Over the next several hundred million years, Earth could face many unforeseen consequences, all because of the slowly but surely developing distance between the planet and its only moon.
In what will be defined as one of the most precisely known physical quantities in all of contemporary planetary science, the Moon is moving away from Earth at a speed of 3.83 cm per year, with a formal uncertainty of about 0.09 mm.This number is based on the current best interpretation of five decades of lunar laser ranging data. It has been measured directly, thousands of times, from multiple observatories on three continents, by firing laser pulses at reflector arrays left on the moon by Apollo astronauts between 1969 and 1971.On the human time scale, the rate is roughly the rate at which human fingernails grow. Over the course of a human lifespan, it accumulates to approximately three metres. Over the course of recorded human civilization, nearly five thousand years, it has accumulated to approximately 190 metres. In these timelines, the Moon sits, for all practical purposes, where it has always sat. But on geological time scales, the picture is different and quite troubling.
Measurements
The moon is waning away and the value has been strong to sub-millimeter accuracy for about three decades.
Apollo 11 astronauts Neil Armstrong and Buzz Aldrin deployed the first laser reflector array on the lunar surface in July 1969, in the Sea of Tranquility. The array contained one hundred cubic prisms of quartz glass, small pyramidal reflectors with the specific optical property that any incident light ray bounced exactly along its incident direction. The Apollo 14 and Apollo 15 missions deployed additional arrays in 1971.
The Apollo 15 array, the largest, contained three hundred angular cubes and remains the backbone of modern measurement campaigns. The two Soviet lunar modules, Lunokhod 1 and Lunocode 2, carried their own French-made reflector panels.According to a summary of the ongoing program published by NASA’s Jet Propulsion Laboratory, four observatories currently perform routine laser measurement of the Moon: one in New Mexico, one in France, one in Italy, and one in Germany.
The most precise of these is the lunar laser ranging of the Apache Point Observatory, or Apollo, at the 3.5-meter telescope in southern New Mexico. According to UC San Diego’s own Apollo project documentation, range resolution is now about a millimeter in the distance between Earth and the Moon, based on timing the round-trip travel of a laser pulse with an accuracy of a few picoseconds.According to a report by Science Blog, this measurement comes back every year with a slightly larger number. The moon is waning away and the value has been strong to sub-millimeter accuracy for about three decades.
Why yet?
The physical process behind stasis has been explained as tidal friction, which acts through a specific combination of Earth’s rotation, the Moon’s gravity and the geometry of Earth’s oceans and crust.
The Moon’s gravity is stronger on the side facing it from Earth than on the far side, because gravity decreases with distance. The differential drag of the land extends into a thin oval, with tidal bulges on the near and far sides.
In Earth’s oceans, these bulges are what produce the tides. In the Earth’s rocky interior, the same process produces smaller but measurable tides in the solid crust.Furthermore, the Earth rotates faster than the Moon’s orbit. One Earth cycle takes 24 hours, while one lunar cycle takes 27.3 days.
As the Earth rotates on its axis, it pulls the tidal bulges slightly forward from the line dividing the center of the Earth and the Moon. The bulge near the Moon is not located directly. It is located slightly in front of the Moon in the direction of the Earth’s rotation. This off-axis bulge exerts a small gravitational force on the Moon in the forward direction of its orbit.
With the added energy, the orbit is expanded. Thus, the Moon rises to a higher orbit every year.By conserving angular momentum, the energy gained by the Moon has to come from somewhere, and it comes from the Earth’s rotation. The Earth is slowing down. The day is getting longer. The moon is waning.
What will happen to the Earth when the Moon goes farther?
Longer days: The first observable consequence of lunar stasis is that the Earth’s rotation day becomes longer at a rate of one to one millisecond per century. Based on the evidence accumulated from those records, the Earth’s day was about 19 hours long about a billion years ago, when the Moon was closer and tidal friction was likely weaker but the geometry was different.
The day has gotten longer since then, and will continue to get longer as long as the moon continues to wane.No more total solar eclipse: A total solar eclipse on the planet is only possible because the Moon appears at roughly the same angular size in Earth’s sky as the Sun. The Sun’s diameter is about four hundred times larger than the Moon’s diameter, but the Sun is also about four hundred times farther away.
The two shapes cancel out almost perfectly, resulting in a specific geometric coincidence that allows the Moon to sometimes completely cover the Sun’s disk.
But as the Moon recedes, its angular size will decrease and so will a total solar eclipse. According to the best current calculations, the last total solar eclipse observable from Earth’s surface will occur about 600 million years from now. After that, the moon will appear so small in Earth’s sky that it will not completely cover the disk of the sun.Weak tides: The amplitude of the tides produced by the Moon on Earth’s oceans will decrease over time. Spring tides and low tides will continue to exist, but the range between them will shrink as the Moon’s gravitational grip on Earth weakens with distance. The specific rhythms that shape coastal ecosystems, marine reproduction, and fisheries will slowly weaken.The moon is leaving; It has been like this for the past four and a half billion years and will continue to recede away from itself as long as the Earth continues to rotate.