Throughout the day and night the level of water in the world’s oceans and other large bodies of water rises and falls. This change is due to tides, which are caused by the gravitational pull of the Moon on Earth as our planet rotates on its axis. Water on the part of Earth directly facing the Moon bulges as the Moon’s gravity pulls it, creating high tide, or high water levels. On the opposite side of Earth a secondary, smaller high tide occurs, again related to the gravitational pull of the Moon. So at any given time during the day or night as the planet rotates there are two high tides, and two low tides (and all the levels in between) in the large bodies of water that cover Earth. High tides occur every 12 hours and 26 minutes, which is a bit more than half the time it takes Earth to make a complete turn on its axis. (That extra 26 minutes comes from the fact that the Moon is moving as well; it rotates around Earth once every 30 days or so.) While the pull of the Moon’s gravity is only strong enough to change the levels of the oceans about two feet, tides can make a bigger difference near a coast. At low tide (when water levels are least affected by the gravity of the Moon) you may be able to walk far out on a beach, while at high tide, the beach may be almost completely covered by water. The Bay of Fundy, located in Canada, has the biggest difference in water levels between high and low tides more than 40 feet.