Where Has All the Water Gone? (in the wild)
The gravitational pull of the sun and moon on the earth causes low tide and high tide. On average there are 4 tides a day; 2 high, and 2 low, that are roughly 6 hours apart from each other. The Earth spins on its axis once a day. As any point on the Earth comes into alignment with the moon it will experience a high tide due to the pull from the moon causing a tidal bulge. The opposite side of the earth will also experience a high tide and this is why we have 2 high tides a day. The gravitational pull of the moon on the earth, where G is the gravitational constant (6.674 x 10^-11 m^3 kg^-1 s^-2), M is the mass of the moon (7.346 x 10^24kg) and r is the distance from the centre of the moon.
The sun also contributes to how high the tide is. The greater the gravitational pull the higher the tide. When the sun, moon and earth are in a straight line, the gravitational force is at its strongest and we experience the highest tides. If we know the position of the moon and the sun we can roughly figure out when and how high the tide will be. Predicting exact tide times and heights is very complicated.
Sir William Thomson built a tide predicting machine - an early computer — in 1872. This could predict the tides up to a year in advance and saved countless lives at sea.
— Georgina Murray (S2)