The moon and the sun cause the Earth's tides, but the moon has a much greater effect. The moon's gravity pulls on the Earth's oceans. High tide aligns with the moon as the Earth spins underneath it. Another high tide happens on the other side of Earth because gravity pulls the Earth toward the moon more than it pulls the water. When the sun, Earth and moon line up at full and new moons, they produce higher than normal tides.
With a diameter of approximately 2,160 miles, Earth's moon is the biggest in our solar system. It may have a "little sister" in the sky, an asteroid called Cruithne.
Without the moon to stabilize Earth's rotation, our planet might wobble so much that it couldn't support life — at least as we know it.