1. 5.1: Earth as a Planet
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2. Why study Earth in an Astronomy class?
We know Earth better than any other astronomical body, and we can learn from it about many other properties that shape other worlds.

3. 2. By studying Earth we can understand the composition and properties of other planets.

4. 3. Earth is the only planet we know (so far) that supports what we define as “life.” By understanding WHY life developed on Earth, we can more easily search for and identify exoplanets that may also support life.

5. The Shape and Size of Earth
Astronomers in ancient Greece knew Earth was a sphere with a radius of about 6,400 km (or 4,000 miles)

6. Earth is easily large enough to pull its matter towards its center
Earth is easily large enough to pull its matter towards its center. (This critical radius measures is about 350 kms, or 217 miles.)
The asteroid Gaspra is about 19 kms (12 miles) across, too small to be pulled into a sphere by gravity.

8. Is Earth really spherical?!??
Earth is not a perfect sphere; it bulges at its equator, due to inertia as it rotates around its axis.

9. Composition of the Earth
Earth is essentially a big ball of rock, which is not as simple as it sounds. Many different kinds of rocks exist.

10. Compositions of Earth’s rocks
The most common elements in terrestrial rocks are oxygen, silicon, aluminum, magnesium and iron.

11. Silicon and oxygen usually occur together in silicates
Silicon and oxygen usually occur together in silicates. Earth’s surface rocks are mostly silicates.

12. Much of Earth’s interior contains the mineral olivine, which also a silicate with iron and magnesium.

13. Density of the Earth
Density is a measure of how much material is packed into a given volume. It is defined by an object’s mass divided by its volume.

14. Density gives us some clue to an object’s composition.
Earth’s average density is about 5.5 g/cm3, which is much denser than the rocks found at Earth’s surface.
What does this tell you about the density of Earth’s interior?