Thank you for mooning us!

In anticipation of the solar eclipse about cut a swath across America, it is a good time to take a look at our friend the moon. In doing so we can ask the metaphysical question: was it luck or was it providence?

First of all we note that we should not at all take solar eclipses for granted. It requires something that there is no a priori reason to expect: the sun and moon are the same size in the sky. Our moon is 400 times smaller than the sun. The sun, conveniently, is 400 times farther away. That makes them the same apparent size and voilà!  perfect eclipses. No other planet in our solar system has perfect eclipses.

The moon orbits around the earth in a plane different from the earth (and the planets) about the sun, a clear sign that the moon was not formed at the same time as the earth. (If it were in the same orbital plane, we'd have eclipses  once a month!)

In addition to the undeniable aesthetic quality of a perfect eclipse, it is of significant scientific benefit. A solar eclipse provide the first great test of Einstein's General Relativity, for it allowed us to see a star behind the sun. The gravity of the sun, as Einstein predicted, bent the starlight like a lens. The test required an eclipse because normally the sun is (of course) too bright to see the feeble starlight.

Furthermore, the study of the chromosphere, made possible by solar eclipses, has been instrumental in the advancement of the field of astrophysics.

But Wait, There's More!

Without our moon it is not at all certain that the earth could support complex life. Consider:

• Generally, the greater a planet’s gravity and distance from the sun, the thicker its atmosphere. Earth violates this rule, with an atmosphere 40 times lighter that Venus.

• Usually moons are much smaller than the planet and are formed of the same material. Earth has a huge moon which is not made of the same material as the earth.

What happened? It would seem that object the size of Mars collided with the young (250 My) earth, and was mostly absorbed into the core. The collision blasted most of the original overly-thick atmosphere into space. The cloud of debris coalesced into the moon. ¹

What are the fortuitous results of this cataclysm? Well, the collision:

• Destroyed a thick, poisonous atmosphere.

• Increased in the earth’s mass just enough; its gravity can retain necessary water vapor (mass = 18), but not the lighter poisonous ammonia (mass = 17) or methane (mass = 16).

• Boosted iron content of the earth (magnetic field) and made the oceans nutrient rich. Without the earth's magnetic field we'd be dead. The magnetic field deflects dangerous radiation around the earth.

• Slowed the earth’s rotation rate, which stabilized weather patterns.

• Because the resulting moon is big, its tidal effect cleans coastal waters and replenishes ocean nutrients.

• However the moon is not too big –no excessive erosion, no excessive alteration of earth’s orbit.2

• The moon also stabilized the tilt of the earth’s axis, preventing climactic extremes.

No matter how you look at it, we are so lucky to have been mooned.

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¹ Belbruno and Gott, The Ast. J., 129, 1724-1745, 2005 

² A fairly recent calculation shows that if the moon were slightly bigger, the earth-moon system

would go unstable.  See Dave Waltham, Astrobiology 4, No. 4: 460-468 (2004)