Unit 1: Earths Dimensions

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Unit 1 Earths Dimensions

• Topic Earths Shape
• Objectives
• Introduce evidence for Earths Shape

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How did they prove the Earth to be round?

• Ships appearing to sink as it travels over the
  horizon
• Polaris (North Star) changes altitude directly
  with you latitude
• Eclipses of the Earths shadow on the moon
• Satellite Imagery

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Ships appear to sink
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Latitude Lines
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Polaris Changes in Altitude with Latitude
90º
49º
Note 90 º in altitude means it is directly
overhead (vocabulary term- ZENITH)
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Locating Polaris
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Is the Earth Perfectly round?

• The Earth is called an oblate spheroid
• It is flattened at the poles and bulged at the
  equator

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Why is the earth slightly oblate?
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How do we know that it is oblate?

• Weight measurements compared from pole to equator
• Weight is based on gravitation
• The closer we are to the center of the Earth
  more gravitation more weight
• We are further from the center of the Earth at
  the equator
• This means we weigh LESS!!!
• We are closer at the poles
• We weigh MORE!!!!

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• At the Poles we are closer to the center
  stronger gravity more weight

• At the Equator we are farther to the center
  weaker gravity less weight

Center of Earth center of gravity
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The Oblate Spheroid

• The difference is small
• We can not see it with the naked eye
• It still appears like a sphere

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Longitude
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Longitude- Based on Suns apparent motions

• How long does it take the Earth to rotate once on
  its axis?
• 24 hours (1 day)
• How many degrees does it rotate during this time?
• 360 degrees
• So how many degrees/hour does the earth rotate?
• 360/24 15 degrees/hour

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Apparent Motions

• So the Sun does not really moveit only appears
  to move do to the Earths rotation

Click- http//spiff.rit.edu/classes/phys301/lectur
es/sunmoon/Lunar_Nav.swf
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Greenwich Mean Time (GMT)

• Prime Meridian runs right through Greenwich
  England (0º Longitude)
• All days (time) begins in Greenwich

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Local Noon (1200PM)

• When the sun is the highest at your location

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Calculating Longitude

• Remember, Earth rotates 15º/hr. Therefore, the
  sun appears to move through the sky at the same
  rate
• So, if you know what time it is in Greenwich when
  local noon is at your location, you can calculate
  your longitude
• Heres how

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Calculating Longitude (cont.)

• It was 200PM in Greenwich (GMT) when the sun
  reach the highest point at your location (local
  noon- 1200PM)
• Take the difference in time 2 hours
• Multiply difference by 15º/hr (2x15 30)
• Your longitude 30ºW
• Note AM East PM West

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Practice

• Polaris 60º GMT 4PM

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Time Zones