It has not been easy for humanity to develop consistent theories about what happens in the sky. If there were no Moon, it would have been more difficult. If the telescope had never been invented, the puzzle would have been tougher still. The Moon goes through obvious changes even to the most casual observer, but the reasons for these changes were not obvious to most people 50 generations ago. Neither the Sun nor the Moon is a smooth globe. Both have complicated surfaces. The Moon has craters, mountains, plains, and cliffs. The Sun has a mottled surface that is often strewn with spots. The Sun and Moon together perform a cosmic dance that, once in a while, puts on a show to rival anything else in nature.
Earth has countless natural satellites—meteors captured by gravity and orbiting in all manner of elliptical paths. The only natural satellite of significance and the only one that can be detected without powerful observing aids, however, is the Moon. It’s interesting that we have never come up with a better name for Earth’s Moon; we speak about the moons of Jupiter and the moons of Saturn, and then we call our own Queen of the Night “the Moon.” It is as if someone had a daughter and named her “Daughter.” Sometimes the Moon is called “Luna,” but that name conjures up visions of madness and worship and is not used by astronomers.
The Moon orbits Earth at a distance of about 381,000 kilometers (237,000 miles). Sometimes it’s a little closer, and sometimes it’s a little farther away. The Moon’s diameter is 27.2 percent that of Earth, roughly 3480 kilometers (2160 miles). That’s large for a moon relative to its parent planet. The Earth-Moon system is sometimes considered a double planet, and some astronomers think the pair formed that way. But Earth is 81 times more massive than the Moon, and the Moon has essentially no atmosphere. Thus, in planetary terms, the Moon is a dull place.
Perhaps you have seen drawings of the Earth-Moon system and have come to envision the Moon as much closer to Earth than is actually the case. (The drawings in this chapter, except for Fig. 4-1, are examples of such misleading data.) There is a reason for this distortion. If the Earth-Moon system were always drawn true to scale, the illustration would be of little use for most instructive purposes. Earth is a bit less than 12,800 kilometers (7,930 miles) in diameter, and the Moon is about 381,000 kilometers (237,000 miles) away on average. That’s 30 Earth diameters. If drawn to scale, the Earth-Moon system would look like Fig. 4-1. Think of the Earth and the Moon as pieces of fruit. Suppose that Earth is a 10-centimeter-diameter grapefruit and the Moon is a 27-millimeter-diameter plum (4 inches and 1 inch across, respectively). To make a scale model, you must set the two fruits 3 meters (10 feet) apart.
Figure 4-1. Earth-moon system, drawn to true scale.
With respect to the Sun, the Moon takes 29½ days to make one orbit around Earth. The exact synodic (sun-based) lunar orbital period varies slightly from one orbit to the next because the orbit of the Moon around Earth is not a perfect circle and the orbit of Earth around the Sun is not a perfect circle either. However, for most amateur astronomers (including us), 29½ days is a good enough figure. Relative to the stars, the Moon’s orbit is faster; the sidereal lunar orbital period is about 27 days and 7 hours.
The synodic and sidereal lunar orbital periods differ for the same reason the synodic day is longer than the sidereal day. Every time the Moon makes one trip around Earth, our planet has moved approximately one-twelfth of the way around the Sun. The Moon has to travel further to come into line with the Sun from one orbit to the next than it must travel to come into line again with some distant star (Fig. 4-2).
Figure 4-2. The synodic lunar period is longer than the sidereal lunar period. (This drawing is not to scale.)
Have you ever looked at the Moon, especially the full Moon, and imagined it to be closer than you remember previous full Moons to have been? Maybe it’s not your imagination. The Moon orbits Earth in an elliptical path, with Earth at one focus. The Moon can get as close as 356,000 kilometers (221,000 miles) and as distant as 407,000 kilometers (253,000 miles) from Earth. This is a difference of 13.5 percent of the Moon’s mean distance. Sometimes the Moon’s disk appears 13.5 percent larger than at other times. This is enough to make a difference, especially when the Moon passes precisely between an observer and the Sun. The Moon’s closest approach is the lunar perigee; this term also applies to the minimum-distance figure. The Moon’s furthest retreat is the lunar apogee, a term that also is used in reference to the maximum-distance figure.
As the Moon makes its way around Earth, it keeps the same face toward us, more or less. This is so because the Moon’s mass is not uniformly distributed within the globe, and Earth’s gravity has managed, over millions of centuries, to tug the Moon’s rotation rate into near-perfect lockstep with its revolution. But the Moon still wobbles back and forth a little; it has not completely “settled down.” We can see 59 percent of the Moon’s surface from Earth if we make enough observations. The wobbling of the Moon’s face relative to Earth is called libration (not to be confused with libation).
Libration can give rise to interesting phenomena. For example, when amateur radio operators bounce their signals off the Moon to communicate with their fellows on the opposite side of the world, libration produces multiple signal paths whose lengths vary constantly, making the radio waves add and cancel in a manner so complicated that precise analysis would challenge any computer. The resulting received signals sound like someone babbling or hooting underwater. The wavelengths of light are too short for this effect to be observed visually. If we could see at radio wavelengths, the Moon would seem to sparkle and scintillate as if fireworks were constantly being set off all over its surface.
The appearance of the Moon is drastically affected by its orientation relative to the Sun. When the Earth, the Moon, and the Sun are in line or nearly in line, the Moon is said to be new, and its existence is not visually apparent unless there happens to be a solar eclipse. As the Moon orbits Earth, a journey that takes place in a counterclockwise direction as viewed from high above Earth’s north pole (Fig. 4-3), it presents more and more and then less and less of its lit-up face to us. Three or four days after the new Moon, it is a waxing crescent. About a week after the new Moon, we see half its globe illuminated by the Sun; this is first quarter. Three or four days after that, most of the Moon is illuminated as we see it; this is waxing gibbous. Two weeks and 18 hours after the new Moon, it is entirely illuminated for us unless a lunar eclipse happens to be taking place. This is the full Moon. Phases proceed in timely fashion after the full Moon through waning gibbous, last quarter, waning crescent, and finally, back to new again.
Figure 4-3. The Moon’s phases result from the relative orientation of the Moon and the Sun, as seen from Earth.
Almost nobody lives in this world without getting to know the lunar phases before they get into kindergarten. The waxing crescent is visible just after sunset; the first-quarter Moon can be seen until midnight. The waxing gibbous Moon stays in the sky into the wee hours of the morning, and the full Moon is above the horizon all night, setting as the Sun rises. After the full phase, the waning gibbous Moon rises a couple of hours after sunset; the last-quarter Moon rises around midnight; the waning crescent Moon waits until the predawn hours to rise. Moonset in the waning phases takes place in the daytime, and some people say that there is “no moonset” during this half of the lunar cycle.
Most people envision the Moon’s phase-to-phase progress and appearance as seen from the northern hemisphere. This is natural because there are more people living north of the equator than south of it. As far as Earth itself is concerned, however, this is only half the story.
Figure 4-4 shows the way the Moon looks at various stages in its orbit around Earth as seen from a midlatitude northern location such as Kansas City, Missouri, or Rome, Italy. (There is some variance in the tilt, depending on the season of the year; moonrise and moonset occur somewhat north or south of due east or west.) The waxing crescent appears in the southwestern or western sky just after sunset and sets 2 to 4 hours after the Sun. The Moon at first quarter is in the southern sky at sunset, moves generally westward, and sets around midnight. The waxing gibbous Moon is in the southeast at sunset, moves generally westward, and sets in the predawn hours. The full Moon is opposite the Sun, rising at sunset and setting at or near sunrise. The waning gibbous Moon rises some time after sunset and sets after sunrise the next day. The last-quarter Moon rises around midnight and sets around noon. The waning crescent waits until the predawn hours to rise and sets in the afternoon.
Figure 4-4. Lunar phases as seen from middle latitudes in the northern hemisphere. The Moon’s tilt varies somewhat, depending on the season and the time of night.
Figure 4-5 illustrates the appearance of lunar phases as seen from a midlatitude southern location such as Perth, Australia, or Napier, New Zealand. (As with the northern-hemispheric situation, there is some variance in the Moon’s tilt. Depending on the season of the year, moonrise and moonset occur somewhat north or south of due east or west.) The waxing crescent is in the northwestern or western sky just after sunset and sets 2 to 4 hours after the Sun. The Moon at first quarter is in the northern sky at sunset, moves generally westward, and sets around midnight. The waxing gibbous Moon is in the northeast at sunset, moves to the west, and sets a couple of hours before dawn. The full Moon rises around sunset and sets around sunrise, tracking across the northern half of the sky during the night. The waning gibbous Moon rises shortly after sunset and sets after sunrise the next day. The last-quarter Moon rises at about midnight and sets around noon. The waning crescent rises a couple of hours before dawn and sets in the afternoon.
Figure 4-5. Lunar phases as seen from middle latitudes in the southern hemisphere. The Moon’s tilt varies somewhat, depending on the season and the time of night.
When you look at the Moon without the binoculars or a telescope, it’s impossible to know much about the true nature of the surface. Before Galileo Galilei and other astronomers began looking at the heavens through “spy glasses” a few hundred years ago, no one could be certain that the terrain was dry, scarred, and lifeless. In fact, the true austerity of the Moon would surprise even the most pessimistic dreamers of old.
The naked-eye Moon, especially the full Moon, has light and dark features. In absolute terms, the whole Moon is a rather dark object; it reflects only a few percent of the solar light that strikes it. If the Moon were as white as snow or powdered sugar, it would shine several times more brightly. Even without the help of telescopes, people long ago surmised that the Moon’s light areas represent irregular terrain and the dark regions are flat by comparison. Some people thought the light regions were clouds and the dark zones were areas of clear weather, but after observing the Moon for many nights and seeing that the “clouds” never moved, most people rejected that theory. However, these general ideas were as far as pretelescopic people got. Many people considered the dark areas to be liquid oceans made up of water. They were called maria (pronounced “MAH-ree-uh”), a word that means “seas.” To this day, flat plains or plateaus on the Moon have names such as the Sea of Tranquillity and the Sea of Crises. The light areas were assumed to be land masses, but few people supposed they were strewn with mountain ranges and crater fields.
When Galileo and others began looking at the sky with telescopes in the seventeenth century, humanity’s ideas did not change overnight, as they might have if our race had been driven more by hunger for knowledge and less by ego, fear, and superstition. People’s imaginations were more active in Galileo’s day than they had been a few centuries earlier, but they were not quite as daring as we are now. When Galileo announced that the Moon had craters and mountains, his fellow scientists became interested right away, but those who held power over people’s lives had other notions. To them, Galileo was a troublemaker, and he was treated as one. He ended up spending his last years under house arrest. It was not a tyrannical government dictator that subjected him to this, but the Pope. Imagine the reaction the Pope would get today if he demanded that some scientist spend the rest of his life under confinement!
The Earth-Moon system stays together because of gravitation. Earth pulls on the Moon, keeping it from flying off into interplanetary space. The Moon also pulls on Earth, although we are not aware of it unless we make certain observations. The Moon’s gravitational effects vary depending on where in the sky (or beneath the horizon) it happens to be at any given time.
The lunar day is about an hour longer than the synodic day—roughly 25 hours—but the Moon, like the Sun, appears to revolve around any stationary earthbound observer. The effects of gravity propagate through space at the speed of light, some 299,792 kilometers (186,282 miles) per second. This covers the Earth-Moon distance in only a little more than 1 second, so there is essentially no lag between the Moon’s position and the direction in which its “tugging” takes place. The Moon’s pull is extremely weak compared with the gravitation of Earth at the surface; it is nowhere near enough to affect the reading you get when you stand on a scale and weigh yourself, for example. But this does not mean that the gravitational effect of the Moon can be disregarded, as any ocean beach dweller will attest.
The Moon’s gravitation, and to a lesser extent the Sun’s gravitation too, cause Earth’s oceans to be slightly distorted relative to the solid globe of the planet. When considered to scale, the oceans form a thin, viscous coating on Earth. Even the deepest undersea trenches reach less than 0.2 percent from Earth’s surface down to the center, and most of the oceans are far shallower than this. Even so, the depth of the oceans is affected by the combined external gravitational effects of the Sun and the Moon. The effect is greatest when the Sun, the Moon, and Earth are all in line, that is, at the times of new and full Moon. When the Moon is at first or last quarter, its gravitational field acts at right angles with respect to that of the Sun, and the two almost cancel each other out, although the Moon’s effect is a little greater than the Sun’s.
As Earth rotates under its slightly distorted “coating” of ocean, the level of the sea rises and falls at specific geographic locations. In certain places, the rise and fall is dramatic, and people who live on the shore must take its effects seriously. In other places, these tides are much less extreme. There are two high tides and two low tides during the course of every lunar day; the reason for this can be envisioned by looking at Fig. 4-6. However, these drawings represent an oversimplification. The actual tides are delayed by the fact that on a planetary scale water behaves more like molasses than the freely running liquid with which we are familiar. Also, the contours of the ocean floor and the continental shelves have an effect. This is not all: Land masses break the planetary ocean up, so wave effects cannot propagate unimpeded around the world. The tides are waves, although they are very long, having two crests and two troughs with the passage of every lunar day. Actually, the tides consist of two waves of different frequencies. Superimposed on the lunar tidal waves, which have a period of about 25 hours, are solar tidal waves (truly tidal in nature, unlike tsunamis, which are caused by undersea earthquakes, not by tides) with a period of 24 hours, but whose crests and troughs have smaller magnitude.
Figure 4-6. Simplified diagrams showing why lunar tides occur. These views are from high above Earth’s north geographic pole.
The Moon and Sun are not the only natural entities that affect sea level. Weather systems, especially ocean-going storms, have an effect, and in some places a storm surge can cause the sea to rise 10 times as much as the astronomical tide. A tsunami comes in and pounds away at the shoreline in a manner similar to that of a storm surge, except that the tsunami is caused by a jarring of the sea floor (or, occasionally, by a volcanic eruption) rather than by high winds piling the water up onto shore. Neither of these phenomena are true tides.
Tides don’t occur to a significant extent in land-locked seas and lakes. This is so because in order for water to rise in one location, it must fall somewhere else where the lunar-solar gravitational composite is different. This can’t happen in a small body of water, on which, relative to every point, the Moon and Sun are in the same positions at any given time.
There has been some debate about the effects of the solar and lunar gravitational fields on the behavior of living cells. No one has yet come out with a respected scientific study that quantifies and defines exactly how such effects, if any, are manifest. For example, so-called Moon madness (lunacy) has not been explained on the basis of increased intracellular tidal effects during the full Moon. Because a similar loss of reasoning power does not seem to grip its perennial victims during the new Moon, it is almost certain that Moon madness (if it really exists) is not caused by gravitation.
Earth-Moon pair is unique in the Solar System; no other major planet has a satellite so large in proportion to itself. (Pluto and its moon Charon are sometimes called a double-planet system, but both of them are hardly larger than asteroids and might better be classified as such.) If the Moon were only a few hundred miles across, say the size of the asteroid Ceres, there would be little doubt that it was captured from solar orbit by Earth’s gravitation. However, because the Moon practically qualifies as a planet, being almost as large as Mercury, there are competing theories about its origin.
One theory holds that the Moon was once a planet in its own right, orbiting the Sun rather than Earth, but something, such as a collision with a large asteroid, deflected it from its solar orbit and caused it to pass so close to Earth that Earth captured it permanently. Few astronomers believe this. If it were true, then the Moon’s orbit likely would be an elongated ellipse sharply inclined to the plane of the ecliptic.
Another theory suggests that Earth originally had no Moon but that a Mars-sized object struck Earth a glancing blow and sent vast quantities of material into orbit. This would have produced rings around Earth, something like those around Saturn. These rings eventually would have congealed into the Moon. One problem with this theory is that an impact severe enough to blast that much matter off Earth might have shattered our planet. However, an increasing number of astronomers accept this theory, and it is lent support by the fact that the Moon lacks a metallic core; most or all of the blown-off material would have come from Earth’s mantle and crust. It also might explain why Earth’s equator is tilted significantly with respect to the plane of the ecliptic. A major impact could have jarred Earth and caused its axis to shift by 23.5 angular degrees.
Still another theory contends that Earth and the Moon formed as a double planetary system over the same period of time as the other planets, condensed from the disk of gas and dust orbiting the newborn Sun. This is a popular theory and is fairly consistent with what we observe about the Moon. It explains why the Moon orbits Earth in a nearly circular path and why the Moon’s orbit is tilted only a little (about 5 angular degrees) with respect to Earth’s orbit around the Sun.
The Sun has been worshipped, despised, and feared by humans throughout history. Ideas about the Sun are as strange nowadays as they have ever been, but there are some statements about our parent star that we can make with confidence.
The Sun is the largest nuclear reactor from which humanity has ever derived energy, and until we venture into other parts of the galaxy, this will remain the case. The Sun has a radius of about 695,000 kilometers (432,000 miles), more than 100 times the radius of Earth. If Earth were placed at the center of the Sun (assuming the planet would not vaporize), the orbit of the Moon would fit inside the Sun with room to spare (Fig. 4-7).
The commonly accepted mean distance from Earth to the Sun is 150,000,000 kilometers (93,000,000 miles) in round numbers. But the day-to-day distance varies up to a couple of million kilometers either way. Earth’s orbit around the Sun, like the Moon’s orbit around Earth, is not a perfect circle but is an ellipse with the Sun at one focus. Earth’s closest approach to the Sun is called perihelion, and it occurs during the month of January. Earth is farthest away from the Sun—aphelion—in July. Surprisingly enough, for those of us in the northern hemisphere, the Sun is closest in the dead of the winter. It is not Earth-Sun distance that primarily affects our seasons but the tilt of Earth on its axis.
Centuries ago, people did not know how large the Sun was, nor how far away it was. Estimates ranged from a few thousand miles (kilometers hadn’t been invented yet) to a few million miles. The distance to the Sun could not be measured by parallax relative to the background of stars because the Sun’s brilliance obliterated the stars near it. The distance to the Moon had been measured by parallax, as well as the distances to Mars and Venus at various times, but the Sun defied attempts to measure its distance until someone thought of finding it by logical deduction. What follows is an example showing the sort of thought process that was used, and can still be used, to infer the distance to the Sun. Let’s update the measurement techniques from those of our forebears and suppose that we have access to a powerful radar telescope, with which we can measure interplanetary distances by bouncing radio beams off distant planets and measuring the time it takes for the signals to come back to us.
Figure 4-7. Earth-moon system would easily fit inside the Sun.
Given a central body having a known, constant mass, such as the Sun, all its satellites obey certain physical laws with respect to their orbits. One of these principles, called Kepler’s third law, states that the square of the orbital period of any satellite is proportional to the cube of its average distance from the central mass. This is true no matter what the mass of the orbiting object; a small meteoroid obeys the rule just as does Earth, Venus, Mars, and Jupiter. We know the length of Earth’s year and the length of Venus’s year; from this we can calculate the ratio (but not the actual values) of the two planets’ mean orbital radii. Knowing this ratio is not enough, all by itself, to solve the riddle of Earth’s mean distance from the Sun, but it solves half the problem.
The next step involves measuring the distance to Venus. If we could do this when Venus is exactly in line with the Sun, then we could figure out our own distance by simple mathematics. Unfortunately, the Sun produces powerful radio waves, and our radar telescope won’t work when Venus is at inferior conjunction (between us and the Sun) because the Sun’s radio noise drowns out the echoes. However, when Venus is at its maximum elongation (its angular separation from the Sun is greatest either eastward or westward), the radar works because the Sun is out of the way. At maximum elongation, note (Fig. 4-8) that Venus, Earth, and the Sun lie at the vertices of a right triangle, with the right angle at the vertex defined by Venus. One of the oldest laws of geometry, credited to a Greek named Pythagoras, states that the square of the length of the longest side of a right triangle is equal to the sum of the squares of the other two sides. In Fig. 4-8 this means that a2 + b2 = x2, where x is the elusive thing we seek, the average distance of Earth from the Sun.
Figure 4-8. If we know the radius of Venus’ solar orbit and we can measure the distance to Venus at its maximum elongation, then we can calculate our own distance from the Sun.
Now that we know the value of a in the equation (by direct measurement) and also the ratio of b to x (by Kepler’s third law), we can calculate the values of both b and x because we have a set of two equations in two variables. Let’s not drag ourselves through a detailed mathematical derivation here. If you’ve had high school algebra, you can do the derivation for yourself. It should suffice to say that this scheme can give us a fairly good idea of Earth’s mean distance from the Sun if the measurement is repeated at several maximum elongations and the results averaged. However, even this will only give us an approximation because the orbits of Earth and Venus are not perfect circles. In recent decades, astronomers have made increasingly accurate measurements of the distance from Earth to the Sun using a variety of techniques.
Once the distance from Earth to the Sun was known, the Sun’s actual radius was determined by measuring the angular radius of its disk and employing surveyors’ triangulation in reverse (Fig. 4-9).
The Sun “shines” by constantly “burning” hydrogen, the most abundant element in the universe. You know, if you have taken chemistry classes, that hydrogen is flammable and that it burns clean and hot. (It might someday replace natural gas for heating if a method can be found to cheaply and abundantly produce it and safely distribute it.) However, the Sun “burns” hydrogen in a far more efficient and torrid fashion: by means of nuclear fusion. The enormous pressure deep in the Sun, caused by gravity, drives hydrogen atoms into one another. Hydrogen atoms combine to form helium atoms, and in the process, some of the original mass is converted directly into energy according to Einstein’s famous equation E = mc2 (energy equals mass times the speed of light squared).
The earliest theories concerning the Sun involved ordinary combustion, the only question being what, exactly, was burning. Coal was suggested as a fuel for the Sun, but if this were the case, the Sun would have burned out long ago. Besides this, there was the little problem of how all that coal got up there into space. Another idea involved the direct combination of matter with antimatter, resulting in total annihilation. However, if this theory were true, the Sun would be far brighter and hotter than it is. The hydrogen-fusion theory accounts for what we see and is consistent with theories concerning the age of the Universe and the age of Earth.
Figure 4-9. The Sun’s actual radius can be determined by reverse triangulation, on the basis of its apparent radius and its known distance from Earth.
How long will the Sun’s supply of hydrogen fuel last? Should we worry about the possibility that the supply will run out soon and Earth will cool off and freeze over?
Eventually, the Sun will burn out, but it will not happen for quite awhile. In fact, most scientists believe that the Sun will continue to shine for at least 1 billion more years at about the same level of brilliance as it does today. There are a lot of hydrogen atoms in that globe. It has a radius, remember, of 695,000 kilometers, or 69,500,000,000 centimeters. Scientists would write that as 6.95 × 1010 cm. Remember the formula for calculating the volume of a sphere from your middle school geometry class: V = 4/3πr3, where V is the volume and r is the radius. Using your calculator, you can figure out, using 3.14 as the value of π, that there are about 1.4×1033, or 1.4 decillion, cubic centimeters of matter in the Sun. Written out in full, that number looks like this:
1,400,000,000,000,000,000,000,000,000,000,000
With this number in mind—or out of mind, because it’s incomprehensibly large—you might be willing to accept practically any claim as to the Sun’s longevity, except, of course, life everlasting. The Sun will perish. The symptoms of aging will begin in 1000 or 2000 million years.
As the supply of hydrogen runs out, the Sun will expand, and its surface will cool off. However, Earth will heat up because the bloated Sun will appear much larger in the sky and will send far more energy to Earth’s surface than is the case now. The climate will become intolerably hot; the polar ice caps will melt; wildfires will reduce all plant life to ashes. Sometime during this process, any remaining humans and other mammals will die off. The oceans, lakes, and rivers will boil dry. All living things, even the hardiest bacteria and viruses, will die. The atmosphere will be blown off into space. Some astronomers think that the Sun’s radius will grow until it exceeds the radius of Earth’s orbit so that the Sun will swallow Earth up and vaporize it.
Don’t let this scenario depress you. By then humanity will have colonized a couple of dozen other planets and will grieve no more about the fate of Earth than we do today about the buried houses of ancient cities. If our descendants remember us at all, it will be with fascination. Knowledge of our present society might be conveyed by legend, by stories told to children at bedtime, by tales about a place called Terra that sank beneath the surface of a stormy star after its inhabitants had fled, a place where people burned the decomposed by-products of dead plants and animals in order to propel surface transport vehicles. And the children will laugh and say that such a ridiculous place couldn’t have existed.
After the red-giant phase, the Sun will fuse helium into carbon, iron, and other elements, and will shrink as gravity once again gains dominance over the pressure of nuclear heat. However, this process cannot continue forever. A point will be reached at which no further nuclear reactions can take place, and then gravitation will assert its ultimate power. The Sun will be crushed into an orb of planetary size and, as the last of its heat dissipates, will fade away and spend the rest of cosmic time as an incredibly dense, dark ball.
If you’ve never looked at the Sun through an appropriately filtered telescope, don’t pass up the chance. Be sure that you use the proper type of filter (one that fits over the telescope’s objective lens, not in the eyepiece or the star diagonal), and use it according to the manufacturer’s instructions. Chances are good that you will see at least one dark spot on that bright disk.
Before the time of Galileo in the seventeenth century, the idea of sunspots did not cross the minds of people in Western civilizations—or if it did, no one ever voiced their thoughts aloud. The Sun was regarded as perfect, and if someone had suggested otherwise, they would have been disciplined or put to death. There is reason to believe that the ancient Chinese knew about sunspots and accepted their existence, having seen them, most likely, when the Sun was rising or setting in a hazy sky.
Sunspots enable astronomers to calculate the rotational period of the Sun because the blemishes seem to move across the solar disk, disappear over the edge, and then reappear a couple of weeks later on the opposite limb. The Sun rotates approximately once a month. Careful observation of spots has revealed that the Sun spins faster at lower latitudes than at higher latitudes. Using spectroscopy, this has been verified, and the solar “day” is in fact 25 Earth days long at the equator and 34 Earth days long at the poles. This fact has been used to prove that the Sun is not a solid body like Earth or the Moon.
Sunspots have an appearance that reminds some observers of biological cells (Fig. 4-10). The darkest part, at the center, is called the umbra; it is surrounded by a brighter region called the penumbra. Sunspot sizes vary, but they can be, and often are, much larger in diameter than Earth. The spots tend to form in groups and are believed to be depressions in the solar surface resulting from magnetic disturbances. They are, in a sense, storms on the Sun. The overall average number of sunspots rises and falls in a cycle of roughly 11 years. The most recent sunspot maximum took place in late 2000 and early 2001.
Figure 4-10. A sunspot can have a diameter greater than that of Earth.
If sunspots are like hurricanes on the Sun, then solar flares are like nuclear explosions: sudden, bright eruptions that send high-speed, charged subatomic particles flying off into space. Solar flares are more difficult to see with an ordinary telescope than are sunspots, but astronomers constantly watch the Sun for signs of these outbursts. A large solar flare is followed, in a day or so, by destabilization of Earth’s magnetic field. As the charged particles come near our planet, they accelerate toward the north and south geomagnetic poles. This acceleration produces its own magnetic field, which interacts with that of Earth. As a consequence of this, the ionized layers of Earth’s upper atmosphere change dramatically; this can create vast halos of light around the geomagnetic poles. If you happen to live at a high latitude, especially in North America, you are familiar with this glow as the aurora borealis (northern lights). Similar effects take place in the south polar regions, but the aurora australis (southern lights) are not spectacular, except as seen from Antarctica and from the far southern ocean on rare occasions when the sky over them is not socked in with grim overcast. The ionospheric disturbances affect radio communications and broadcast, especially on the so-called shortwave bands. In the extreme, even wire, cable, and satellite communications systems are disrupted by the powerful, erratic magnetic fields.
The Moon’s orbit around Earth does not lie exactly in the plane of the ecliptic but instead is tilted about 5 degrees. The angular diameter of the Moon’s disk, as seen from Earth, is only about half a degree; the same is true of the Sun’s disk. Therefore, when the Moon passes between Earth and the Sun, the Moon’s shadow almost always misses Earth, and when Earth passes between the Moon and the Sun, the Moon usually misses Earth’s shadow. Once in a while, however, shadow effects occur and are observed by people. When the Moon’s shadow falls on Earth, we have a solar eclipse or eclipse of the Sun. When Earth’s shadow falls on the Moon, we have a lunar eclipse or eclipse of the Moon.
The fact that the Sun and the Moon are almost exactly the same angular size as we see them from Earth is a convenient coincidence, especially when it comes to eclipses of the Sun. The Moon is just about exactly the right size to blot out the Sun’s disk but little or none of the surrounding space, allowing us to see, during total solar eclipses, features of the Sun that would otherwise be invisible except from outer space. These include the corona, a pearly white mane of glowing, rarefied gases, and solar prominences, which are bright red or orange and look like flames leaping thousands of miles up from the solar surface.
The Moon’s shadow consists of two parts, called the penumbra and the umbra (the same names as are used with sunspots but with different meanings). Any object exposed to sunlight casts a shadow consisting of an umbra and a penumbra. From the point of view of an observer in the Moon’s penumbral shadow, part of the Sun is obscured, but not all. Observers within the Moon’s umbra see the entire solar disk covered up by the Moon.
The umbra of the Moon’s shadow is a long cone extending approximately 395,000 kilometers (245,000 miles) into space opposite the Sun. The penumbra extends much further from the Moon and gets wider and wider with increasing distance, as shown in Fig. 4-11. The penumbra, while much longer than the umbra, does not have infinite length; it fades away as the Moon’s apparent diameter shrinks with respect to that of the Sun. At a distance of several million miles, the penumbra dissipates as the Moon’s disk as viewed against the Sun becomes comparable with the size of a sunspot.
When an observer is located within the penumbra of the Moon’s shadow, a partial solar eclipse is observed. At the edge of the penumbra, the Moon seems to “take a bite out of the Sun.” Further within the penumbra, the Sun’s disk takes the shape of a crescent. The closer to the umbra the observer gets, the slimmer the crescent Sun becomes. However, even a narrow crescent Sun is bright and has the same observed brilliance per unit area as the full Sun. Even with three-quarters of the Sun covered by the Moon, daylight still looks quite ordinary. This is where careless observers get into trouble. The exposed portion of the Sun is likely to cause permanent eye damage if a partial eclipse is viewed directly. This is true even if 99.9 percent of the Sun is obscured. The only safe way to look at an eclipse is the hole-projection method. Punch a 1- or 2-millimeter hole in a piece of cardboard, let the Sun shine through the hole, and look at the Sun’s image on a piece of white paper held in the shadow of the cardboard. Never look at the Sun directly during an eclipse.
Figure 4-11. The umbra and penumbra of the Moon’s shadow.
The length of the Moon’s umbral shadow is almost exactly the same as the mean distance between Earth and the Moon. Thus, when the Moon passes between Earth and the Sun, the tip of the umbra sometimes reaches Earth’s surface, but not always. If the Moon is at perigee and Earth is at aphelion when a solar eclipse takes place, the Moon is at its largest possible angular diameter, whereas the Sun is at its smallest. This results in a spectacular total solar eclipse, and, under ideal conditions, it can last about 7 minutes.
The worst conditions for eclipses of the Sun occur when the Moon is at apogee and Earth is at perihelion. Then the Moon is at its smallest possible apparent size, and the Sun is at its largest. In this case, a total eclipse does not occur anywhere on Earth. For observers fortunate enough to have the tip of the Moon’s umbral shadow pass overhead, an annular eclipse takes place. The term annular means “ring-shaped,” a good description of the appearance of the Sun during such an event. During an annular eclipse, the landscape and sky appear as they would in ordinary daylight through dark sunglasses.
Earth, like the Moon, casts a shadow into space, but Earth’s shadow is longer and wider. The full Moon usually misses Earth’s shadow, but it sometimes enters the penumbra, and once in while it makes it into the umbra. If the Moon passes into Earth’s penumbral shadow, we usually don’t notice anything; for a few hours the full Moon might shine a little less brightly than it ought, but that is all. However, if part or all of the Moon moves into Earth’s umbral shadow, a lunar eclipse occurs, and anyone on Earth who can view the Moon will see the eclipse.
When the Moon first begins to enter Earth’s umbral shadow, the darkness “takes a bite out of the Moon” in much the same way as the Moon obscures the Sun during the beginning of a solar eclipse. The Moon might pass beneath or above the shadow core, so darkness never covers the Moon completely; in these cases we see a partial lunar eclipse. At mideclipse, the Moon seems to either “smile” or “frown” depending on which side of the umbra it passes.
The diameter of Earth’s umbral shadow at the Moon’s distance (381,000 kilometers, or 237,000 miles, on average) is several times the diameter of the Moon itself. For this reason, there is a fair chance that the Moon will plunge entirely into the umbra for a time, causing a total lunar eclipse (Fig. 4-12). These happen more often than total solar eclipses occur on Earth. The Moon rarely goes black during totality but exhibits a dim coppery or rusty glow caused by sunlight passing through Earth’s atmosphere; the redness occurs for the same reason that some sunrises or sunsets appear red. An observer on the Moon would see a total solar eclipse of a truly alien sort. Imagine it: a thin red, orange, and yellow ring hanging in a black sky filled with unblinking stars, the Moonscape aglow as if with energy of its own.
Figure 4-12. Progress of a lunar eclipse. This drawing is not to scale.
QuizRefer to the text if necessary. A good score is 8 correct. Answers are in the back of the book.
1. The rotational period of the Sun
(a) is synchronized with the orbit of Earth.
(b) is shorter at the equator than at the poles.
(c) varies in an 11-year cycle.
(d) is not defined; the Sun does not rotate at all.
2. Which of the following theories concerning the Moon’s formation is most popular?
(a) The Moon condensed from the ejecta of thousands of earthly volcanoes many millions of years ago.
(b) The Moon spun off Earth because of centrifugal force when Earth was in its early, molten state.
(c) The Moon and Earth formed as a double planet.
(d) All three of the preceding theories have been disproven conclusively.
3. Eclipses would be more common if the Moon’s orbit
(a) were more tilted with respect to the plane of the ecliptic.
(b) were in the plane of the ecliptic.
(c) had a longer period.
(d) were more elongated.
4. The northern lights owe their existence to
(a) Earth’s magnetic field.
(b) charged particles ejected from the Sun.
(c) solar flares.
(d) more than one of the above.
5. On a particular day, the Moon sets around high noon. What phase is the Moon in or near?
(a) New
(b) First quarter
(c) Full
(d) Last quarter
6. The composite tidal pull of the Sun and Moon is greatest when
(a) Earth, the Sun, and the Moon are at the vertices of an equilateral triangle.
(b) Earth, the Sun, and the Moon all lie along the same straight line.
(c) Earth, the Sun, and the Moon are at the vertices of a right triangle.
(d) This question is irrelevant; the composite tidal pull of the Sun and the Moon never varies.
7. Earth is
(a) about 81 times more massive than the Moon.
(b) about 10 times the diameter of the Moon.
(c) always in full phase as seen from the Moon.
(d) the same angular diameter as the Sun, as seen from the Moon.
8. In an annular eclipse of the Sun,
(a) the path of totality is narrow.
(b) the Sun’s disk is totally covered for only a few minutes.
(c) the Sun’s disk is never totally covered.
(d) the Moon takes on a copper-colored glow.
9. Suppose that a small stone with a mass of 10 grams and a large boulder with a mass of 10,000 kilograms are both put into circular orbits 15,000 kilometers above Earth’s surface. Which of the following statements is true?
(a) The stone will take longer to orbit Earth than the boulder.
(b) The boulder will take longer to orbit Earth than the stone.
(c) The stone and the boulder will take the same amount of time to orbit Earth.
(d) The relative orbital periods will depend on how close together the stone and the boulder are placed.
10. The sidereal lunar orbital period (around Earth) is
(a) longer than the synodic lunar orbital period.
(b) the same as the synodic lunar orbital period.
(c) shorter than the synodic lunar orbital period.
(d) sometimes longer than and sometimes shorter than the synodic lunar orbital period.
