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1. The distance to the moon is most accurately measured by laserranging; using reflectors placed on the moon during the Apollo
program to reflect laser beams fired from earth.
2. Mercury is seldom seen with the naked eye because it is so close
to sun that is usually hidden in the sun’s glare.
3. Early astronomers probably thought Mercury was two planets
because of the speed with which it appeared on either side of the
sun.
4. The average molecular speed of atmospheric gases exceeds the
escape speed of both the moon and Mercury, thus neither can
hold an atmosphere.
5. Lunar maria appear smoother, flatter and darker than
surrounding lunar highlands, and from a great distance bear a
resemblance to bodies of water.
6. Mercury and the moon look very similar, they each are small &
have essentially no atmosphere.
Differences: Mercury has warmer sun side than the moon, it
doesn’t appear to have maria-like features, and it does have scarps.
7. Synchronous orbit means moon rotates once per orbit; Earth’s
gravity deformed the moon slightly, and continues pulling on that
deformity. The moon is said to be “tidally locked” to the earth (as
are most moons in the solar system).
8. 3:2 orbit spin resonance means Mercury rotates two full
times during every three orbits; Mercury could not establish a
synchronous orbit (as the moon did) because Mercury’s orbit is too
elliptical.
9. A scarp is a “wrinkle” (but if you were there it would look like
a cliff!) in the surface of Mercury, caused when the planet’s
warm surface cooled and shrank. Scarps cut across craters, which
means scarps formed AFTER the craters had formed.
10. Meteorite impacts are the only real sources of “erosion” on
the moon; with no atmosphere or running water most surface
features are there forever unless disturbed by an asteroid impact.
11. As of the publication date of this book, ice evidence is
circumstantial: Lunar Prospector detected large amounts of
hydrogen at the lunar poles in 1998. Today we have found craters
that reflect light in dramatically different way than other craters do.
Also, when we have crashed satellites onto craters and the impact
plumes: ice is apparent.
12. The maria have FAR fewer craters, and these craters
generally are smaller than those in the highlands. Since large
craters are rare, for the highlands to have many of them
indicates great age.
13. The moon and Mercury have no atmosphere to moderate daily
temperature ranges the way earth does. (Earth ranges from 180 K
to about 330 K = -90˚C to 60˚C)
14. Both moon & Mercury have been geologically dead for several
billion years. The moon has a small iron core while mercury’s core
is believed to be mostly iron.
15. The impact theory of Moon Origin is currently in vogue among
astronomers and astrogeologists: A Mars-sized object collided with
earth when earth was young; the material that was torn loose from
the impact (literally) gravitated back together to form the moon.
16. The moon orbits the earth so lunar observers would see the
earth go through phases; crescent, quarter, gibbous, full and new.
17. The best part of the moon to observe with a telescope or
binoculars is the terminator line. The long shadows in this area
highlight the varied texture of the moon’s craters, maria and
highlands. If you were standing on the moon’s terminator (or the
terminator of ANY body, by definition) the sun would appear on
the horizon. On earth, you are on the terminator twice daily, at
dawn and at dusk.
18. Were I to establish an observatory on the moon, I would want
the least solar glare, so I would chose the bottom of a polar crater,
where the sun never reaches. Any other location would have the
sun in the sky for 14 earth days at a time, but of course the bottom
of a polar crater on the moon is one of the coldest places in the
entire solar system!
19. At midnight in the night sky, the sun is under your feet, on the
other side of the earth! Mercury never gets more than 28˚ away
from the sun (a bit more than pinky-tip to thumb-tip on a fully
spread hand), so there’s no way you could ever see it high
overhead in the night sky.
20. The lava that came to the surface to form the smooth maria did
so on the near side because the crust on the near side was much
thinner.