A Celestial Visitor Returns
Halley’s Comet—A Glory to God
OF ALL the splendors God has put in his starry heavens, a great comet, with its tail sweeping majestically across the sky, has been acclaimed the most magnificent. Few people alive today remember the spectacle of Halley’s comet in 1910, but three generations since have heard its fame, and many have hoped that they might live to see its return.
Now, 75 years later, Halley’s comet is again coming to keep its appointment. Astronomers have already spotted it. They first identified it in a photograph taken three years ago with the giant telescope on Mount Palomar. They will keep close watch on it as it approaches to where all of us can see it.
Halley’s comet will receive a welcome this time quite different from anything on its past visits. Technical advances not even dreamed of in 1910 make it possible to send cameras and instruments out into space to meet Halley. Already, in December last year, two Russian spacecraft were launched, and in July two others, one from Europe and one from Japan, were sent into orbits that will intersect Halley’s path in April next year. Thus scientists hope to become much better acquainted with this spectacular, but little understood, celestial wonder.
However, we are cautioned not to expect the display of Halley’s comet this time to equal the one in 1910. Some astronomers even say that this appearance may be the most disappointing in 2,000 years. Why should one appearance differ from another? It depends mostly on where the earth is in its orbit when the comet goes by. Of course, the closer we are, the better our view of the comet. Also, the larger the angle between the comet and the sun, the longer it will be in the night sky. Now it happens that on February 9, 1986, when the comet makes its nearest approach to the sun, called perihelion, and is at the peak of its performance, the earth will be almost exactly on the opposite side of the sun. That means we will be separated by the maximum distance, nearly 150,000,000 miles,a while the comet is directly behind the sun! That is the worst possible lineup.
But the comet will be inside the earth’s orbit for several months, passing closer to us both before and after perihelion. On its way in, the comet will be in a good position for those living in the northern hemisphere, but then it will not be very bright. It will be brighter and will pass closer to the earth on the way out. Then it will be high in the southern skies but not so well placed for northern observers.
Why Is Halley’s Comet the Most Famous?
Isn’t this just one of many comets that have adorned the skies down through the years? Comets are really not rare. There are usually at least one or two in the sky at any given time, and a dozen or more may be seen in a year. But most of these are distant, visible only with a telescope, and they look like faint, somewhat fuzzy stars. Only an occasional one comes close enough to be seen with the naked eye. Seldom does a really spectacular comet appear, with a long, diaphanous tail spread gracefully across the sky. There were half a dozen such comets in the 19th century that rivaled or surpassed Halley’s in brilliance.
Yet, when the average person hears the word “comet,” the name Halley immediately comes to mind. Why? Halley’s comet in 1910 was truly the finest in our century. There have been a few bright comets since then, but not one has matched Halley’s glory.
Aside from its resplendence, however, its unique claim to fame is that it was the first comet recognized to be a periodic visitor, returning on a regular schedule. It fell to the lot of Edmond Halley, an English astronomer, to make this surprising discovery. Halley (his name rhymes with alley, not with daily) was a colleague of Isaac Newton, and he used Newton’s new theories of gravitation and elliptical planetary orbits to calculate the orbits of comets previously observed. Halley noted that the paths of two historic comets, in the years 1531 and 1607, and a third that he himself had seen in 1682, were very nearly alike. Was this just a coincidence? No, he conjectured, they were all the same comet, returning every three quarters of a century. He predicted that the comet would be seen again about the year 1758.
Halley did not live to see it—he died in 1742 at the age of 86—but true to his prediction, the comet appeared on the world stage in 1758. It was first sighted by a German peasant in December 1758, and it reached its perihelion in March of 1759. It was promptly given the name Halley’s comet, and so it has been known to this day.
Thus it was established that Halley’s comet is a bona fide member of the solar system. Was it possible that it could be identified with other comets seen in earlier times? Surely so prominent an object would not have escaped notice on earlier visits. Halley himself noted that the comet of 1456 must have been the same. Delving through historical records, scholars have found that the comet had been sighted on every one of 23 revolutions before that, all the way back to the year 240 B.C.E., when it was reported by Chinese astronomers. The coming appearance, then, will be the 30th of an unbroken series of sightings, every 75 to 78 years for over two millenniums.
A Wanderer in the Solar System
Halley’s comet rides in a high orbit. It is not at all circular but is a long narrow ellipse. It stretches out across the orbits of all the planets from Venus to Neptune. At perihelion it is only 54,000,000 miles from the sun, but at its most distant point, it is more than 3 billion miles away.
At about the distance of Jupiter, the comet comes within range of telescopes, and after it crosses the orbit of Mars, it reaches naked-eye visibility. About this time the tail begins to form. It grows larger as the comet approaches the sun. It always points away from the sun, blown by a solar wind and by solar radiation.
Tips for Comet Watchers
When can you expect to see the comet, and where should you look for it? Whether you can see it and how bright it will appear depend on many factors. Look before dawn or after the evening twilight has faded, when it does not appear too close to the sun. The darkness of the sky is an important factor. Bright city lights will spoil the view. Where you live, can you see the Milky Way distinctly on a clear night? If not, find a place where you can if you expect to see the comet at its best.
Bright moonlight overwhelms the pale light of the comet. It will best be seen when the moon is below the horizon or in a phase within a few days of new moon. Also, the comet should be high enough in the sky to be out of the haze and dust. Its elevation will depend on what latitude you are in, north or south of the equator. Finally, what will the weather be like? A cloudy sky will frustrate your best plans.
Taking everything into account, when should you plan to look? The comet will become visible to the naked eye in December. It will be easier to see in the northern hemisphere. It will be in the evening sky, high in the southwest. After the full moon is out of the sky the first of December, for two weeks the skies will be dark. Don’t expect a brilliant show at this stage. Look for a faint, fuzzy light that moves westward from night to night.
By the end of December, the full moon will be out of the way again. By then the comet will be brighter, and the tail should be visible, but it will be moving closer to the western horizon. In the latter part of January, it will sink into the evening twilight and be lost to view on its way to rendezvous with the sun.
Do You Have Binoculars?
Binoculars will greatly enhance your view and appreciation of the comet, especially during its approach phase. For the nonspecialist they are better than a telescope because of their wider field of view. With binoculars, you might be able to catch sight of the comet before it reaches naked-eye visibility. Of course, you must know where to look. A good chance to find it will come November 15 to 17, when it will be passing just south of the famed Pleiades.b It will be nearest on the 16th, close enough to be seen in the same binocular field of view with the Pleiades. Look for a fuzzy star, and note its position among other nearby stars. Then look again an hour or two later and see whether it has moved westwardly. If it has, you will know that you are seeing the long-awaited Halley’s comet.
After perihelion, Halley’s comet should reach its greatest length and brightness early in April. Before that, in March, it may be seen rising, tail first, in the predawn sky. For viewers in northern countries, Japan, the United States, and Europe, it will be disappointingly low in the southern sky. But watchers in South America, southern Africa, and Australia will have a splendid view. The first week in April, Halley should be at its best, high in the sky with its tail arching across the zenith. The moon will be in its last quarter, and as its crescent shrinks toward the new moon on the 9th, the ever darker sky will provide the best background for enjoying all the splendor of our celestial visitor.
What Makes a Comet Behave as It Does?
The interest aroused by this exhibition in the sky will naturally suggest many questions about the mysterious object, so different from the stars and the planets. What is a comet? Where does it come from? What does it look like up close? What is its tail made of? Why does it undergo such remarkable changes as it comes close to the sun and then backs away?
These questions have fascinated generations of astronomers, but even now the answers remain most uncertain and speculative. When a comet comes close enough that we might hope to see some detailed features through a telescope, it veils its head (nucleus) in a diffuse cloud (coma), so that all we see is a fuzzy ball of mist. The light from the coma, analyzed in a spectroscope, tells us some of the things it is made of: water vapor, ammonia, methane, cyanogen. Also, atoms of common metals are revealed: iron, nickel, manganese, calcium, magnesium, sodium, and so on. All of these are driven out of the coma by the sun’s radiation to form the tail. The tail shines, as does the coma, by fluorescence and reflection of sunlight.
The size of comets is tremendous. The coma is often larger than the planets, sometimes even as large as the sun. The tails of comets are tens of millions of miles long; some have been over a hundred million miles, long enough to reach from the earth to the sun. The solid nucleus, however, is tiny in comparison. It is probably no more than a few miles across.
In keeping with the small nucleus, the bulk of the entire comet is billions of times less massive than the earth. The tail of a comet so great that it seems to fill the sky has so little substance that stars shine right through it. It is more rarefied than the best vacuum man can make. Realizing this has allayed earlier fears that the earth’s passage through a comet’s tail might be disastrous. Such a scare spread when Halley’s comet was last here. People panicked at the thought that the gases in the tail would poison the atmosphere and sought to protect themselves before the fateful day of May 18, 1910. But the comet’s tail brushed right over the earth without the slightest discernible effect.
The Birth and Death of Comets
It used to be thought that comets were onetime visitors from interstellar space. Occasionally, one would pass close enough to a large planet, like Jupiter, to be drawn into a closed elliptical orbit in our solar system. However, more recent research seems to imply that comets are gravitationally bound to the Sun like other members of the solar system. At times comets are hurled into a hyperbolic, or open-ended, orbit, and these leave the solar system forever.
In the currently popular theory of comets, the nucleus is described as a “dirty snowball,” made up of ices of water, methane, and ammonia, mixed with solid grains containing metallic elements. As the comet approaches the sun, it sublimates, emitting vapors and spewing out dust particles, to form the cloudlike coma. Closer to the sun, the vapors and dust are blown away from the coma by a solar wind of particles and by solar radiation to form the tail.
During this passage of Halley, astronomers hope to learn how nearly correct this picture is. Maneuvering space probes up beside the comet, they will take close-up pictures and measurements. Thus they expect to clear up some of the mysteries about comets.
Comets are not eternal. They are not even dependable for keeping time. A comet’s schedule is subject to change because of the repeated pull of planets near its path. In fact, a close passage may sling it permanently out of the system, as was done deliberately with the Voyager space vehicles. Also, a periodic comet suffers from aging. Every time it passes the sun, it expends some of its substance to recreate its coma and tail. Some short-period comets have disappeared after repeated circuits, leaving nothing behind but a shower of meteors. Halley’s comet is large enough to have outlived dozens of circuits without marked loss of luster, but it must eventually come to an end.
Comets Praise Their Creator
When you see Halley’s comet, think of the 19th Psalm 19 in the Bible. Surely, this comet is one of the marvels of the heavens, declaring the glory of God, even in silent splendor, without speech or words.
a 1 mile equals 1.6 km.
b If you are not familiar with the Pleiades, consult a star guidebook in your local library or refer to the July 8, 1977, issue of Awake!
[Diagrams on page 15]
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The elliptical orbit of Halley’s comet
White box indicates area of the comet’s orbit visible from the earth