Why are there both high and low tides? (Intermediate)

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.

This page was last updated on July 18, 2015.

About the Author

Karen Masters

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

Twitter:  @KarenLMasters
Website:  http://icg.port.ac.uk/~mastersk/

Our Reddit AMAs

AMA = Ask Me (Us) Anything