April 2, 2021
Moons Needed
Originally published August 14, 2011
images by Chris Kotsiopoulos, Sounio, Greece
I have observational impediments. My old house is on Main Street of a small American town.
Directly in front of the house is a sidewalk and then a busy street. I have no side yards, and
the back yard is hemmed in by two large, beautiful but sky-blocking trees to the west, and a
view of nearby roofs to the east. Opportunities to view the Moon cover an arc of about 45°.
I can't see the Moon during days 1 to 3 of a lunation, and once it reaches about 12 days old
it is too far east to be observed at a reasonable hour. I could get a lot more observing accomplished if Earth had two moons, not just one. Here is one arrangement that would work pretty
well - a full Moon rises in the east at sunset while a crescent prepares to set in the west. These
two moons might be more dynamically stable if they were exactly opposite in their orbits,
resulting in one always being visible in the sky. Of course, Luna2 would have had a somewhat
different history, although many things might be similar. Recent studies have suggested two
or more proto-moons might have been formed by the big whack, so they would start the same
and probably have similar impact histories, but each would differ in the details. It would be
more interesting if Luna2 formed in the outer asteroid belt and had been perturbed inward by
a growing Jupiter and got captured by Earth. Its composition would have been richer in carbon-
aeous materials and even water ice, buried under an insolating regolith, resulting in comet-like
degassing due to impacts and solar heating. If it were nearly exactly in a equatorial orbit, like
Phobos, then we would have lunar and solar eclipses every month - think how that would have
impacted religion. Maybe though, it would be easier for me to move out into the countryside.
Chuck Wood
Technical Details
Images from LPOD PhotoGallery
Related Links
Chris' GreekSky website
Yesterday's LPOD: First Light
Tomorrow's LPOD: A Better View
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