In most coastal areas there are two high tides every day. What causes the two tides?

Answer 1

The Moon's gravity causes the high tides and low tides. Two tides happen each day, because the Earth and Moon actually form an orbital pair: that is, the Earth is not exactly 'stationary' with the Moon circling around it. Rather, it 'wobbles' a bit, away from the Moon as it orbits. This means that on the side of the Earth opposite the Moon, a 'bulge' of water is created due to Centrifugal Force. This is the other high tide, and it is always somewhat smaller than the high caused directly by the gravity of the moon. The actual height of the tides varies due to distance of the Moon (like all orbits, it is not exactly circular, but elliptical, and the distance between Earth and Moon varies). The Sun also exerts a considerable force, hence tides are bigger at Full Moon (Moon and Sun opposite each other), and even larger at New Moon (Moon and Sun pulling together on the same side of Earth). Because the Moon orbits a spinning Earth, it rises later each day at any given location, and so the tides get later (by approx 50 minutes) each day. By coincidence (?) this means that in a week, the tides have become later by approx 6 hours (7X50 minutes) so if high tide is, say, 8 o'clock today, it will be low tide at 8 o'clock (approx) in a week's time.