The Sky Observer's Guide: A Fully Illustrated, Authoritative and Easy-to-Use Guide
160
The Sky Observer's Guide: A Fully Illustrated, Authoritative and Easy-to-Use Guide
160eBook
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Overview
This eBook is best viewed on a color device.
Filled with practical information for the amateur astronomer, this Sky Observer's Golden Guide explains:
-How to select and use binoculars and telescopes
-How to best observe stars, the moon, planets, comets, meteors, and other celestial objects
-How to use star charts
Profusely illustrated with photographs, diagrams, charts, and tables, this guide is recommended by leading astronomers.
Product Details
| ISBN-13: | 9781466864870 |
|---|---|
| Publisher: | St. Martin's Publishing Group |
| Publication date: | 02/25/2014 |
| Series: | A Golden Guide from St. Martin's Press |
| Sold by: | Macmillan |
| Format: | eBook |
| Pages: | 160 |
| File size: | 10 MB |
About the Author
Golden Guides first appeared in 1949 and quickly established themselves as authorities on subjects from Natural History to Science. Relaunched in 2000, Golden Guides from St. Martin's Press feature modern, new covers as part of a multi-year, million-dollar program to revise, update, and expand the complete line of guides for a new generation of students.
R. Newton Mayall contributed to nature guides from Golden Guides and St. Martin's Press.
Margaret Mayall contributed to nature guides from Golden Guides and St. Martin's Press.
Jerome Wyckoff contributed to nature guides from Golden Guides and St. Martin's Press.
John Polgreen contributed to nature guides from Golden Guides and St. Martin's Press.
Read an Excerpt
CHAPTER 1
Becoming a Sky Observer
All of us, from childhood, have gazed at the sky in wonder. Sun and Moon, the wandering planets, the fiery trails of comets and meteors — these are things to marvel at. Man will never tire of looking up into the tremendous, sparkling bowl of space.
Sky watching was undoubtedly a pastime of prehistoric man. The ancient Egyptians and Babylonians, several thousand years ago, observed the heavens carefully enough to devise quite accurate calendars. Observations by Copernicus, Galileo, and others in the sixteenth and seventeenth centuries were among the first great steps to modern science. Even today, the science of astronomy depends on observation.
ASTRONOMY FOR EVERYBODY Astronomy is for the amateur as well as the professional. The amateur can see for himself the sights that stirred Galileo, the Herschels, and other great astronomers. A high-school boy may be the first to see a comet, a rug salesman may discover a nova, and a housewife can observe and map meteor showers. An amateur's faithful observations of a variable star may be just the data an observatory needs.
Although in some regions weather and climate are often unfavorable, any interested person in any part of the world can become a sky observer. The aspect of the sky differs from place to place, but the majesty of Sun and Moon, of stars and planets and nebulas, is to be seen everywhere.
This book is a guide to observing — to the use of binoculars and telescopes, the locating of sky objects, and what objects to look for and how best to see them. The beginning observer should have also a book on general astronomy. Even a little knowledge greatly increases the pleasure of observing, and it prepares us to undertake real astronomical projects. Most old hands have found that the fun of amateur astronomy is greatest when they are working on observation programs that are scientifically useful.
OBSERVING WITH UNAIDED EYES Even an observer without binoculars or a telescope can see many wonders of the heavens. The important thing is to know how tolook and what to look for. The constellations can be traced and identified. Some star clusters can be located, and eclipses and some comets observed. The changing positions of Sun, Moon, and the brighter planets can be closely watched, and some artificial satellites can be seen. The brightness and length of meteor trails can be estimated. Get used to finding your way about the sky with the eyes alone before trying a telescope.
BINOCULARS AND TELESCOPE Your first look at the heavens through good binoculars can be exciting. Binoculars with 50mm. lenses gather about 40 times as much light as the eye alone, revealing such features as mountains and craters of the Moon, sunspots, the four larger satellites of Jupiter, double stars and star clusters, and luminous clouds of cosmic gas such as the famous nebula in Orion. (Before observing the sun, see here)
With no more than binoculars, some observers do useful scientific work, such as recording light changes in variable stars and watching for novas and comets. A telescope is obtained by every serious amateur sooner or later. Refractors, with lenses 1&189; to 4 inches diameter, and reflectors, with mirrors of 3 to 6 inches, are popular types. The light- gathering and magnifying power of telescopes brings out details of the Moon's surface. It reveals Jupiter's larger satellites and its banded clouds, as well as markings on Mars and the rings of Saturn. With telescopes we can "split" double stars and distinguish star clusters, nebulas, comets, and sunspots. We can watch the Moon occult (that is, pass in front of) stars and planets. Light fluctuations of faint variable stars and novas can be detected.
Good small telescopes can give surprising performance. When conditions are right, an observer with a good 3-inch refractor or 6-inch reflector can see some features of Jupiter and Saturn more distinctly than they appear in observatory photographs.
FUN WITH THE CAMERA Many amateurs make use of the camera. The eye is sensitive only to the light it is receiving in the present instant, but photographic film is sensitive to light received over a long period of exposure. An amateur's camera can detect faint objects which the eye, even with the aid of a telescope, could never see. Even a simple camera gives exciting and useful results.
MAKING A TELESCOPE Some serious amateurs, not content with factory-made telescopes, make their own. They grind the lenses and mirrors, and design the mountings. It takes special knowledge and skill, yet hundreds of amateurs have made instruments that perform splendidly. Telescope-making classes are held at some planetariums, universities, and observatories. Books on telescope making are available from booksellers.
ORGANIZATIONS OF AMATEURS Many amateur observers belong to national organizations. These give members information on equipment, observing techniques, and standard methods of reporting their work. They set up observing programs and receive observational data from members. Data are sent to observatories for use in programs of research. Some organizations publish news of developments that interest amateurs. Local groups observe together, compare equipment, and promote public interest in astronomy.
The Observer's Equipment
CHARTS AND BOOKS Just as we gather a supply of maps and booklets before touring the country, so we must gather certain sources of information before touring the sky.
This book provides all necessary information for a good start in sky observing. The index will guide you to explanations of observing techniques and equipment, to lists of interesting objects to look for, and to tables indicating where and when to look for planets, eclipses, meteor showers, and periodic comets. For more background in astronomy, the reader may turn to books and periodicals recommended here.
Hundreds of stars, nebulas, and other objects can be located with the aid of the maps here. For fainter objects the more detailed charts to be found in a star atlas become indispensable. There are atlases of convenient size that show nearly all stars as faint as can be seen with binoculars. For serious work with a telescope, more detailed charts are needed.
Some beginners use a planisphere to learn constellations. One type has a "wheel" on which is printed a map of the constellations. The wheel is rotated within an envelope that has a window. When the wheel is set for any particular month, day, and hour, the window shows the positions of the constellations at that time.
BINOCULAR FACTS Every observer should own a good pair of binoculars. These gather far more light than the eye; they magnify images and use the capacity of both eyes.
Opera-glass binoculars consist essentially of two small refracting telescopes mounted together. At the front of each is a large lens, the objective, which gathers the light. At the rear is a smaller lens, the eyepiece or ocular, which does the magnifying. In the front part of the eyepiece is a third element, the erecting lens, which is necessary to prevent our getting an upside-down view.
In the large prism binoculars, the objectives are centered farther apart. The light rays from them must be brought closer together before they reach the eyepieces. This is done by a pair of prisms in each tube.
Opera glasses have objectives of about one inch diameter and a magnifying power of 2 to 3. Prism binoculars, with their larger objectives and higher magnification, are preferable for astronomical observing. Popular types have objectives of 35 to 50 millimeters (about 1? to 2 inches), and magnify 6 to 10 times.
Binoculars labeled ?7'50? magnify 7 times and have an objective 50 millimeters in diameter. The area of the objective determines light- gathering ability; so 7×50 binoculars gather more than 7×35's.
Binoculars vary also as to field of view. The field is the whole circular area we see through the instrument. Thus in binoculars with a 6° field we can see an area of sky 6° in diameter — equal to an area 100 feet in diameter at 1,000 feet.
Heavy binoculars make the arms tired and unsteady. The magnification increases the effect of unsteadiness. Usually 7-power glasses are the limit for ease in handling. Bigger ones ordinarily require a support.
TELESCOPE PRINCIPLES Astronomical telescopes are of two main types: refracting and reflecting.
In a simple refractor, light is gathered by a lens, and magnification is done by the eyepiece. There is no erecting lens, because this would cut down the amount of light delivered to the eye. The image seen by the observer is inverted, but this makes no difference in observation of most celestial objects.
With the telescope the observer usually gets several removable eyepieces. These are used for different degrees of magnification, as desired.
Every good astronomical telescope has a finder — a small telescope, usually of 5 or 6 power, with a wide field, mounted on the main tube. It is used for aiming the telescope, because the field seen through a high-power telescope is very small. Astronomical refractors generally have a star diagonal, also, to bend the light at right angles before it reaches the eyepiece. This allows us to observe objects overhead with comfort.
Reflecting telescopes use a mirror, not a lens, for the objective. It is a highly polished concave glass disk coated usually with aluminum or silver. Light from the star falls upon this mirror and is reflected to a smaller diagonal mirror or prism in the tube. This reflects the light to the eyepiece.
Refractors get out of adjustment less easily than reflectors. Less maintenance, such as realignment or the resurfacing of mirrors, is necessary. But reflectors are less expensive and more readily made by amateurs.
LIGHT-GATHERING POWER The telescope's ability to reveal faint objects depends mainly upon the size of its objective. A lens or mirror 3 inches in diameter will gather two times as much light as a 2- inch, and a 6-inch will gather four times as much as a 3-inch. Figures given in the table here are only approximate. Some telescopes can do better. Actual performance depends partly upon seeing conditions, quality of the instrument, and the observer's vision.
MAGNIFYING POWER The eyepiece of a telescope bends the light rays so that they form a larger image on the retina of the eye than would be formed if no eyepiece were used. The image size depends upon the focal length of the eyepiece. The focal length is the distance between the eyepiece and the point at which the converging rays of light meet. The shorter the focal length, the larger the image. Focal lengths of typical telescopic eyepieces range from 1/4 inch to 1&189; or 2 inches.
Magnification given by a telescope depends not only upon the eyepiece being used, but also upon the focal length of the objective. The longer the focal length of the objective, the greater the magnification obtained with any given eyepiece.
To determine the magnification being obtained, we divide the focal length of the eyepiece into the focal length of the objective. For example, if the focal length of the objective is 50 inches, a &189;-inch eyepiece will give 100 power (?100×?).
Theoretically, there is no limit to the magnifying power of an instrument. Practically, there is. As we use eyepieces of higher power, the image becomes more and more fuzzy, though larger. Finally the fuzziness becomes so extreme that the object is seen less distinctly than at a lower power.
The practical magnifying limit depends mainly upon the diameter of the objective. For well-made telescopes the limit is about 50 times the diameter of the objective, in inches. This means about 150× for a 3-inch telescope, or 300× for a 6-inch. As the observer becomes familiar with his own telescope, he may find it has a somewhat different limit — say, 40 or 60. The exact figure will depend partly upon the atmospheric conditions.
RESOLVING POWER The resolving power of an instrument is its ability to show fine detail — for example, markings on planets. To determine the theoretical resolving power of an objective, divide the number 4.5 by the diameter of the objective in inches. The answer (known as "Dawes' limit") is the distance, in seconds of arc, between the closest objects that can be distinguished.
A good 3-inch lens should separate objects about 1.5? (seconds) apart. One second of arc is 1/60' (minute) or 1/3600° (degree). A degree is 1/90th of the distance from the horizon to the zenith (point in the sky directly overhead). The average unaided human eye, under good conditions, can distinguish stars that are about 180? apart. The performance of an objective depends upon quality of the glass, optical surfaces, seeing conditions, and proper alignment of the telescope.
TELESCOPE MOUNTINGS Since it gives such high magnification, a telescope must have a strong, steady mounting. The two main types of mounting are the altazimuth and the equatorial.
The altazimuth mounting is the simpler. It allows two motions of the telescope — up and down, an "altitude" motion; and horizontal, an "azimuth" motion.
This is a good general-purpose mounting. It is light, portable, and easily taken down and set up; usually it rests on a tripod. Most telescopes with objectives of less than 3 inches have this type of mounting.
The equatorial mounting is designed to be set up in a certain way in a specially prepared location, though it too is used for some small portable telescopes. In its simplest form, the equatorial has two axes at right angles to each other. It is an all-purpose mounting, generally used for serious work. To make the most of it, we must set it up properly (see here). Some equatorials have setting circles, which make it possible to aim the instrument automatically at the right point in the heavens (see here).
QUALITY OF EQUIPMENT Both for serious astronomical work and for plain fun, quality in equipment is all-important. Test instruments before buying. Haziness, milkiness, or rainbow colors in the field are a sign of poor optical parts. Good instruments will reduce stars to neat points of light, and show distant print without distortion. If an object as viewed "dances" when the telescope is lightly touched, the mounting is below par. A poor mounting spells inconvenience and frustration.
Price is not always an indicator of quality. Some low-cost instruments turn out well, but there is always risk in buying them. If possible, the buyer should have the advice of an expert. Some buyers exaggerate the importance of "power." They buy the biggest telescope available at a given price — only to learn, later, that a smaller instrument of better quality would have given greater satisfaction.
Understanding the Sky
A yardful of expensive equipment cannot make up for an ignorance of astronomy. Every observer should have a basic astronomy guide (see here) and read it. But here is a review of facts that directly affect observing.
Astronomers call the sky, as seen from Earth, the "celestial sphere." It can be imagined as an enormous hollow ball with Earth at the center, and with the stars on the inside surface. As Earth rotates, the stars seem to parade by.
Exactly what section of sky the observer can see depends partly upon his location. The sky seen from the North Pole is completely different from the sky seen from the South Pole. Between the poles there is an overlapping. An observer looking south from New York sees a portion of the northern part of the sky that is seen from Rio de Janeiro. People in Rio can see only the southern part of the sky area that is seen from New York. Theoretically, a person at the Equator can see the whole sky, but he can see only half of it at once.
At night the sky appears to pass steadily overhead, east to west. This seeming motion is due to Earth's rotation. From the North Pole, the sky appears to turn like an enormous wheel, counterclockwise, with its hub directly overhead at the so-called North Star. From the South Pole the sky appears as a wheel turning clockwise. For an observer halfway between the Equator and the North or South Pole, the hub is just halfway up (45°) from the horizon. Stars within 45° of the hub remain in view all night as they move around it. Objects farther than 45° from the hub rise and set. Objects near the center of the wheel seem to move slower than stars farther out. All, however, are moving at the same speed in degrees — about 360° in 24 hours.
STARS BY SEASONS A star or planet appears to move at a speed of about 15° per hour along its circle. But each evening it rises about 4 minutes earlier than the evening before. This daily gain is due to the progress of Earth in its journey around the Sun, and amounts to a gain of a day in the course of a year. Stars that rise and set at any particular latitude, therefore, are not visible all year. During some of the year, the time between rising and setting will occur during daylight. The star charts here show that each constellation, at a given hour, is farther west in summer than in spring, farther west in fall than in summer, and so on.
"FIXED" STARS AND MOVING PLANETS Stars are so distant that, though traveling many miles per second, they look motionless. Constellations remain the same year after year. Only over centuries could changes in their shapes be noticed by the unaided eye.
But all objects within our solar system are much closer. As seen from Earth, they move against the background of the constellations. The Moon, during most of the year, rises an average of 50 minutes later each night, and the height of its path in the heavens changes with the seasons. Positions of all the planets, asteroids, and comets change as well. The Sun's motion against the background of stars is not so noticeable, but does occur.
ECLIPTIC AND ZODIAC The path of the Sun against the background of stars is called the ecliptic. In the course of each day, the Sun moves about 1° against the background. In a year it makes the full circuit of 360°.
(Continues…)
Excerpted from "The Sky Observer's Guide"
by .
Copyright © 2002 St. Martin's Press.
Excerpted by permission of St. Martin's Press.
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Table of Contents
Becoming a Sky Observer,
The Observer's Equipment,
Understanding the Sky,
First Steps in Observing,
Using a Telescope,
Star Charts and Setting Circles,
The Moon,
The Sun,
Sky Colors,
The Planets and Asteroids,
Comets,
Meteors,
Stars,
Nebulas,
Drawing Sky Objects,
The Sky Observer's Camera,
Using Astronomical Time,
Accessories and Maintenance,
Incidental Information,
Maps of the Heavens,
Index,