Despite being much smaller than the Earth, the Moon still has a great deal of influence on its parent planet. Its gravity is constantly pulling on Earth’s surface. This is not noticeable in relation to solid ground, but can clearly be seen in the movement of Earth’s tides. Twice a day, the oceans on Earth rise and fall. This is because the Moon’s gravitational pull is strongest on the side of Earth that is facing the Moon. Oceans on this side will be pulled into a bulge — high tide. Water on the opposite side is least affected by the Moon’s gravity, so it flows away from Earth in another bulge, resulting in another high tide. Areas of Earth at right angles to the Moon will have low tide.
A bigger instant effect would be on the ocean’s tides. But to understand the impact we need to know about how tides work. Tides are the result of the gravitational tug from the Moon and Sun that the Earth feels. If we disregard the Sun for now, the Earth’s oceans facing the Moon bulge up in response to the lunar gravitational force: a high tide. The difference in gravitational attraction on the near and far sides of the Earth means that, at the same time, there is also a high tide on the side furthest from the Moon. And because the ocean is liquid, between these two high tides there are two low tides. As the Earth is spinning, these high and low tides move across the globe over 24 hours, meaning each coastal location experiences two high tides and two low tides every day.
In reality, it is a little more complicated. The Moon’s 27-day orbit of the Earth means the times at which high and low tides occur change. You have to wait 12 hours plus 25 minutes between each high tide. And the Sun plays its part too. The Sun’s influence on tides is just under half as strong as the Moon’s.
When the Sun, Moon and Earth are all lined up, the Sun and Moon work together to produce ‘spring’ tides (though confusingly they don’t have to happen in spring). During spring tides, high tides are a little higher and low tides a little lower than normal. In contrast, when the Sun and Moon are at right angles to one another, the tides from the Sun partially cancel those from the Moon and we have the opposite: ‘neap’ tides. Here, high tides are a little lower and low tides a little higher than average.
