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# Earth and the Universe - Rotation

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by

## Jim Webb

on 15 September 2016

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#### Transcript of Earth and the Universe - Rotation

Which way does the Earth rotate?
Eastward
How long does it take for the Earth to rotate one full time?
24 hours
How long does it take for the Moon to rotate once on its axis.
27 days
In your own words, why is the Moon in a synchronous rotation with the Earth?
Because the Moon's rotational period and its period of revolution are both 27 days.
Explain how day and night happens on Earth.
The Earth constantly rotates on an axis exposing sunlight to half of the Earth while the other half experiences darkness in the shadow.
Time for a review...

Define rotation
- a spinning motion around an axis.
Earth and the Universe -
All planets, including the Earth, rotate around an imaginary line running through their centers called an "axis of rotation".
Rotation
The Earth rotates eastward which makes the sun appear to move westward across the sky.
A spinning motion around an axis.
Earth takes 24 hours to rotate once in a 360 degree circle around its axis.

That is Earth's "rotational period".

In physics, we define a period as "the interval of time necessary for a regularly recurring motion to make a complete cycle".

The rotation of a planet is called its "rotational period".
Jupiter by New Horizons in 2007
10-hour day
Mercury's rotational period is 59 days.

Mercury’s temperature range is greater than any other planet in the solar system. The side facing the sun reaches temperatures of 430˚C (806˚F).

Because Mercury has almost no atmosphere, at night its heat escapes into space. Then its temperature plummets to below -170˚C (-274˚F).
It takes Venus 243 Earth-days to make one rotation.
Intensity of Daylight
Mars' rotational period is 24 hours.

Mars’ temperature ranges from 20˚C (68˚F) during the day to -140˚C (-220˚F) at night.
Intensity of Daylight
What is Earth's rotational period?

On Earth, the temperature can vary greatly depending on where you are located.

The polar regions do not capture direct heat as well as the equatorial regions. Think of a blow dryer blowing on your head. The area taking the direct heat is very hot while other parts of your head are only warm.
Intensity of Daylight
It takes our Moon approximately 27 days to rotate once on its axis.

It orbits the Earth once every 27.322 days.

As a result, the moon does not seem to be spinning but appears to observers from Earth to be keeping almost perfectly still.

Scientists call this sychronous rotation.
The rotational period of the moon wasn't always equal to its orbit around the planet.

Just like the gravity of the moon affects ocean tides on the Earth, gravity from Earth affects the moon.

Because the moon lacks an ocean, Earth pulls on its crust, creating a tidal bulge at the line that points toward Earth.
Gravity from Earth pulls on the closest tidal bulge, trying to keep it aligned.

This creates tidal friction that slows the moon's rotation.

Over time, the rotation was slowed enough that the moon's orbit and rotation matched, and the same face became tidally locked, forever pointed toward Earth.
They also rotate in predictable patterns.
Does our Moon have a tilt?
So does our Sun rotate?
Galileo discovered from the movement of sunspots that the Sun rotates on its own axis, and each revolution takes about an earth month.

However, because the Sun is fluid not solid, it spins faster at its equator (27 days) than at the poles (35 days).