Sponge: Tell three reasons reflectors are better than refractors.

The Earth has a circumference of 40,000 km and a radius of 6500 km (6378 km).

The volume of the solid and liquid parts is 1027 cm3. (no gases or magnetosphere)

Its mass is 6.0 x 1027 g.

(6000 billion billion tons)

(Atmosphere and magnetosphere make up less than 0.1% of mass)

Density is 6 x 1027g divided by 1027cm3.

Avg density = 6 g/cm3

For comparison, water has a density of 1 g/cm3, crust rock is 2 to 4 g/cm3. Since the average is 6 g/cm3, the Earth must be much denser below the crust.

The Earth has an equatorial bulge 40 km larger than the polar diameter (because of rotation speed).

This is 40 km compared to a total diameter of 13,000 km; the Earth is smoother and more spherical than a billiard ball.

The Earth has six main regions:

1) Magnetosphere - zone of charged particles trapped by the Earth’s magnetic field.

2) Atmosphere - air

3) Crust - solid continents and sea floor

4) Hydrosphere - oceans

5) Mantle - huge layer of rocky material below the crust and above the core.

6) Core - contains a liquid outer layer and a solid inner layer.

Hydrosphere 3/4 of the Earth’s surface is covered by water to an average depth of 3,682 meters.

2 % of the water on Earth is in lakes and rivers, 98% in oceans.

Tides - the rising and lowering of ocean water levels due to the gravitational pull of the Moon.

There are two high and two low tides each day. The typical change in level from low to high is 1 meter. (In some places it can be as high as 20 m.)

The gravitational influence of the Moon causes a tidal bulge on the side toward the Moon and the side away from the Moon.

Rotation actually causes the tidal bulges to be offset in the direction of rotation.

Spring tides - higher tides because the Sun’s gravity is adding to the tidal bulge.

Neap tides - lower because the Sun is pulling at a right angle.

Spring tides occur twice per month when the Moon, Earth, and Sun are lined up.

Neap tides occur twice per month when the Moon has moved to a position at a right angle to the Earth-Sun line.

Tidal forces - refers to any variation of gravity across a large body like the Earth or Moon.

This variation of the Moon’s gravitational pull which causes the ocean tides is just one example.