Along most coasts the sea rises and falls twice a day. These tides come from gravity, mostly the Moon's. The Moon pulls hardest on the side of Earth facing it and weakest on the far side, and that difference stretches Earth's oceans into two bulges.
Key idea
There are two tidal bulges, one facing the Moon and one on the opposite side. As Earth turns, a coast passes through both bulges, so it gets two high tides and two low tides about every 24 hours 50 minutes.
Why it works
The Moon pulls the near-side ocean a bit more than it pulls Earth's center, so that water heaps up toward the Moon. It pulls Earth's center a bit more than the far-side ocean, so Earth is pulled away from that water, leaving a second bulge behind. Why 24 h 50 min and not 24 h? While Earth spins once, the Moon moves along its orbit, so Earth needs about 50 extra minutes to face the Moon again. High tides are about 12 h 25 min apart.
Worked example: The next high tide
High tide at a harbor is at 3:10 a.m. About when is the next high tide?
- High tides come about 12 hours 25 minutes apart.
- 3:10 a.m. plus 12 h is 3:10 p.m.; plus 25 min more is 3:35 p.m.
Answer: About 3:35 p.m.
Worked example: Spring or neap?
Tonight there is a full moon. Will the tides be bigger or smaller than usual?
- At full moon (and new moon) the Sun, Earth and Moon are in a line.
- The Sun's smaller tide adds to the Moon's, so highs are higher and lows are lower: spring tides. (The name has nothing to do with the season.)
Answer: Bigger: spring tides
Common mistake
Picturing one bulge facing the Moon and expecting one high tide a day. There is a second bulge on the far side, so most coasts get two high tides a day. Check: tide tables for most beaches list two highs and two lows each day, each about 50 minutes later than the day before.