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Earth Science 11
Earth’s Motion
• Rotation
 Rotates around its axis
 Period = 24 hours = 1 day
 Axis tilted at 23.5ᵒ
• Revolution
 Revolves around the sun
 Period = 365.25 days = 1 year
• Precession
 A “wobble” in Earth’s motion
 The conical motion of Earth’s axis
 Period = 26 000 years
Earth’s Shape
• Evidence for Spherical Earth
 Can go all the way around without falling off
 The top of a ship’s mast is the first thing you see as it
approaches your location (discovered by Magellan, 1522)
 Earth casts a curved shadow on the moon during a lunar eclipse
 Position of constellations change as you move north or south
• Earth is an Oblate Spheroid = “flattened sphere”
 Due to rotation
 Diameter at equator is 42 km greater than diameter at poles
 Circumference around equator is 67 cm greater than
circumference around poles
Earth’s Density
• Earth’s Specific Gravity (Relative Density)
 Density as compared to water
- water = 1.0 g/mL
- Earth’s crust = 3.0 g/mL
- Earth’s core = 8.0 g/mL
- Average = 5.5 g/mL (Earth’s specific gravity)
Earth’s Heat
• Earth’s Internal Heat
Surface temperatures do not penetrate below 20
Past 20 m, temperature in the crust increases at
about 1ᵒC for every 40 m depth (=geothermal
Sources of heat:
- Radioactive isotopes
- Leftover heat from Earth’s formation, trapped in by crustal
- Friction between tectonic plates
- Tidal stress from moon’s gravity
Earth’s Energy (Heat) Sources
• The Earth has 2 sources of energy:
1. Sun
 The fusion (putting together) of hydrogen atoms into helium
 Releases tremendous amounts of energy which the Earth
receives as sunlight
 Is the energy source for our weather, waves and ocean currents
2. Radioactive decay
 The fission (breaking down) of large elements within the Earth,
releasing tremendous energy
 Results in convection currents in the mantle, driving the plates
against each other, and causing the land to rise up out of the
 Earthquakes and volcanoes are associated with this
• Complete Questions #5-7 on pg. 19
• Answers can be found in the notes or on
pages 16-18