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[IMAGE: Earthrise]

(4a) The Moon: A Closer Look

The View Through the Telescope


2a. The Sundial

3. The Seasons

3a. Angle
        of sunlight

4. The Moon (1)

4a. The Moon (2)

4b. Moon Libration

5.Latitude and

5a. Navigation

5b. Cross-Staff

5c. Coordinates

6. The Calendar

6a. Jewish Calendar


8. The Round Earth
    When Galileo became the first human to view the Moon through a telescope, our understanding of the Moon changed forever. No longer a mysterious object in the sky, but a sister-world full of ring-shaped mountains and other formations!

    Giovanni Riccioli in 1651 named the more prominent features after famous astronomers, while the large dark and smooth areas he called "seas" or "maria" (singular "mare," mah-reh). Some of the names he used for the Moon's crater are of persons discussed in "Stargazers"--Tycho (distinguished by bright streaks that radiate from it), Ptolemy ("Ptolemaeus"), Copernicus, Kepler, Aristarchus, Hipparchus, Erathosthenes; Meton and Pythagoras are on the edge, near the northern pole.

    Late-comers who lived after the 17th century had to make do with left-overs: the craters Newton and Cavendish are at the southern edge of the visible disk, Goddard and Lagrange too are near the edge. Also, "Galilaei" is a small undistinguished crater (because of Galileo's banishment?). However, since the Russians were the first to observe the rear side of the Moon, a prominent crater there bears the name of Tsiolkovsky, who at the end of the 19th century promoted the idea of spaceflight.

    Note: Strictly for moon junkies--all you ever wanted to know, perhaps even more. From Cambridge University Press (1999), Mapping and naming the moon: A history of lunar cartography and nomenclature by Ewen A. Whitaker, xix+242 pp., $59.95.

The Craters

What had created those strange round "craters"? ("Krater" is Greek for a bowl or wide-mouthed goblet.) They reminded some observers of volcanic craters on Earth, or better, of the large "calderas" (cauldrons) formed by the internal collapse of volcanos, e.g.
Crater Lake in Oregon. Others suggested that they were formed by the impact of large meteorites, but this was countered by the argument that most meteorites probably arrived at a slanting angle, and were expected to leave not a round ring but an elongated gouge.

[IMAGE: Lake Manicougan crater in Canada]  
We now know that the impact explanation was right. The craters are round because at the enormous velocities with which meteorites arrive, the impact resembles a local explosion, and the signature of the impact is determined by the energy released rather than by the momentum transmitted.

Part of the evidence has come from the nicely rounded impact remnants found on Earth, e.g. Meteor Crater (Canyon Diablo) in Arizona and Manicougan lake in Canada, in northern Quebec (picture on right), which is about 100 km (60 miles) wide and 214 million years old. Note that rather than having a pit in its center, the Manicougan lake has a round island. After the impact, the land rose again to the level of its surroundings, pushed by fluid pressure of the material below it, which acts like a viscous fluid and tries to establish equilibrium between the different loads which it supports. (For another picture of Lake Manicougan, and more about it, click here.)

Other solid bodies of the solar system also diplay round craters. On the large ice-covered moons of Jupiter, the return to equilibrium is much more pronounced, because ice sags and flows much more readily than rock. Those moons display "palimpsest" craters which are merely surface markings, because as time passed, the walls which originally existed sagged back onto the flat surface.

The Airless Moon

In the centuries after Galileo's discoveries, the Moon was extensively studied by astronomers using telescopes. One thing soon became clear: it had no atmosphere. When a star was eclipsed by the Moon, it vanished suddenly and its light showed no refraction or absorption by an atmosphere.

Why? By the laws of motion, the Moon orbits not the center of the Earth, but the center of gravity of the Earth and Moon (this will be discussed in section #11a, and the center of gravity is defined in section #25). The location of that point allows astronomers to deduce the mass of the Moon, and from that, the pull of the Moon's gravity. At the surface of the Moon, it turned out, gravity is only 1/6 as strong as at the surface of the Earth.

Gravity is important for the retention of an atmosphere. It holds an atmosphere down, while heat is what can make it escape.

Heat is atomic or molecular motion. In a hot solid or liquid, it can be viewed as a shaking motion of atoms or molecules around their average position, like the rustling of leaves in a wind. The higher the temperature, the more vigorous the motion, until the material boils or evaporates, at which point its particles shake loose altogether. In a gas atoms and molecules fly around randomly, colliding constantly (if the gas is as dense as it is in the atmosphere), and their collisions lead to a very good explanation ("the kinetic theory of gases") of the observed properties of a gas.

The average velocity of a gas molecule depends on the temperature of the gas, and at room temperature it is comparable to that of the speeding bullet, quite below the "escape velocity" needed for escaping Earth's gravity. However, that is just an average: actual velocities are expected to be distributed around that average, following the "Maxwellian distribution" first derived by James Clerk Maxwell, whom we meet again in the discovery of the three color theory of light (section #S-4) and the prediction of electromagnetic waves (section #S-5). According to that distribution, a few molecules always move fast enough to escape, and if they happen to be near the top of the atmosphere, moving upwards and and avoiding any further collisions, such molecules would be lost.

For Earth, their number is too small to matter, but with the Moon, having only 1/6 of the surface gravity, it can be shown that any atmosphere would be lost within geological time. The planet Mercury, only slightly larger, also lacks any atmosphere, while Mars, with 1/3 the Earth's surface gravity, only retains a very thin atmosphere.

Water evaporates easily and once in gas form, is quickly lost by the same process. That suggested the "maria" could not possibly be oceans, though their name remained. They actually turned out to be basaltic flows, hardened lava which long ago flowed out of fissures on the Moon; no present-day volcanism on the Moon has been reliably identified. The vast majority of craters probably date back to the early days of the solar system, because the lava of the maria has very few craters on it, suggesting it flooded and obliterated older ones.

The picture of a dry Moon was reinforced by Moon rocks brought back by US astronauts. Earth rocks may contain water bound chemically ("water of hydration"), but not these. Water, of course, would be essential to any human outpost on the Moon. Yet small amounts of water may still exist, brought by comets which occasionally hit the Moon. All this water is sure to evaporate in the heat of the collision, but some of it may re-condense in deep craters near the Moon's pole, which are permanently in the shade and therefore extremely cold. Observations by the "Clementine" spacecraft suggest that one such crater may indeed contain a layer of ice.

[IMAGE: First photo of far side]

In the Space Age

From the beginning of spaceflight, the Moon was a prime target, but this chapter in space exploration is too long to be covered here in any detail. The first spacecraft to reach the Moon were Luna 1, 2 and 3 of the Soviet Union, in 1959. Of these, Luna 3 rounded the Moon, took photographs of the far side which is not seen from Earth, and later scanned and transmitted those images (on the right); unfortunately, their quality was poor. In the decade that followed, 19 other Soviet missions were aimed at the Moon.

In 1970 a Soviet spacecraft landed and returned a rock sample, and later that year a remotely controlled "Lunokhod" vehicle was landed, exploring its surroundings for nearly a year. Other sample returns and Lunokhods followed, the series ending in 1976. However, failures marked tests of a large rocket developed for human Moon flights, ending any plans of manned lunar exploration by the Soviet Union.

Early attempts by the US to send unmanned spacecraft to the Moon (1958-64) either failed or returned scanty data. In July 1964, however, Ranger 7 returned clear TV pictures of its impact on the Moon, as did Rangers 8 and 9. Of the 7 "soft landers" in the "Surveyor" series (1966-8), 5 performed well and sent back data and pictures. In November 1969, after Apollo 12 landed 500 feet (160 meters) from the "Surveyor 3" lander, astronauts retrieved its camera and brought if back to Earth. In addition to the Surveyor project, 5 lunar orbiters photographed the Moon and helped produce accurate maps of its surface.

[IMAGE:Stepping onto Moon] On May 25, 1961, about one month after Russia's Yuri Gagarin became the first human to orbit the globe, US president John F. Kennedy proposed to the US Congress "that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to Earth. "

The Apollo missions followed, with Apollo 8 rounding the Moon in 1968 and Apollo 11 finally landing there, on July 20, 1969. Five other lunar landings followed, the last of them in December 1972. Only Apollo 13 failed to land, its crew members narrowly escaping with their lives after an explosion aboard their craft on the way to the Moon.

Achievements of "Project Apollo."

Among the activities of Apollo astronauts on the Moon were:

[IMAGE: Moonbuggy]

  • Bringing back to Earth extensive samples of lunar rock and soil. The rocks turned out to be ancient, suggesting no significant change since the surface of the Moon formed, about 4.5 billion years ago. The "soil" (regolith) had probably been pulverized by impacts, but as the "Surveyor" missions showed, it was firm enough to provide support.

  • Crews of Apollo 15, 16 and 17 explored the Moon aboard an electrically driven "moon buggy. " (picture on the right).

  • Extensive video pictures from the Moon were beamed to Earth--even one (by a remotely controlled camera) of the take-off from the Moon by the Apollo 17 crew. Also, the Earth and its "geocorona" of glowing hydrogen were photographed by a special camera using ultraviolet light.

  • A seismometer was placed on the Moon, showing that the Moon was seismically much quieter than Earth.

  • Metal foils were hung out (like a flag) to receive the solar wind. They were then returned to Earth where the composition of the ions caught in them was analyzed.

  • Corner reflectors were placed on the Moon, so that laser beams reflected from them could accurately measure the distance.

    No humans have visited the Moon from 1972 until now, but some orbital missions have studied the Moon's magnetic field as well as X-ray and gamma-ray emissions, from which some variations of the surface composition could be inferred.

    The Moon was found to have no global magnetic field like the Earth, but its surface was weakly magnetized in some patches. Molten rock can become permanently magnetized if it solidifies in the presence of an external magnetic field, suggesting that in some ancient era the Moon, like Earth now, had a molten metallic core in which electric currents generated a magnetic field. Somewhat similar observations were made near Mars in 1998-2000.

    Some excitement was caused by indications from the Lunar Prospector spacecraft, which suggested that ice may exist on the moon, inside a deep crater near the Moon's south pole. A possible explanation was that some time in the past (perhaps long ago) a comet had crashed into the Moon, and comets contain considerable amounts of water ice. The energy of the impact, turned into heat, would of course evaporate the ice. However, some of the water vapor would form a temporary atmosphere around the Moon, and might condense again to ice in very cold locations, like craters near the pole, which are permanently shaded from sunlight.

    At the end of its mission, on July 31, 1999, Lunar Prospector was therefore steered to deliberately crash inside the crater. It was hoped the impact might create (briefly) a cloud of water vapor, which could be observed from Earth, but none was detected.

    There is little doubt that the future will see further lunar exploration, though a "lunar base" is probably far off. Astronomical and other observations can readily be made from Earth orbit, and providing life support on the Moon is not easy. Such a base will probably become attractive only after ways are developed for utilizing local lunar materials for construction and for fuel.

Exploring Further

Entire books about "Project Apollo" can be found of the web. Some good ones:

You can also watch about 40 film selections of NASA history, including John F. Kennedy's 1962 speech "We choose to go to the Moon" and many clips from the US Moon landings.

A timeline of lunar landings with links to further details of the missions.

A site about impact craters on Earth.

Questions from Users:  
Was the Moon landing a hoax?
                Also asked:  Why does the Moon have the bigger craters?
                One more:  Have any changes been observed on the Moon?.
                        And:  Does the temperature reach absolute zero during lunar night?.
                        ***  What if Earth had no mountains?.
                    ***  US Flag on the Moon

Next Regular Stop: #5. Latitude and Longitude

Optional excursion: #4b Librations of the Moon

            Timeline                     Glossary                     Back to the Master List

Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   audavstern("at" symbol) .

Last updated: 12.13.2001