Just how much do the planets affect the tides?

The influence of the planets, and planetary alignment on the Earth’s tides is addressed. Starting from Newton’s law of gravitation, the tidal influence of any celestial body is expressed in terms of its apparent size and its density. From this, planetary alignment can be seen to contribute at most tenths of a millimetre to a tide and so is unlikely to be a significant contributor to exceptional tidal events. The likely causes of extreme tidal events are outlined: when the Sun and Moon are each closest to Earth, equinox, and weather—in particular extreme air pressure, rainfall, and wind. We conclude with the long-term influence of the planets on the Earth’s orbit and thus on the Sun’s tidal effect (with timescales of the order of 100 000 years), and planetary influence on the Moon’s orbit.