 times more massive than the moon, their mutual gravitational attraction accounts for most of the tidal bulge in Earth’s oceans.
The moon travels around the Earth in an orbit that lasts an average of 27 days (the sidereal orbit). Relative to its position against the background stars, however, the moon’s orbit around the Earth appears to last 29 days. This is known as the synodic orbit, and is used as the basis of lunar calendars throughout the world.
The moon is not the only celestial body whose gravitational attraction affects tides on Earth. Although the sun is located 93 million miles from Earth, our star is gigantic, representing 99% of the mass of the entire solar system. Because Earth is held in orbit by the sun’s gravity, it stands to reason that this gravitational pull affects Earth’s tides. Although the sun’s gravitational effect on Earth’s tides is less pronounced than that of the moon, this effect manifests itself every two weeks as a phenomenon called spring tide.
This phenomenon occurs twice a month at new moon and full moon. High tides are higher than usual (and low tides are lower) because the sun, Earth, and moon are roughly in alignment at these times. As a result of this linear arrangement, the gravity of the sun and moon produces an additive effect on Earth’s tidal bulge.
Neap tides occur at first quarter and last quarter moon. At these times, the sun, earth, and moon are located at right angles to each other. The result is that the difference between high and low tide is less pronounced at neap tide than at any other time of the month.