Guide to Stars and Planets (Collins Field Guides) - Hardcover

Ridpath, Ian; Tirion, Wil

Hardcover

ISBN 10: 0002190710 ISBN 13: 9780002190718

Publisher: HarperCollins Publishers, 1984

Synopsis

Stars and Planets is a sturdy, thorough field guide for amateur astronomers. The book's first section is a general introduction to astronomy. A solar system primer and constellation catalog are followed by a month-by-month night sky guide. Filled with clear, easy-to-read star charts, photos, and diagrams, this is the perfect starter for beginning astronomers, and a handy reference for those with a little more experience. You'll find information on stargazing equipment, a glossary of terminology, and the history of each cosmic feature's discovery. Like all the Eyewitness Handbooks , this one will be a terrific addition to your family science library. --Therese Littleton

About this title

About the Author

Sir Patrick Moore is the author of 60 books including Firefly Atlas of the Universe and hosts the BBC-TV series, The Sky at Night.

Excerpt. © Reprinted by permission. All rights reserved.

Foreword

Astronomy today is a fast-moving science. This applies to amateur work as well as to professionals, and books become out of date very quickly. Much has happened even since 2001, when the Guide to Stars and Planets was last revised. For example, many small telescopes are much less expensive than they used to be, and many amateurs can afford something reasonably effective, with the result that mirror grinding is no longer so popular, and I have amended the text accordingly. I have said very little about prices, because they alter so rapidly. Moreover, electronic aids such as CCDs (Charge-Coupled Devices) have also become available, and are to a large extent superseding photographic plates, as some of the pictures shown in this new edition make clear.

There are also many other modifications, and I have done my best to make the text as up to date as possible.

Patrick Moore. Selsey, 2005

Preface

I have often been asked "How does one go about making astronomy one's hobby?" I can best answer this by saying what I did when I was very young. First, do some reading and make sure that you understand the basic principles. Next, take an elementary star map, go outdoors at night and learn your way around the sky: it does not take long, and the stars become so much more interesting when you know which is which. Then obtain a pair of binoculars, and turn them skyward (taking great care not to look anywhere near the Sun). Join a local and/or national society. By now you will begin to see where your main interests lie, and it will be time to think about obtaining a telescope.

This book is strictly for newcomers. I hope that you will find it useful; I have tried to give a comprehensive survey.

--

Chapter 1 Introduction to Astronomy

Astronomy is the study of the sky and everything we see there. It is the oldest science in the world, and fascinates all kinds of people -- young and old, amateurs and professionals.

Understanding the skies

The Earth is a planet, 12,756 kilometers in diameter, which moves round the Sun once every 365.86 days at a mean distance of 150 million kilometers. It is a member of the Sun's family or Solar System and there are eight other planets in the system, some of which have satellites orbiting them. The Earth has one satellite, the familiar Moon. The stars themselves are suns, and these are so remote that even their light, moving at 300,000 kilometers per second, takes years to reach the Earth.

Because the stars are so far away, their individual or "proper" motions are very slight, and the patterns or constellations now known to astronomers are the same as those that must have been seen by prehistoric man. It is convenient to picture the Earth as being surrounded by a sphere, the celestial sphere, which has a center the same as the center of the Earth. The various objects in the sky may then be positioned on this imaginary sphere.

[The book includes a series of diagrams, expanding on and illustrating the following text.]

In the sky the equivalent of latitude on Earth is known as declination: that is, the angular distance north or south of the celestial equator. The celestial equivalent of longitude is right ascension, and this is the angular distance of a body from the First Point of Aries (see diagram above), measured eastward; it is usually measured in units of time rather than in degrees. As the Earth rotates, a star will seem to rise, reach its highest point or "culmination" and then set. Any other fixed point on the celestial sphere also culminates once a day. The right ascension of an object is defined as the time interval between the culmination of the First Point of Aries and the culmination of the object concerned.

Apart from the effects of "precession," a minor shift in position of the celestial poles and equator due to a slight "wobbling" of the Earth's axis, the right ascensions and declinations of the stars do not change appreciably. Thus a star's celestial coordinates can be sufficiently well established using the celestial equator as the reference plane.

Over a sufficiently long period of time, however, the effects of precession become appreciable. At the moment astronomical catalogs give stellar positions for "epoch 2000," but older catalogs give them for "epoch 1950" -- so when really accurate setting of a telescope is needed, make sure that you have an up-to-date catalog.

Astronomers are used to vast distances and immense spans of tune. The Moon, at a mean distance of 384,400 kilometers from the Earth, is its closest neighbor; even the Sun (150 million kilometers away) is relatively very near the Earth. The average Sun-Earth distance is called the astronomical unit (a.u.) and is used in describing distances on the scale of the Solar System; but in measuring the distances of the stars, even this unit becomes hopelessly inconvenient.

Instead, astronomers use the light-year, which is the distance traveled by light in a year. Light moves at 300,000 kilometers a second, so one light-year is equal to 9.46 million million kilometers. Even the nearest star beyond the Sun is over four light-years away; this is why the "proper motions" of the stars -- their apparent motions across the celestial sphere -- are so slight. The star with the greatest known proper motion (Barnard's Star, a faint red dwarf) takes 180 years to crawl a distance equal to the apparent diameter of the full Moon.

Another unit used by astronomers is the parsec. One parsec is equal to 3.26 light-years. It is defined in such a way that a star at a distance of one parsec would appear to move through one second of arc during the year, as a result of the Earth's motion around the Sun (see below).

A "constellation" has no real meaning, as the stars are not at equal distances from the Earth; it is simply a pattern of stars that happen to lie in much the same direction, as seen from Earth. For example, the stars of the Big Dipper are part of the constellation of Ursa Major. Though they all look equally far away, their distances actually differ enormously, ranging from 59 light-years (Mizar) to 108 light-years for Alkaid, so Alkaid is much further from Mizar than might be thought. From a different vantage point in the universe, the constellation patterns would look quite different.

The stars are suns, but there is a tremendous range in both size and luminosity. The apparent magnitude of a star is a measure of its brightness as seen from Earth. The scale works like a golfer's handicap, with the more brilliant performers having the lower values. Thus bright stars are of magnitude 1 or even less (the four brightest stars in the sky have negative magnitudes), while stars of magnitude 6 represent the limit of naked-eye visibility under good conditions, and large telescopes can reach well below magnitude +20.

Everything, however, depends upon the star's distance: for example, Vega (magnitude 0.04) looks much brighter than Deneb (magnitude 1.3), yet Vega is a mere 55 times as luminous as the Sun, while Deneb equals over 200,000 Suns. Deneb is more remote, and its absolute magnitude is far brighter. (Absolute magnitude is defined as the apparent magnitude that a star would have at a distance of 10 parsecs.)

Edition details

• Publisher: HarperCollins Publishers
• Publication date: 1984
• Language: English
• ISBN 10: 0002190710
• ISBN 13: 9780002190718
• Binding: Hardcover
• Number of pages: 320