The moon's effect on the tides and gravity?

[jpfassari]

I'm a newbie, so this may seem basic to many of you, but here goes.

 

Does the moon's pull affect the law of gravity? (I'm thinking since the moon's pull affects the tides, perhaps it also affects gravity to the slightest degree?)

Or are those laws constant, independent of the moon's pull?

If so, how are these laws affected?

 

Thanks

[insane_alien]

no, the moon doesn't affect the laws of gravity.

 

the moons affects the tides BECAUSE of the laws of gravity.

 

the moon has mass, hence it has a gravitational field.

 

since the gravity of the earth and moon ar pulling in opposite directions(when the moon is overhead) the net force on you is less. this is what makes tides happen.

[D H]

no, the moon doesn't affect the laws of gravity.

 

the moons affects the tides BECAUSE of the laws of gravity.

 

the moon has mass, hence it has a gravitational field.

This first part is perfectly correct.

 

since the gravity of the earth and moon ar pulling in opposite directions(when the moon is overhead) the net force on you is less. this is what makes tides happen.

This part isn't right. By this logic, the tidal force would make you weigh more when the Moon is directly underfoot. It doesn't. The tidal force from the Moon when the Moon is directly underfoot is nearly equal in magnitude to that when the Moon is directly overhead. However, it is directed away from the center of the Earth in both cases. The tidal force from the Moon does make you a tad heavier when the Moon is on the horizon, in which case the magnitude is about half that when the Moon is directly overhead or directly underfoot.

 

The tidal acceleration due to the Moon at some point on the surface of the Earth is the difference between the gravitational acceleration toward the Moon at the point in question and the gravitational acceleration of the Earth as a whole toward the Moon.

[jpfassari]

Thanks for the detailed answer!

So, if I understood correctly when the tides go up, we weigh slightly less...And when they go down, we weigh more?

[D H]

You weigh slightly (and very, very slightly) less when the Moon is directly overhead / underfoot compared to when the Moon is on the horizon. However, that does not mean you weight less when the tides go up / more when they come down. The ocean tides don't necessarily rise and fall with the Moon. It's a lot more complex than that.

 

The image below displays the amplitude and timing of principal lunar semidiurnal (or M₂) component of the ocean tides.

 

Source: http://svs.gsfc.nasa.gov/stories/topex/tides.html

 

Note that there are several dark blue patches at the center of which several white lines coalesce. The points at which those lines come together are called nodes: Points where the M₂ component of the tide is zero. The white lines coming out of those nodes are called cotidal lines. All points on a given cotidal line will have high and low tides occur at exactly the same time. Move toward the next cotidal line and the tides will be earlier or later, depending on which direction you are heading.

 

You will also see several spots on the Earth such as the North Sea, Hudson's Bay, southeast South America, and the east coast of Maylaysia where there are a lot of white cotidal lines. The timing of the tides is very complex in those areas. For example, at any given time of day, there is always a high tide somewhere in the North Sea.