# How much do tides affect earth's rotational velocity?

Many of us are familiar with an exaggerated picture of the Moon's effect on Earth's tides:

Since the Earth is not uniformly covered with water, however, I imagine that the tidal effect will depend on which part of the Earth is facing the Moon (for simplicity, ignore the Sun's contribution). I'm sure that the tidal effect still affects the Earth's landmasses, but I'm guessing that it would be to a lesser degree (i.e.: the landmasses would bulge out less than the oceans in the image above). Now, by conservation of angular momentum, if the landmasses bulge out less, the Earth should rotate more quickly, hence this question's title.

How does the tidal effect of the Moon's gravity affect the Earth's landmasses differently from its oceans? And how big is this difference?

• Look up solid earth tides. The Earth does bulge but not as much as the oceans. About 20 cm on average, in places and at times over 30 cm but that's probably sun and moon together. (I'd post the link but it's giving me trouble, not hard to look up). Also, interesting point on rotation velocity/conservation of angular momentum, and apparently that happens too, brief summary here: bowie.gsfc.nasa.gov/ggfc/tides/intro.html Commented Jun 20, 2016 at 13:43
• The "bulge under the moon" model is probably too simplistic to use here, since the tidal wave is not uninterrupted around the planet. I guess the pertinent question is "Is the change of the planet's angular momentum over a tidal cycle more than a negligible fraction of the total angular momentum of the planet?" - whether that change be from differences in global mean sea surface elevation, or differences in speed or direction or whatever. I haven't done the maths, hence not a full answer, but my instinct is that the answer is "no". Commented Jun 21, 2016 at 9:26

This question is handeled pretty throughly in Physics site; https://physics.stackexchange.com/questions/214042/earths-kinetic-energy-change

Your question; "How big is the difference?"

Body Tides Summarize
0.3 ms for a 14 day period.
0.16 ms for a 28 day period
0.14 ms for 1/2 year period
0.02 ms for a year period.
0.126 ms for Lunar node period; (18.6 years)

Ocean Tides
Max 0.05 ms for a 14 days period
0.026 ms for a Years period
0.024 ms for lunar node period