The spinning of Earth on its axis is called revolution.

The study of the moon, stars, and other objects in space is called _____.

The strength of the force of gravity between two objects depends on two factors: the masses of the objects and the distance between them.

_____ is the amount of matter in an object.

The force of gravity on an object is known as its ______.

Tides are caused mainly by differences in how much the moon's gravity pulls on different parts of Earth.

Only one side of the moon from Earth because _______.

What force must a rocket overcome to be launched into space?

The four inner planets are small and dense and have rocky surfaces.

1 astronomical unit is equal to _______.

If the orbital period is known for a planet, its distance from the Sun can be found.

The planets' orbits are elliptical.

How did Johannes Kepler determine a planet's distance from the Sun?

A planet needs to orbit a star, be big enough that its own gravity shapes it as a sphere, be small enough that it isn’t a star itself, and have cleared its orbit of smaller objects.

A dwarf planet needs to orbit a star, have enough mass to be a sphere, not have cleared its orbit of smaller objects, and not be a moon.

Pluto is considered a dwarf planet because _______.

Thus famous was first to discover the principal of gravity..

If you're standing on the top of a mountain and drop a rock, it will fall to the ground, even though the rock is gravitationally attracted to you. Why?