Quirks and Quarks

If the moon had oceans, what would its tides be like?

The Earth would raise tides on the moon's imaginary oceans 22 times greater than the tides of moon on the Earth.
The moon rises over the Chugach Mountains in 2012 in Anchorage, Alaska. The location of the moon in its orbit and landscapes on Earth affect tides on this planet. (The Associated Press)

We know the moon has a tremendous impact on the Earth's tides. Laughlin Eddy in Merritt, B.C., asks: "What would happen, hypothetically speaking, if the moon also had enough liquid water to have tides? What impact would the Earth have on the moon's tides? Could it change the Earth's tides in any way?"

To answer the question, assume the moon has the same mass and radius as it does now, said Judith Irwin, a professor of Physics and Astronomy at Queen's University in Kingston, Ont. To create tides, the moon would need to have continents and oceans like the Earth does.

Everything else being equal, the tides raised by the the Earth on the moon would be 22 times greater than the tides of moon on the Earth, Irwin calculated. She said that means if you get one or two feet of height from the tides then on the moon then that would be 22 to 44 feet higher at the moon's tide or nearly seven to 13 meters higher. 

One important difference is that since the moon always puts the same face towards the Earth, there would be a permanent bulge on the lunar oceans on the Earth-facing side. 

There is, however, one change to consider: the orbit of the moon around the Earth isn't perfectly circular. It is a slight eclipse. Over the course of a month the moon moves from closer to the Earth (perigee) to farther away. That difference would affect the height of the moon tide as the tides would be higher, the closer the moon and Earth were. It's a difference that you would notice and it's fairly significant, Irwin said.

As for the moon's tides changing the Earth's tides, there shouldn't be any direct effect. It's more local conditions on the Earth and the location of the moon in its orbit that matters.