Earth’s Nightlight

As a naturally orbiting object, the Moon orbits Earth in an elliptical path, with the center of the Earth at one focus – more precisely, both Earth and Moon orbit each other around what it called the barycenter, the imaginary point about 1800 km below the surface of the Earth that constitutes their mutual center of gravity.  Since the Moon is only about one percent the mass of Earth, the barycenter is much closer to the center of Earth than it is to the center of the Moon.

When the Moon comes closest to Earth in its elliptical orbit it is said to be at perigee.  If the Sun, Earth and Moon come into alignment along a straight-line, a condition occurs that astronomers perversely have named syzygy (a great word to keep in your hip pocket the next time you play Scrabble, though you’ll need a blank to get there).  Syzygy (alignment) is not the same as perigee (the closest approach of Moon to Earth) but on the occasion when syzygy and perigee coincide, we have what’s called a “Super Moon.”

During perigee, the Moon’s elliptical orbit causes it to be about 45,000 km closer to Earth than at farthest point (apogee).  As the average distance between the two is about ten times that distance, the visual effects of this variation, though not large, is measurable.  I’ve been skeptical about noticing this size difference by “eyeballing” the Moon at perigee (closest) and apogee (farthest).  However, during an early morning walk with the dog this last weekend, I was somewhat startled to see the full Moon low in the sky, definitely appearing larger than usual.

In part, this appearance results because of the “Moon illusion,” whereby the Moon appears much larger on or near the horizon than when it is overhead, near zenith.  The traditional explanation for this illusion is that when the Moon is near the horizon, we can compare the size of the Moon’s apparent disk to known objects on the Earth (such as a house, distant tree or hill).  When the Moon is directly overhead, there is no nearby object with which to compare it.  Many depictions in art show the Moon as an enormous lunar disc, glowing the night sky; it is to this optical illusion that such portrayals refer.

The Moon’s apparent diameter is about one-half of a degree of arc (same as the Sun), or roughly the dimensions of a small pea held at arm’s length.  Although the biggest object in our sky, that size is much too small for the naked eye to resolve most surface features (except for the vague markings of light and dark that comprise the lunar maria, the “Man in the Moon”).  In full phase, the Moon can be quite bright, illuminating the landscape at about -12 visual magnitude.  While no one would mistake such conditions with daylight (the Sun is about -26 visual magnitude, about 400,000 times brighter than the full Moon), full moonlight is bright enough to cast strong shadows and to read by.  This is one of the reasons astronomers “hate” the Moon – during full phase, the sky is typically too bright to reveal any but the very brightest stars and it interrupts their views of coinciding meteor showers.   However, they’ll “love” the views that await them from the far side of the Moon, the only place in our Solar System where radio noise from Earth is silent and at times, when Earth blocks the Sun, the sky-viewing would be unsurpassed.

The most important effect of a “Super Moon” is on tides, which can be extraordinarily high during perigee.  This effect can be especially significant in coastal areas that experience high tides, such as the famous Bay of Fundy in Canada.  In this area, the combination of shore depth and geometry, prevailing winds and position create tidal height variations as high as 16 meters (over 52 feet) in the course of a day.  At Super Moon, tidal variations are at their largest; during the passage of Hurricane Sandy up the East Coast last year, landfall occurred during full Moon (syzygy), resulting in both a storm surge (i.e., a large dome of water caused by low atmospheric pressure and wind) and high gravitational tides.  As witnessed with Hurricane Sandy, the combination of both occurring together can be devastating.

Tidal effects are most notable in large bodies of water, but the solid Earth also deforms in response to the pull of the Moon’s gravity.  A tidal bulge extends slightly above the mean radius of both Earth and Moon.  This bulge is not perfectly aligned with the geometric line that connects the centers of the two objects because both Earth and Moon are rotating, and it takes time for the solid bodies to deform plastically.  Thus, the tidal bulge of the rapidly spinning Earth slightly leads the Earth-Moon line, resulting in a constant increased tug at the Earth by the Moon, slightly slowing the rate of Earth’s rotation down.  At the same time, this leading tidal bulge attracts the Moon more, making it speed up in its orbital path slightly and thus, move outward, away from the Earth.  So over time, as the Earth spin rate slows, the Moon gradually recedes away from its grip; this rate of recession is about 4 cm per year.  The Moon is currently about 60 Earth radii away; it was once much closer, possibly as close as a few Earth radii.  It could not be closer than about 3 radii (the Roche limit) because at distances closer than the Roche limit, tidal forces would tear the Moon apart.  In a few hundred million years, the Moon will be too far away to permit a total solar eclipse to be seen from Earth.  A timely and good thing that we came along when we did!

Using information from a lunar seismic network deployed on the Moon during the Apollo missions, we know that “moonquakes” often correlate with the tidal flexing of the solid Moon induced by the Earth (which is much larger than the terrestrial bulge because Earth is much more massive).  In fact, although there is a slight suggestion that the Moon might induce the initiation of an earthquake, in most cases there is no obvious connection.  The Earth is an active, dynamic body and its great internal heat and complexity of configuration appear to be more important in determining when and where an earthquake occurs than by tidal effects caused by the Moon.  But if the proper tidal conditions and the alignment of stress and magnitude of effect coincided, there is no reason that either syzygy or Super Moon could not induce an earthquake.

Our Moon is much more than the familiar, comforting nightlight orbiting Earth.  Beyond touching us emotionally and affecting our planet physically, the Moon is also an orbiting treasure trove of, as yet unrealized (some imagined but mostly yet unimagined) scientific discoveries and technological breakthroughs.  But before we make it our goal to settle the Moon, we must make it our goal to sail beyond it.

Paul D. Spudis | READ MORE

Paul D. Spudis (1952-2018) was a senior staff scientist at the Lunar and Planetary Institute in Houston, Texas, and a prolific author on the subject of the moon. His books include The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources.