Ruby Brown, of Port Chalmers School, asks :-

Zola Hale, of Whangarei Primary School, asked:- Tilli Kofe, of St Andrew's College, asked:- Room nine, of Addington Primary School, asked:- Rosa Henderson, of Greta Valley School, asked:-

What makes the tide go in and out?

Dick Dowden, a physicist at the University of Otago, responded.

Kids in England see us in New Zealand as "down under". To them we are upside down and ought to fall off the Earth! We don't because everything on Earth is attracted towards the centre of the Earth by gravity. All massive things like the Sun and the Moon pull at us by gravity but the pull is very weak if the things are very distant. If the Moon or Sun (or both) happened to be right above you, you would be a tiny bit lighter because of the upward pull of the Moon (or Sun). So you might expect the sea around you to be a bit lighter too and so flow in from elsewhere to make a high tide a metre or higher than if the Moon were low on the horizon instead of right above us.

Happy with that? If you are very observant, you'll say: "Hey! Wait on! There are TWO high tides a day as the Earth rotates, not just one! That explanation can't be the whole story!" And you would be right!

The Moon and the Earth are massive solid things which pull on one another from and at their centres. The sea on the side of the Earth nearest the Moon at any time feels a stronger pull by the Moon than the Earth feels because that bit of the sea is closer to the moon. So there is a high tide there, sucked up a little by the Moon. Aha, but what about the sea on the opposite side of the Earth, furthest from the Moon? THERE the sea feels a WEAKER pull by the Moon than the Earth feels because that bit of the sea is further from the Moon than the Earth is, so it also feels a little lighter and makes a high tide there too.

That explains why there are two high and two low tides each day instead of one, and why the high tides are higher and the low tides lower when the Sun and Moon pull together at Full Moon and at New Moon.