Transcript Earth`s Rotation

Earth’s Rotation
• Earth rotates counterclockwise
• Earth is tilted on its axis 23.5 degrees
Earth’s Rotation Vocabulary
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Coriolis Effect: the tendency of matter moving across Earth’s surface to be deflected
from a straight-line path.
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Foucault pendulum: a freely swinging pendulum whose path appears to change
over time in a predictable manner; provides evidence of Earth’s rotation.
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Rotation: the turning of an object on its own axis.
Earth’s Rotation
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Earth rotates (spins one time on it’s axis) once every 23 hours 56 minutes 4
seconds.
Earth’s axis runs directly through it’s north and south poles.
To determine Earth’s rotational speed (how fast our planet spins on its axis) we take
the 360° in a circle then divide by the 24 hours in our rotational period
360° ÷ 24 hours = 15°/hour
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So Earth’s actual rotational speed is 15°/hour
Because Earth is rotating 15° every hour, this makes objects in our sky (the sun,
the moon, stars) appear to move at this same speed (15°/hour).
– This is the difference between “actual motion” and “apparent motion”
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Earth's Rotation (hyperlink) (Just Earrth rotating)
Proof of Earth’s Rotation – Foucault Pendulum
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When a Foucault pendulum is set in motion, it will swing at a constant speed in a constant
direction (back and forth, back and forth). As the pendulum is swinging, the Earth rotates
underneath it. The actual motion of Earth’s rotation makes the pendulum appear to change it’s
direction.
The pendulum appears to move 15°/hour.
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Foucault Pendulum - with pins (hyperlink)
Foucault Pendulum - top view (hyperlink)
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The apparent movement of the pendulum
of 15° each hour proves that Earth is rotating.
REMEMBER!
Actual motion – vs – apparent motion
(What is happening – vs – what we see)
Proof of Earth’s Rotation – Coriolis Effect
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The Coriolis Effect is the deflection, or movement, of gases and liquids at the
surface because of Earth’s rotation.
Particles are deflected to the right north of the equator and to the left south of
the equator.
Coriolis effect demonstration (hyperlink)
Shade this diagram – leave the intended path
(the straight arrows) black. Highlight the apparent
path (the curved arrows). The apparent path of
motion (the curved arrows) shows the
Coriolis Effect – proof that Earth is rotating.
Proof of Earth’s Rotation – Star Trails
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Earth’s actual motion is 15° each hour. Because of this, stars appear to move
through the night sky at a rate of 15° / hour.
There is one exception to this in the northern hemisphere – Polaris, which always
appears in the same place - all night, every night.
There are two ways you might see a star trail question:
– Given degrees of movement, calculate how much time passed
• Ex: How long does it take for a star to move 60° through the sky?
60° ÷ 15° / hour = 4 hours
– Given time passed, calculate how many degrees the star moved
• Ex: A student observes the position of a single star for three hours. How
many degrees does it appear to move?
3 hr x 15° / hour = 45°
Look at the example. How much time is
represented by each star trail?
4 hours
The apparent motion of stars proves that Earth
is rotating
Star Trails (hyperlink)
Star Trails (hyperlink)
Proof of Earth’s Rotation – Shadow Length
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As the Earth rotates first towards and then away from the sun each day, the
altitude of the sun above the horizon changes.
Solar noon is the time of day when the sun is at it’s highest point. This time varies
throughout the year.
A shadow is always found on the opposite side of the object from the sun.
– Ex: If the sun is to the south, the shadow will be towards the north
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When the sun is lower in the sky, shadows are long.
– Low sun at sunrise and sunset = longer shadows
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When the sun is higher in the sky, shadows are short.
– High sun at solar noon = shortest shadow
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Daily Shadow Length (hyperlink)
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The changing length of shadows
proves that Earth is rotating