Naked Science Forum
Life Sciences => Marine Science => Topic started by: MarianaM on 15/10/2019 16:01:25
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Richard is wondering...
What does the moon and earth do that results in tides?
Why do tides differ more in high to low difference at the higher latitudes than at tropic ones?
When the tide is high, is there a corresponding low tide in the opposite side of that body of water? You know, like a bowl of water swishing from side to side. High tide on the Eastern coast of the US, a low tide on the Atlantic coast of Europe?
Are there tides in inland lakes?
Can you help?
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Tides are the result of the difference in gravitational pull across the Earth caused by the Sun and Moon.
So for example, the Moon pulls on the near side of the Earth more than it pulls on its center, and pulls on the center harder than it pulls on the far side. Also, since the Moon's gravity pulls towards the center of the Moon, it also tends to slightly squeeze in on the sides of the Earth. This results in the two tidal bulges we get. So that when you have a high tide on one side of the Earth you will have a high tide on the other side. ( this is why we get 2 high tides and 2 low tides in roughly 24 hrs.)
The Earth is tilted with respect to the both its orbit around the Sun and the Moon's orbit around it. This results is the tidal bulge peaks to be at latitudes away from the equator most of the time. Sometimes, like during full and new Moons, the Sun Earth and Moon are more or less in a line and the pulls of the Sun and Moon align. This results in their tidal effects adding up and we get higher than average high tides and lower than average low tides, known as Spring tides. During quarter Moons, the tidal effects tend to cancel each other out and we get the least variation between low and high tides, these are known as Neap tides.
Inland lakes can have tides, but the lake has to be fairly large for them to be measurable. Lake Superior has a tide, but it's Spring tides are only 5 cm in height.
Another effect is that open ocean tides are only about 1 meter in height. The much larger tides we get at the coasts of continents is due to a "sloshing" effect. If you are on an Island in the tropics, you will only see the open ocean tidal difference.
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The common view is that tides rotate around the Earth with the Sun and Moon.
However, this does not happen because:
- The continents get in the way
- The average depth of the oceans is too shallow to allow tides to propagate at 1600km/h (at the equator)
In fact, tides circulate in a circular motion within ocean basins.
- This circulation is usually centered on an "amphidromic point" in the center of the ocean, which has virtually no tide
- However, in some cases, the circulation is centered on a sizeable island which has opposite tides on opposite sides of the island (like New Zealand and Madagascar)
Sometimes multiple circulations occur in a basin (eg the Mediterranean). In this case, adjacent tides circulate in opposite directions, like clockwork gears.
This pattern of tides could only be pictured once there were Earth-observing radar satellites
See: https://en.wikipedia.org/wiki/Amphidromic_point