6.6 Tides Tides and the Moon

It’s the Difference in Gravity! Gravity obeys the Inverse Square Law. If the distance between an object and the gravity source is doubled, a given area receives only one-fourth as much gravity; if the distance is tripled, only one-ninth.

Imagine the Earth as a large ball of rock suspended in a “bubble” of ocean water to make the next few slides a bit easier to understand...

High Tide The Moon’s gravity pulls on both the Earth AND Earth’s waters. It pulls harder on the water on the near side of the Earth than it does on the Earth itself.

This pulls the water on the near side away from the Earth, which makes a High Tide on the Earth’s side nearest the Moon.

The Moon’s gravity also pulls harder on the Earth than it does on the water on the far side of Earth. This pulls the Earth away from the water on the far side… and also creates a high (but not quite as high) tide on its far side. Therefore…. Earth always has two high tides at any time.

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Low Tides vs High Tides Low Tides High Tides Occur on the sides of the Earth which neither face nor are opposite from the Moon. Moon pulls on the Earth and water with the same force. Occur on the sides of the Earth either facing toward or on the opposite side from the Moon. On the near side, the Moon pulls away water from the Earth. On the opposite side the Earth is pulled away from the water.

The Sun and Tides The Sun also causes tides! These tides are much weaker than those caused by the Moon because the difference between the gravity of the Sun and Moon. Since the Earth is so small compared to its distance from the Sun, the gravitational pull on Earth’s waters is less than that of the Moon, even though the Sun pulls on Earth with more force than the Moon does.

The Sun’s tides affect Moon’s tidal forces by adding to or subtracting from them.

Do notice that the water bulge follows the Moon (and Sun) Do notice that the water bulge follows the Moon (and Sun)! Even though we say the tides “ebbs” and “flows,” it’s really the Earth that is revolving inside of the water bulge!

Spring and Neap Tides During the full moon or new moon lunar phases, the Earth and Moon “line up” with the Sun. Sun and Moon tidal pulls work together, so high tide is at its highest while low tide is at its lowest. We call these Spring Tides.

When the Lunar Phase is at a first or last quarter, the Sun, Earth and Moon form a rigt angle. The Sun’s tidal force cancels some of the Moon’s stronger tides, so the high tide is not so high and the low tide isn’t so low. We call these Neap Tides.

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The next slide is a stopmotion video of Halls Harbour, located on the Bay of Fundy in Nova Scotia, Canada. (Nova Scotia tends to have extreme tides because it is so close to the Arctic circle.) It is a 12-hour timelapse of the highest Spring Tide of the year. Notice what happens to the water levels!

The Timing of the Tides The Tide at any point on Earth should be highest when that point is nearest...or farthest from the Moon. That is when the moon is at its highest altitude and its lowest. What time this is depends on the phase of the Moon. For example: When the Moon is full it reaches its highest altitude at midnight, so that should be High Tide. The exact time is affected by a delay due to friction with the Earth, and local geography.

Tidal Braking The Earth rotates faster than the Moon orbits the Earth (west to east). The friction between the water and the solid Earth drags the Tidal Bulge forward. The Tidal Bulge will pull the Earth backward and makes the Earth rotate more slowly. This is called Tidal Braking.

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Earth’s day gets about 0. 00164 seconds longer every year. (That’s 16 Earth’s day gets about 0.00164 seconds longer every year! (That’s 16.4 secs every million years.) The Tidal Bulge pulls back on the Moon, which causes: The Moon’s orbital velocity to speed up, moving it away from Earth. The Moon revolution and rotation to be synchronous with Earth, making it tidally locked.