Facts of the Matter

Richard Brill

The moon has
a fascinating role
in everyday life

The moon has been source of boundless fascination throughout recorded history because is it the biggest and brightest thing in the night sky, it changes its location in the sky faster than any other object, and its changing phases distinguish it even further from all other celestial objects. There is little doubt that our most ancient, prehistoric ancestors likewise regarded it as something special, mysterious, and mystical.

Every culture has its myths and legends about the moon, which underscores its dominance in the night sky and our fascination with it. Among other things the moon is responsible the seven-day week, seven being the closest divisor to the number of days in a month, thus dividing the month into neat quarters that correspond to moon phases.

Countless lovers have courted to the full moon, tin pan alley turned out hundreds of songs, and legends abound surrounding the moon, especially the full moon, and its effects on people. The words lunacy and lunatic derive from the supposed craziness that the full moon evokes. Even today among police, civil defense, and emergency room personnel there are anecdotal tales galore of full-moon lunacy, even though research has failed to make any firm statistical correlations with unusual behavior.

Many religious and cultural celebrations depend on the moon, including Easter, which was decreed by Constantine I in AD 325 to fall on the first Sunday following the first full moon following the vernal equinox. Actually it was the calculated dates of the full moon, known as the Ecclesiastical Full Moon that was used until the calendar was revised by Pope Gregory in the 17th century.

The Romans knew the moon as Luna, equivalent to Selene, the white-faced moon goddess of Greek mythology who was known for her countless love affairs, with the shepherd Endymion, Zeus with whom she had three daughters, and Pan by whom she bore a herd of white oxen. According to the myth, Selene begins her journey across the sky only after bathing in the sea then following her brother Helios in his daily westward march.

Early attempts to use the moon as a calendar were eventually abandoned in favor of the solar calendar because there are 13.4 new moons per year, which makes it impossible to keep a lunar calendar tuned to the seasons.

The moon orbits the earth once a month (literally "moonth"), but there are different ways to define the month, including those that relate to the calendar such as "monthly" payments.

Astronomers define the sidereal month as the time it takes for the moon to complete one full orbit of Earth with respect to the distant stars, equal to 27.3 days.

The more familiar month is the time it takes for the moon to complete one cycle of phases. This is the synodic month, the time between successive new moons. The synodic month is measured with respect to the sun and is approximately 29.5 days.

The phases of the moon are lighting effects caused by the angle between the sun, Earth, and the moon.

A new moon occurs when all three are lined up with the moon between Earth and the sun. Most of the time the alignment is not perfect, so the moon is close to the sun in the sky but does not block it. Occasionally it does block the sun and casts a shadow on Earth, which we observe as a solar eclipse if we happen to be in the shadow.

Although the new moon rises and sets with the sun, we can't see it except during eclipses because the sky is too bright and the side of the moon facing us is dark.

The moon changes its position against the background stars noticeably in real time. It moves the distance of its own diameter in about one hour, at which rate it will "lose" about 50 minutes per day to the sun to rise later and later each day as it drifts eastward.

As the month progresses, more and more of the moon's lighted hemisphere comes into view and we see it in the sky further and further to the east of the sun.

In one day it moves about 12 degrees (one-thirtieth of 360 degrees), and after one week has moved through one-fourth of its orbit to become a first-quarter moon, which rises around noon.

During this two-week-long waxing period, the moon is visible in the daytime sky as it gradually grows from thin crescent to full disk. By the time it has completed one-half of an orbit it is on the opposite side of Earth from the sun and we see it rising around sunset.

In the waning half of the monthly cycle the moon shrinks back to crescent as it appears to the west of the sun and continues to rise 50 minutes later each day. By the end of the third week, the third-quarter moon is rising around midnight.

The moon's orbit is inclined five degrees to the ecliptic, so it does not follow the same path through the sky as the sun. Whereas the sunrise and sunset drift slowly back and forth across the horizon from north to south, crossing due east and west on the March and September equinoxes, the moon's rise and set positions change much more rapidly, making its path through the sky much more variable than that of the sun.

The moon is an unusual feature of Earth's environment. More than 100 satellites orbit the various planets, but none can compare with Luna. Not only does it appear large and bright in the sky, it is actually quite large compared to Earth, both in radius and in mass.

Its diameter is just a bit more than one-quarter of Earth's (27%) and it's mass is just a little more than one-percent of Earth's (1.2%). The only other moon that is so large compared to its planet is Charon, the lone moon of distant Pluto, which in the eyes of some astronomers doesn't qualify as a planet anyway since Pluto itself is somewhat smaller than Luna and much less massive.

The gaseous giants, Jupiter, Saturn, Uranus, and Neptune may have more moons, but with the possible exception of Jupiter's volcanic "pizza moon," Io, none of those moons would come close the splendor of our own Luna in its awe-inspiring dominance of the night sky.

Luna's density is greater than any other planet or moon in the solar system except for the four inner planets, Mercury, Venus, Earth, and Mars (collectively known as the "terrestrial planets") and Io.

Earth and Mars are the only terrestrial planets that have moons, but the two small moons of Mars are more like captured asteroids than "moons" in their own right. Earth and Luna are more like a double planetary system than planet and moon, and some astronomers consider Luna to be a terrestrial planet.

In its slightly elliptical orbit, the moon's average distance from Earth is about 239,000 miles. Because of their size and proximity, the two exert significant tidal effects on one another.

Earth's tidal forces on Luna have locked its rotation and revolution rates, causing Luna to show us the same face regardless of its phase.

The moon exerts significant tidal forces on Earth, but not enough to have stopped our rotation altogether, although we are slowing down as the days get longer by about 0.002 seconds per century. At that rate it takes 50,000 years to add one second to the length of the day.

That's not much, but over geologic time it adds up. At the time of the demise of the dinosaurs 65 million years ago the day would have been almost 20 minutes longer. At the end of the Precambrian era, when multicellular fossils first appear in abundance in the rock record, a day would have been just under 21 hours, making a year 40 days longer at that time.

The dark markings on the moon reflect two primary types of terrain on its surface. The lighter areas are very old, heavily cratered highlands. The darker and the relatively smooth areas are younger maria. The maria (from the Latin word for "sea") are huge impact craters that were later flooded by molten lava. Most of the surface is covered with a mixture of fine dust and rocky debris produced by meteor impacts.

Craters on the moon are a record of thousands upon thousands of impacts, most of which took place after the formation of the planets was nearly complete but still early in the history of the solar system. The planets had been formed from agglomeration of rocks and space dust, but were still gathering loose pieces of debris that littered the space between them.

On Earth, geological processes and erosion by water have smoothed out the remains of these early collisions, but a few scars remain to remind us that the origin and evolution of our home planet have been violent. It also serves to remind us that collisions are the norm rather than the exception, and we are not immune from future collisions that could have catastrophic consequences.

Evidence from several different areas has led to current theories that the Earth-Luna system itself is the result of a violent collision.

Sometime early in the formation of the solar system an object the size of Mars collided with the Earth in a glancing blow. The collision vaporized a portion of both planets and peeled off a sizable blob of material, which remained in orbit around Earth, gradually coalescing to become the moon.

Computers programmed with the laws of physics and properties of materials have allowed us to witness the process through computer modeling. The simulations show the surprising result that multiple collisions may have occurred between Earth and the nascent moon.

Regardless of the reasons behind its existence, our moon Luna has been there long before there were coyotes to howl at it. Ancient corals half a billion years old show distinct evidence of tides, and even confirm the tidal slowing of Earth's rotation.

The moon is as much a part of our history and culture as Earth itself.

If you feel a primitive urge to howl when the full moon rises on May 5, consider it a part of your heritage as one of Earth's creatures.

Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at


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