Chapter 6 6-1, 6-4 thru 6-6 The Earth- Moon System © 2007 Jones and Bartlett Publishers Courtesy of NASA/JPL/Northwestern University

6-1 Measuring the Moon’s Distance and Size The Distance to the Moon 1.Using parallax, Ptolemy determined that the distance from the Earth to the Moon is 27.3 Earth diameters— close to the correct average distance of Since the Earth’s diameter is about 12,800 km, thirty Earth diameters puts the Moon at about 380,000 km from Earth.

The Size of the Moon 1.Angular size of the Moon is close to 0.5°. 2.The Moon’s apparent diameter depends on its distance from the observer and on its angular size. Figure 6.04: The Moon’s angular diameter is about 1/2 degree.

The Small-Angle Formula 1.The diameter (width) of an object is directly proportional to its angular size and its distance from the observer. 2.Small angle formula is accurate for angles less than 5°. 3.Small angle formula yields a value of 3,480 km (2,160 mi) for Moon’s diameter. Figure 6.05: The angular size and diameter of an object are related to its distance from the observer.

Summary: Two Measuring Techniques 1.The triangulation (parallax) method relies on the relationship among size of the baseline, angle of parallax, and distance to the object. 2.Another important relationship exists among angular size, actual diameter, and distance.

The Moon’s Changing Size 1.Larger apparent diameter of the Moon occurs at perigee—the point in the orbit of an Earth satellite where it is closest to Earth—which is at a distance of 363,300 km. 2.Smaller apparent size of the Moon occurs at apogee—the point in the orbit of an Earth satellite where it is farthest from Earth—which is at a distance of 405,500 km.

Figure 6.08: Moon near to and far from Earth Courtesy of Galileo Project, NASA

Question 1 (6-4 thru 6-6 PPT Questions) Because Ptolemy lived long before instant distant communication was possible, he was not able to coordinate his observations of the Moon with someone far around the Earth. Propose a method by which he might have been able to observe parallax of the Moon.

6-4 The Moon’s Surface 1.The surface of the Moon can be divided into maria and mountainous, cratered regions. 2.Mare (plural maria) are any of the lowlands of the Moon or Mars that resemble a sea when viewed from Earth. 3.Most craters on the Moon are the result of impacts by meteorites—an interplanetary chunk of matter that has struck a planet or moon.

4. Earth has few impact craters because its atmosphere keeps all but the largest meteorites from reaching the surface. Over time, erosion and tectonic plate movement has erased all but a relative few of the largest craters. On the airless Moon, Mercury, satellites of other planets, and even asteroids, craters remain intact and visible for billions of years.

Figure 6.26b: A close up of Tycho shows that it has a prominent central peak Photo courtesy of UCO/Lick Observatory

5.Lunar ray is a bright streak on the Moon caused by material ejected from a crater. 6.The Moon’s maria are the result of volcanic action leading to massive lava flows. 7.The Moon’s crust ranges in depth from 60–100 km and is thinner on the side facing the Earth. 8.Mountains on the Moon are the result of extensive cratering over eons. Photo courtesy of UCO/Lick Observatory Figure 6.26a: Light- colored rays can be seen radiating from the Tycho crater on the Moon.

8.Mountains on the Moon are the result of extensive cratering over eons. 9.The Moon’s density is 3.35 g/cm 3. Its core, if composed of iron, must be small. 10. The Moon’s weak magnetic field—10 −4 times that of Earth’s magnetic field—suggests the presence of a small iron core, though this has not been confirmed. 11.Sensors on the Moon have detected very weak natural moonquakes.

Question 2 (6-4 thru 6-6 PPT Questions) Why do you think craters on the Moon could only be from meteorite impacts and not from extensive volcanic activity?

6-5 Theories of the Origin of the Moon 1.Evidence indicates that the Moon formed about 4.6 billion years ago. 2.According to the double planet theory, first suggested in the early 1800s, the Earth and Moon formed at the same time from the same rotating disk of material. The different densities of the Earth and Moon seem to rule out this scenario.

3.According to the fission hypothesis, the large basin of the Pacific Ocean is the place from which the Moon was ejected. This theory cannot explain the Moon’s current orbit nor offer an adequate rationale for what force could have caused the Moon to be torn from the Earth. 4. According to the capture theory, proposed early in the 20th century, the Moon was originally solar system debris that was captured by the Earth’s gravitational field. Dynamically, a third object is required for capture, and the chance of this happening with the Moon and Earth is remote at best.

5.The Moon’s chemical composition is similar to that of the Earth’s crust, but the Moon has smaller proportions of volatile—easily vaporized— substances than the Earth.

The Large Impact Theory 1.According to the large impact theory, proposed in the 1970s, the Moon formed as the result of a glancing impact between a large Mars-sized object and the Earth. This theory can explain the relative compositions of the Earth and Moon, the Moon’s orbit, and the Earth’s rotation rate. This theory has also been successfully modeled on a supercomputer. Figure 6.30

6-6 The History of the Moon 1.Radioactive dating techniques on the 840 pounds of Moon rocks brought back to Earth by the Apollo astronauts have been indispensable in forming a model of the Moon’s history. 2.The Moon formed about 4.6 billion years ago. 3.Most craters formed between 4.2 and 3.9 billion years ago. Giant impacts near the end of the cratering period formed the maria.

6-6 The History of the Moon 4.After cratering ended, the Moon’s interior became hot from radioactive decay and molten lava flowed, ending about 3.1 billion years ago. The Moon has probably remained relatively unchanged since then. 5.Micrometeorites (tiny meteorites) still hit the Moon, but no new large crater has ever been observed.

Figure 6.B04: Astronaut collecting lunar rocks Courtesy of NASA/JPL-Caltech

Figure 6.32a: Lunar eclipse on May 5, 2004 © Kyodo/Landov

Question 3 (6-4 thru 6-6 PPT Questions) Name and describe three different theories of the formation of the Moon. Which of these theories is considered most likely correct? Describe the evidence that leads us to that conclusion.