The Physics
An encyclopedia of scientific essays

Diameter of the Moon

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Bibliographic Entry Result
(w/surrounding text)
Beichner, Robert J. and Serway, Ramond A. Physics for Scientists and Engineers with Modern Physics. Orlando, Florida: Saunders College Publishing, 2000.
Body Mass (kg) Mean Radius (m)
Moon 7.36 × 1022 1.74 × 106
3.48 × 106 m
Gorenstein, Paul. World Book- "Moon." Sydney, Australia: World Book Inc, 1992: 782-795. "The moon measures about 2160 miles (3476 km) across. The distance is about one fourth the diameter of the Earth, and 400 times smaller than that of the sun." 3.476 × 106 m
Arny, Thomas and Panindes, Nicholas A. Introductory Astronomy 2nd Edition. USA: Addison Wesley Publishing Company Inc, 1979: 115. "Solving for d in the proportion d/2πr = 0.5°/360°, gives us the moon's diameter -- about 2162 miles (3479 km)" 3.479 × 106 m
Moore, Patrick and Tirion, Wil. Cambridge Guide to Stars and Planets. United Kingdom: Reed International Books Limited, 1993: 23. "The moon is officially ranked as the Earth's satellite, but since it is relatively large and massive with a diameter of 3476 km and a mass of 0.012 that of the Earth, it may better be regarded as a companion planet." 3.476 × 106 m

The moon is Earth's nearest neighbor in space. Since ancient times, people have measured time by the phases of the moon, from new moon to new moon again (about 29½; days). However, people also thought the moon was a goddess or god (Luna and Diana were the "moon goddesses"), powerful enough to control humans and their actions. To this day, many countries use the moon to determine their months of the year through use of a lunar calendar.

The moon orbits the earth, and completes one rotation after 27 and a third days. The distance between the Earth and the Moon is 384,385 km. In comparison to the Earth however, the moon's weight and diameter are much smaller. The weight of the moon is 1/81 that of the Earth, and its diameter is ¼ of the Earth's. According to the data, the diameter of the moon is relatively centered, at about 3,479 km. This can be determined by a very simple proportion setting circumference equal to 360°, and the diameter of the moon equal to 0.5° (as this is how much of an angle the moon occupies):

(d/2πr) = 0.5°/360°


r = 3.844 × 108 m
d = diameter of the moon

Solving for d gives 3.479 × 106 m

On the moon itself, no life exists, and there is no wind, air or water. While during the day, temperatures rise to over 173 °C and at night, temperatures drop way below freezing.

People have been fascinated by the moon ever since 1609, when Galileo formed a crude telescope to examine it. While as time progressed, drawings of the lunar surface improved, it was never fully explored. When the NASA space program began in 1958, space exploration became a reality. On July 20, 1969, Neil Armstrong became the first human being to land on the moon with Apollo 11 and explore it. Since then more information has been gathered on the moon (i.e., its density, formation, and effect on earth), and experiments on it are still being conducted to this day.

Samuel Bernard -- 2002

Bibliographic Entry Result
(w/surrounding text)
Beichner, Robert. Physics For Scientists and Engineers. Orlando: Saunders College, 2000. 433
Body Mean Radius (m)
Moon 1.74 × 106
3.48 × 106 m
"Moon." The New Encyclopedia Britannica.1997. "The Moon is less than one-third the size of the Earth, having a diameter of only about 3,476 km (2,160 miles) at its equator." 3.48 × 106 m
Spudis, Paul D. The Once and Future Moon.Smithsonian Institution, 1996. 22. "The radius of the moon is 1738 km, whereas the radius of Earth is 6371 km; thus the moon is relatively large in relation to its parent planet." 3.48 × 106 m
Moore, Patrick. The Moon. Mitchell Beazley Publishers, 1981. 6. "It has a diameter of 3,476 km, over a quarter the size of the Earth's diameter, and a mass of 7.3483 × 1022 kg, as compared with a figure roughly 81 times larger for the Earth." 3.48 × 106 m
Science and Technology Desk Reference. Comp. James E. Bobick. Carnegie Library of Pittsburgh, 1996. 609. "The Moon's diameter is 2,159 mi (3,475 km) and its circumference is 6,790 mi (10,864 km).The moon is 27% the size of the earth." 3.48 × 106 m

As the only natural satellite of the Earth, the Moon has always captured the attention of scientists across the world. Aristarchus, (circa 240 BC) often referred to as the "Copernicus of antiquity", was the first to estimate the size of the Earth, the size and distance to our Moon, and the size and distance of our Sun. His ideas were truly radical for his time period, but were eventually accepted by future scientists.

Aristarchus applied modern geometric methods in order to measure the size of celestial bodies such as the moon.

S = Dθ

Where S is the object's diameter, D is the object's distance, and θ (theta) is the angular size of the object in the sky.

By observing a lunar eclipse, Aristarchus found that the Moon moved across the sky an amount equal to 2.5 times the Moon's angular diameter. Consequently, the Earth's shadow on the Moon loses a distance equal to the Moon's diameter. He then concluded:

2½ Moon Diameter = Earth's Diameter − 1 Moon Diameter
Earth's Diameter = 7/2 Moon Diameter
Moon's Diameter = 2/7 Earth's Diameter

Substituting the Earth's diameter into the equation reveals an answer of 3.64 × 106 M. This answer does not match the figure that my sources use, but is very close. The slight deviation is most likely the result of inaccurate tools and measurements.

Richard Lau -- 2002

Bibliographic Entry Result
(w/surrounding text)
Earth Science. Glencoe McGraw Hill, 2002: 753. "Radius (km): 1737.4" 3474.8 km
World Atlas. USA: Rand McNally & Co., 1968: XXIV. "In comparison to our planet, the moon is considerably smaller; its diameter is only 2,160 miles." 3476.2 km
Moore, Patrick, & Wil Tiron. Guide To Stars And Planets. Cambridge, UK: Cambridge University Press, 1997: 24. "The moon is officially ranked as Earth's satellite, but since it is relatively large and massive, with a diameter of 3,476 km, and a mass of 0.012 that of Earth, it may better be regarded as a companion planet." 3476 km
"Moon." The Columbia Viking Desk Encyclopedia. New York: Viking Press, 1953: 838. "Moon, Earth's only satellite (diameter c.2,160 mi)" 3476.2 km
Moon: Physical Characteristics. The Columbia Electronic Encyclopedia, 2000. "The moon's diameter is about 2,160 miles, somewhat more than ¼ the Earth's diameter." 3476.2 km

The moon has been a mystery to man for centuries. Thankfully, in more recent years, we have had the ability to explore this vast world of huge mountain peaks, enormous valleys, and gigantic craters that satellites our planet. The moon which measures 3,476 km, is about ¼ the size of Earth. In comparison to the largest moon in our solar system, Ganymede, which orbits Jupiter and has a diameter of 5,262 km, and the smallest major moon in our solar system, Miranda, which orbits Uranus and has a diameter of 472 km, our moon is considered to be rather large. It takes 27 1/3 days for the moon to fully orbit the Earth's barycenter (center of gravity), and contrary to popular belief, it rotates in an elliptical orbit, not circular.

Most of the characteristics we know about the moon are things that Galileo determined. In 1610, he invented the telescope and studied the moon's various visible characteristics. Since then, science and technology have allowed us to actually send man into space to walk on the moon. Perhaps one of the more interesting things I found out is that even the older books, published before 1969, when the first man landed on the moon, had the correct measurement of the moon's diameter. This shows that most of the observations made about the moon were made through telescopic use, so that most of what we know about the moon can indeed be attributed to Galileo.

Alyssa Brown -- 2002