Many people are unaware of the effect that the Moon’s gravity has on the apparent length of the day on Earth. Rest assured that there is a very large effect if one considers the masses being shifted by this patient, long-term force. Likewise, the effect of the Sun’s gravitational pull, though lesser than that of the closer satellite, is also meaningfully changing the configuration of the Earth-Moon system.
Tidal forces
The gravitational pulls that Earth experiences from its two most obvious neighbors are known as tidal forces. Naming a force usually helps in identifying its most obvious or its largest change points, and that is the case here. Tidal forces cause, as you might expect, the tides of Earth’s oceans from low to high and back again.
What is less obvious is that water is not the only object on Earth moved by these gravity effects: Both land masses and the atmosphere are also dragged into shapes approximating the liquid tides. People generally, unless they have very precise measuring equipment, are unaware of these latter mass motions.
Spring and neap tides
One should expect that any massive body in the solar system will exert some influence over Earth’s tides, and, to varying degrees, all do. These forces are much weaker than the effects of the Sun and the Moon, and there are two identified tidal types that have to do specifically with the competition between the far-distant force of the Sun’s gravity (“less than half that of the Moon”) and the much nearer influence of the Moon: spring and neap tides.
Spring tides occur when the Sun and the Moon are in direct line with one another, whether the Moon is on the far side of the Earth or on the same side as the Sun. Tides resulting from these forces cause the Earth’s mass to assume an egg shape with the main bulges in the same line with the astronomical bodies causing the movement.
Neap tides result when Sun and Moon are 90 degrees from one another with respect to the Earth. Bulges still occur in the Earth’s system (air, water and land), but the Moon “wins” the competition. The larger changes occur on the hemisphere of the planet facing the Moon and that hemisphere 180 degrees away from the Moon.
Gravitational locking
The friction associated with the tidal forces’ effects on air, land and water are very large, though not catastrophically large for those living on the surface of the Earth. Their observed effects will take millions of years to be visible to the naked eye, and perhaps billions to cause Earth to do what the Moon has: stop rotating altogether except to match the orbital period. This consequence of constant tidal forces, in other words, will one day cause Earth and Moon to present only one face to each other. As the Moon has been throughout human memory, so the Earth will someday be. “The present rate of change is that the Earth’s rotation rate is slowing by 16 seconds every million years and the distance of the Moon is increasing by 120 cm each year.”
The Earth seems very solid sometimes. Even at those times, its surface and the fluids (both liquid and gaseous) that cover it are in constant motion. This is natural, and most living things are simply unaware of its happening. It is, however, slowly changing the Earth-Moon-Sun system on a daily basis. Humanity is, at present, at little risk from this reshaping.
