Expert: Jayendra Upadhye - 4/8/2006

Question
When the Earth, Moon and Sun are aligned for Spring Tides, Why they are about the same height whether at New or Full Moon?

Answer
Hi,
Alireza, I was under the impression that Newmoon tides are bigger than Fullmoon tides!

As in one case the sun and moon pull together,in the other case they oppose each other.

But here is the reason! [Though i am not convinced myself..but who am i to argue against newton the great?]
more than 100 years ago he pointed out the answer in his path-breaking book "principia"..

He also outlined modern calculus in that book!

****** reason ********
Tides are periodic rises and falls of large bodies of water. Tides are caused by the gravitational interaction between the Earth and the Moon. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Since the earth is rotating while this is happening, two tides occur each day.

Isaac Newton (1642 -1727) was the first person to explain tides scientifically. His explanation of the tides (and many other phenomena) was published in 1686, in the second volume of the Principia.
******* end *******

Wow is all i can say!
understanding the "since the Earth is also being pulled toward the moon (and away from the water on the far side)." part is a bit difficult, as it is difficult to imagine or visualise.

Fluids behave a bit differently than solids, under uniform (even if weak) continuous point fields such as gravity.
So newton is right and we puny guys are wrong i suppose!

ref:- http://www.enchantedlearning.com/subjects/astronomy/moon/Tides.shtml

Hope that suffcies.
Jayen