The Tidal Force – an essay by Neil deGrasse Tyson
November 1, 1995
Here’s a snippet:
“Meanwhile, Earth’s bulged gravity field, positioned slightly ahead of the Moon in its orbit, acts in return as an energy pump. Like the effect of rocking your legs in rhythm with a playground swing, the Moon slowly ascends into larger and larger orbits. You want proof? In 1969, when the Apollo 11 astronauts Neil Armstrong and Buzz Aldrin visited the Moon’s Sea of Tranquillity, they left behind (among other things) a series of corner reflectors that are designed to reflect light in exactly the same direction that it arrives. Starting shortly after the Moon landing, and continuing today in places such as the McDonald Observatory Laser Ranging Station in west Texas, high-powered lasers on Earth are beamed to the Moon, and the return signal is carefully timed.
Knowing the speed of light, one can compute the Moon’s distance with unprecedented accuracy: with a twenty-five year baseline of measurements we know that the Moon is spiraling away from us at a rate of about two inches per year, just as predicted by tidal theory. Earth’s rotation will continue to slow down, and the Moon will continue to spiral away until the Earth day exactly equals the lunar month. At that time, one Earth rotation will last over 1000 hours, which would require 4 million leap seconds per day. No need to panic just yet. You have over a trillion years to think about it.”