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*The Moon, Gravity, and Tides

A brief explanation on the cause of oceanic tides on Earth.

paola polasek

on 25 October 2018

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Transcript of *The Moon, Gravity, and Tides

What causes the tides on Earth?
What are tides?
What about the sun?
To summarize:
Tides are the natural rise and fall of sea levels
The sun's gravitational pull (which keeps Earth in orbit) also effects the tides. The sun is a lot farther away than the moon, though, so its impact is not as great.
A tide is the rise and fall of Earth's bodies of water.

The rising and falling is caused by gravity from the moon (primarily) and the sun (a little).

Wherever the moon is overhead, that area is experiencing a high tide, as well as the opposite side of the Earth.

The parts of Earth where the moon is not overhead (and not directly on the other side of Earth) are experiencing low tide.

When the moon, Earth, and sun align, "extra high" tides occur known as spring tides (Think: during a full or new moon)

When the three objects are not aligned, the high tides are known as neap tides (Think: during a first and third quarter half moon)
The Moon, Gravity, and Ocean Tides
First, let's review:
How long does it take the moon to complete one around Earth?

How long does it take the moon to complete one
around its axis?
REVOLUTION = 28 days

ROTATION = 28 days
...but how?
So the moon completes one orbit around Earth every time it completes one "spin" around its axis. This is called "synchronous rotation" and it means we see the same side of the moon at all times.
While watching this video, remember that the "red side" of the moon is just a tool to help us distinguish one specific side from the other.
So the moon rotates while it revolves around Earth. This revolution is the main cause for the ocean tides on Earth, but before we can talk about that, we need to answer a question:
Why does the moon orbit Earth?
The simple answer is:
Gravity is the non-contact force that causes all of the planets in our solar system to revolve around the sun and all the moons to orbit their planets.
Newton's 3rd law of motion tells us that an object in motion stays in motion until another force acts on it.
So, the moon WANTS to travel STRAIGHT through space!
While the moon tries to travel in a straight path, the Earth's gravity keeps pulling it towards itself. These two powerful forces--the moon traveling straight, and the Earth pulling it down--cause the moon to settle in between creating this elliptical orbit.
Gee, that's fascinating, but what does it have to do with tides?
The Earth has a gravitational pull.
The moon ALSO has gravitational pull.
What causes tides?
The long answer is...
The moon has gravitational pull
Keep that idea in your mind as we learn about the causes of tides...
Start by reading Pg 32-34 in your Astronomy book
In general, this process happens twice a day.
It keeps the moon in orbit, our feet on the ground, and the water in the oceans.
It keeps rocks and astronauts from floating off into space, but this gravitational pull is weaker than Earth's because the moon is a smaller object . That's why astronauts can jump so high on the moon--weaker gravity!
So, tides...what's going on?
The moon's gravity is pulling on Earth causing:
the waters closest to the moon to rise significantly,
the Earth to move towards the moon a little, and
the waters on the opposite side of Earth to remain where they were therefor "rising" up as a result of the Earth moving.
It's kind of like this:
(Remember that Earth rotates 28 times for every lunar cycle)
See, when the Earth, sun and moon align, the gravitational pull of the moon and the sun combine and the tide is extra high. These are called "spring tides."

When the moon is not in line with the Earth and sun, then its gravitational pull creates a high tide, while the sun still pulls on the low tide area.
What moon phases are occurring during a spring tide?
What moon phases are occurring during a neap tide?
Write down anything you find interesting in your science notebook
Full transcript