Why does the moon's gravity cause tides?

Question

Scientists tell us the moon's gravity is very weak compared to earth's gravity.So why would it cause tides in the seas yet the earth's gravity is stronger?It does not make sense because we are told people can literally float on the moon due to its weakness yet it slightly pulls our water thousands of miles away!

Answer 1

Tides on earth
The gravitational attraction that the Moon exerts on Earth is the cause of tides in the sea. The tidal flow period, but not the phase, is synchronized to the Moon's orbit around Earth. The tidal bulges on Earth, caused by the Moon's gravity, are carried ahead of the apparent position of the Moon by the Earth's rotation, in part because of the friction of the water as it slides over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 millimetres per year. At the same time the Earth's rotation is gradually slowing, the Earth's day thus lengthens by about 15 µs every year. A more detailed discussion follows in the section titled Earth & Moon

Earth & Moon
Tidal effects

The tides on Earth are mostly generated by the Moon's gravitation, with a less significant contribution by the Sun. These gravitational effects are specifically manifested as tidal forces. The combination of the two is responsible for spring and neap tides. Two tidal bulges, one in the direction of the Moon, and one in the opposite direction (figure 1) form as a result of the tidal forces. The buildup of these bulges and their movement around the earth causes an energy loss due to friction. The energy loss decreases the rotational energy of the Earth.

Since the Earth spins faster than the Moon moves around it, the tidal bulges are dragged along with the Earth's surface faster than the Moon moves, and move "in front of the Moon" (figure 2). Because of this, the Earth's gravitational pull on the Moon has a component in the Moon's "forward" direction with respect to its orbit. This component of the gravitational forces between the two bodies acts like a torque on the Earth's rotation, and transfers angular momentum and rotational energy from the Earth's spin to the Moon's orbital movement.


Galileo spacecraft took this picture of the Earth-moon system
Lunar librationBecause the Moon is accelerated in the forward direction, it moves to a higher orbit. As a result, the distance between the Earth and Moon increases, and the Earth's spin slows down (figure 3). Measurements reveal that, at present, the Moon's distance to the Earth increases by 38 mm per year (lunar laser ranging experiments with laser reflectors are used to determine this). Atomic clocks also show that the Earth's day lengthens by about 15 microseconds (µs) every year.

This effect is expected to continue until the spin of the Earth has slowed to match the orbital period of the Moon. At that point the tidal effect of the Sun will dominate, further slowing the Earth and thereafter causing the orbit of the Moon to steadily shrink. However these effects would not be expected to occur until long after the Sun has become a red giant.

However, the formation of tidal bulges on Earth is irregular and not directly related to the frictional energy loss which accompanies the tides. For example, continents on Earth may cause an increase in frictional energy losses and hamper the buildup of tidal bulges (figure 4).

The lunar surface is also subjected to tides from earth, and rises and falls by around 10 cm over 27 days. The lunar tides comprise a mobile component, due to the Sun, and a selenographically fixed one, due to Earth (the Moon keeps the same face turned to the Earth, but not to the Sun). The vertical motion of the Earth-induced component comes entirely from the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would be solar tides only. The magnitude of the Moon's tides corresponds to a Love number of 0.0266, and supports the idea of a partially melted zone around its core. Moonquake waves lose energy below 1,000 kilometres in depth, and this may also show that the deep material is at least partially melted. The Earth’s Love number is 0.3, corresponding to a movement of 0.5 metres per day; for Venus the Love number is also 0.3.[5]

Answer 2

You have to look at the bigger picture. Like the size of the moon and the size of the earth. Oh, and the SUN. The tides that are caused here have little to do with the gravity ON the moon and more to do with the gravitational pull of the planets in general. The planets are all happily floating around the sun in the Sun's gravitation pull and it's a good thing really... otherwise the moon might just slam into the earth or the earth might float into the sun.. which would really suck.... Imagine two magnets if you stick them almost together you get that bouncy little feeling between the two.. well that's sort of what the moon and earth do because of the gravity of the sun. That little bounce causes the tides. I am not a science major, but I think I have the general idea right. Hope this helps.

Answer 3

How Does Gravity Cause Tides