Rotating Question Curious About Astronomy? Ask an Astronomer

Why are there both high and low tides?

I am a teacher. Recently I attended a course. In it there was a question not answered regarding high tide and low tide. There seem to be a lot of confusing facts from various book. My question is what makes low and high tides? Why is it so?

As I'm sure you understand, the tides are caused by the gravitational attraction of the Moon. Most people have no problems understanding why, when the Moon is directly over head there are high tides - but then they ask why there are two tides a day! It is also a good question to ask why there are low tides.

The presence of the Moon deforms the shape of the Earth slightly, and since the water is the oceans moves much more easilly than the land, we mostly see this deformation in the form of tides in the ocean (but the land does move slightly too). The force of gravity varies inversly with the distance from the object (ie. it gets weaker the further away you are), so the part of the Earth closest to the Moon is pulled towards it slightly more than the centre, while the part furthest away is pulled towards it slightly less. This means that the Earth becomes (very slightly) egg shaped, and since there is only so much water to go around this causes high tides on the "front and back" of the Earth (as seen from the Moon) and low tides on the sides.

Below is a nice diagram illustrating this and also why we have Spring tides (much more extreme) and Neap tides (much less extreme) at different times of the year.

Tides diagram

July 2002, Karen Masters (more by Karen Masters) (Like this Answer)

Still Curious?

Get More 'Curious?' with Our New PODCAST:

Related questions:

More questions about The Moon: Previous | Next

More questions about The Earth: Previous | Next

How to ask a question:

If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist

This page has been accessed 133050 times since July 19, 2002.
Last modified: June 4, 2003 9:21:55 PM

Legal questions? See our copyright, disclaimer and privacy policy.
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.

Warning: Your browser is misbehaving! This page might look ugly. (Details)