A Field Guide to the Stars and Planets - Hardcover

Pasachoff, Jay M.

Hardcover

ISBN 10: 039593432X ISBN 13: 9780395934326

Publisher: Houghton Mifflin, 1999

Synopsis

The fourth edition of this best-selling field guide has been completely revised and updated to include the latest information from leading astronomical sources. All the time-sensitive material is new and valid through 2010: solar eclipses, phases of the moon, positions of the planets, and more. Twenty-four Monthly Sky Maps, all newly revised and in color, show exactly what you'll see when facing north or south in the night sky. Fifty-two Atlas Charts, also revised and in color, cover the entire sky, including close-ups of areas of special interest such as the Pleiades and the Orion Nebula. The hundreds of thousands of devoted users of the previous editions of this guide have been eagerly awaiting this new volume so they can continue to enjoy their hobby in the coming decades.

About this title

About the Author:

Jay M. Pasachoff is the Field Memorial Professor of Astronomy and director of the Hopkins Observatory at Williams College in Williamstown, Massachusetts. Professor Pasachoff has done extensive research on the solar corona at total eclipses and has been on 29 eclipse expeditions. He has also used a wide variety of telescopes around the world. He is the author of popular textbooks on astronomy and on other science subjects. He has twice been chair of the Astronomy Division of the American Association for the Advancement of Science and is the chair of the Working Group on Eclipses of the International Astronomical Union and the U.S. national representative to the IAU's Commission on the Teaching of Astronomy.

Excerpt. © Reprinted by permission. All rights reserved.:

1 A First Look At The Sky
I hope you will use this book to become familiar with the sky.
Finding your way around the sky is like finding your way around a
large city- it is easy if you are familiar with the streets and have
navigated there before, but otherwise it takes some time to become
familiar with routes and shortcuts. In this first chapter of A Field
Guide to the Stars and Planets, I will assume that you are new to
observing the heavens. I will start from scratch and show you some of
the basic ways that you can orient yourself when observing. My focus
here will be on some of the most prominent stars and constellations
that you can observe with the naked eye or binoculars.

Before you begin to observe the nighttime sky, you should know which
way north, south, east, and west are. If you don't know the compass
directions for the place from which you are observing, though, you
can find them with the aid of the Big Dipper and the North Star,
Polaris (see p. 19).

One of the first things you will notice when you start to study the
sky is that stars and other objects are of different brightnesses.
Perhaps the easiest way to determine what is what in the sky is to
take advantage of this fact. Except for the moon, the brightest
objects in the nighttime sky are some of the planets. The planets
change their positions slightly from night to night with respect to
the stars in the background; in chapter 9 1 show you how to locate
the planets on any given night.

Three characteristics tell you quickly if an object is a planet.

1. BRIGHTNESS. Some of the planets simply appear too bright to be
stars. Venus, the brightest planet, is an example. It can never be
very far away from the sun in the sky, so whenever an extremely
bright dot of light-the "evening star"-appears in the sky toward the
west after sunset, or in the morning sky toward the east before
sunrise-the "morning star"-it is probably Venus (fig. 1-1).

It is often the first bright object visible, before any stars appear
in the sky. Mercury also appears in these areas of the sky around
sunrise and sunset, but it never looks as bright as Venus nor gets as
far from the sun as Venus does. Mercury appears only during twilight,
and Venus never remains visible through the night. Whenever a very
bright yellowish white point of light appears in the sky in the
middle of the night, it is probably Jupiter. Unlike Mercury and
Venus, Jupiter is not always near the sun in the sky; it can appear
high in the sky at midnight. Mars and Saturn can also appear far from
the sun in the sky, rising well after sunset; Mars rarely outshines
Jupiter, though, and the brightness of Sat-urn never equals that of
Jupiter or Venus. Mars can often be dis-tinguished by the fact that
it has a slight but distinct reddish tinge. Saturn, on the other
hand, appears to be yellowish. The other planets are too faint to be
seen with the naked eye.

2. TWINKLING. Planets usually seem to shine steadily, while stars
twinkle. Twinkling is an effect of turbulence in the earth's atmo-
sphere: the atmosphere bends the starlight passing through it, and,
as small regions of the atmosphere move about, the intensity of a
star's light varies slightly but rapidly. Observations with a
telescope would also reveal that a star appears to move around
slightly. The reason why stars twinkle and planets do not is that
stars are so far away that they look like points even when viewed
through large telescopes; planets, though, are close enough to earth
that their telescopic images are tiny disks. The light from different
parts of a planet's disk averages out and makes the planet appear
relatively steady in both brightness and position.

If the atmosphere is especially turbulent, or if you are looking
through an especially large amount of atmosphere (when you are
looking at an object low in the sky, for example, making your line of
sight pass obliquely through the atmosphere), even planets can seem
to twinkle. Under these conditions, the object you are observing may
even seem to change in color-when Venus is low in the western sky, it
is not uncommon to see it change from greenish to reddish and back
again.

3. LOCATION. All the planets always appear close to an imaginary line
across the sky, so objects located far from that line cannot be
planets. The line is called the ecliptic, and it is followed (more or
less) not only by the planets but also by the moon. (The ecliptic is
actually the path followed by the sun across the background of stars
in the course of the year.) Since the earth is but one of the
planets, and since all the planets orbit the sun in approximately the
same plane, from our point of view the planets and sun must follow
roughly the same line across the sky. The moon orbits the earth at
only a slight angle to the plane of the planets, so it too always
appears close to the ecliptic. The location of the ecliptic is
plotted as a dotted line on the Monthly Sky Maps in chapter 3, which
show how the sky looks to the naked eye at different times.

From northern temperate latitudes, including the continental U.S.,
Canada, and Europe, the ecliptic crosses the southern part of the
sky. This means that any bright objects at the zenith -the point
directly over your head-or in the northern sky cannot be planets.
(There are occasional exceptions to this if you are observing from
the southernmost parts of the U.S.)

Now that you know how to tell whether you are looking at a star or a
planet, you can look around the sky and identify some of the
brightest stars. Some people find it easier to identify a few
individual bright stars. Others prefer to locate a few favorite
constellations or color photo asterisms-a few stars, also roughly in
the same direction from us, that are parts of one or more
constellations.

Many people can identify one or two specific constellations or
asterisms, even though they don't know any other constellations.
(This statement holds true for many professional astronomers.) The
most prominent asterism in the sky is the Big Dipper, whose seven
stars trace out the shape of a dipper in the northern sky (fig. 1 -
2). The Big Dipper is an asterism rather than a constellation be
cause it makes up only part of the constellation Ursa Major, the Big
Bear (fig. 2-2, P. 23)

The four stars in the bowl of the Big Dipper make a squarish
(actually trapezoidal) shape about 10 across. (Ten degrees is about
the width of your fist, if you hold it up at arm's length against the
sky.) Curving away from the bowl are the three stars in the handle,
which cover another 15 degrees. All the stars in the Big Dipper
except the one that connects the handle to the bowl are of about the
same brightness, which makes it easy to single out the Dipper in the
sky.

Sky observers -including both professional and amateur astronomers -
usually express star brightness in magnitudes, the scale of which is
described in detail in chapter 3. The lower the magnitude, the
brighter the star. The brightest stars in the sky are magnitude zero
(o), or in two cases, magnitudes -0-7 and -1.4 Figures 1.4 and 1.5 on
pp. 14-17 show the brightest stars in the sky; the faintest star
shown is magnitude 3 - 5. The naked eye can see stars about 10 times
fainter than this, down to those as dim as 6th magnitude under
perfect sky conditions.

One difference between the maps or charts in this guide and the real
sky is that all the stars in the sky look like points, even though
they have different brightnesses. The charts and maps in this guide
represent these different brightnesses (magnitudes) as circles of
different sizes.

It is often interesting to begin by identifying the brightest star
near the zenith. Table 1 (P. 1 2) lists the 2 1 brightest stars in
the sky. Following the table is a display-a Graphic Timetable (fig. 1-
3)- that shows when the brightest stars visible from midnorthern
latitudes are passing their highest points in the sky. On any given
date, different stars will be overhead at different times of night;
the whole sequence changes with the seasons as the earth orbits the
sun. The positions of the stars repeat from year to year.

To use the Graphic Timetable of the Brightest Stars (fig. 1-3), run
your finger down the side to find your date of observation, then move
across the page to find the time of night when you are observing. You
will see the names of the brightest stars that are transiting at
about that time. An object transits when it passes your meridian-the
imaginary line passing from the point due north on the horizon
through the zenith to the point due south on the horizon.

Figure 1-3 also shows how high the stars are in the sky, in degrees
above the horizon, at their time of transit, for an observer at 40' N
latitude. This altitude above the horizon is the highest point that
each star reaches in the arc it traces across the sky. For example,
Sirius, the brightest star in the sky, reaches a maximum Of 33
degrees above the southern horizon - slightly more than one-third the
altitude of the zenith. Since your fist covers about 10 degrees of
sky (when you place your thumb flat on the outside of the fist and
hold it at arm's length), you can mark off the altitude above the
horizon in 10 degrees segments. You may want to verify first that
about nine of your fists indeed cover go' from horizon to zenith.

In the region near the ecliptic, a bright object could be a star or a
planet. When looking in this part of the sky, do make sure you know
which planets are up (above the horizon). The Graphic Timetables in
chapter 9 provide this information.

Figures 1-4 and 1-5 are pairs of sky maps centered on the north
celestial pole and on the south celestial pole, respectively. The
celestial poles are the imaginary points where the earth's axis, if
extended, would meet the celestial sphere. The north and south
celestial poles lie above the earth's north and south poles,
respectively. As the earth rotates, the sky appears to rotate in the
opposite direction around the celestial poles. The sky thus seems to
rotate once around the celestial poles every 24 hours. Midway between
the celestial poles is the celestial equator, which lies on the
celestial sphere, above the earth's equator. The celestial equator
separates the northern and southern halves of the sky.

Figure 1-4 shows the northern half of the celestial sphere, and is
spread across two pages with some overlap between. This map is
centered on the north celestial pole. Below that pole is the Big
Dipper. Since the sky appears to rotate around the north or south
celestial pole (depending on which hemisphere you are in), whichever
pole you can see always remains at a constant height in the sky. (If
you are observing from a latitude Of 40' N on earth, the north
celestial pole will always be tilted up 40' degrees above due north
on the horizon; if you are observing from a latitude Of 30 degrees N,
the pole will always be tilted up 30 degrees, etc.) Observers at
midnorthern latitudes will see the Big Dipper appear to revolve
around the north celestial pole every 24 hours. For these observers,
the Big Dipper is close enough to the north celestial pole that it
will never set, and is thus an example of a circumpolar asterism.

Figure 1-5 shows the southern half of the celestial sphere. It
includes some stars (near the celestial equator) that midnorthern
observers can sometimes see and some stars that never rise above the
horizon at northern latitudes.

The Big Dipper is a particularly handy asterism to know because you
can follow lines marked out by its stars and trace them across the
sky to other interesting objects. Best known is the line marked by
the two stars at the end of the bowl, which are known as the
Pointers. These two stars point to the North Star, Polaris. To find
Polaris, follow a straight line from the Pointers upward from the
bowl of the Dipper and try to imagine the line curving slightly as it
follows the curve of the sky for about 30 degrees. (This is three
fists' width, or about five times the distance between the Pointers,
which are separated by 5 1/2 degrees.) Polaris is at the end of an
asterism known as the Little Dipper. None of the stars in the Little
Dipper is as bright as the five brightest stars of the Big Dipper;
the back two stars of the bowl and the two stars between the bowl and
Polaris may be hard to see with your naked eye.

Polaris is not an especially bright star, but it is bright enough to
be visible ordinarily. It is the brightest star in that region of the
sky, so it is not easily confused with other stars. Polaris is within
1 degree of the true north celestial pole and is thus of help not
only to navigators at sea but also to land-based amateurs navigating
around the sky. If you face Polaris, you are facing north. Thus it is
best to find Polaris in order to orient yourself before you use any
of the charts or maps in this guide.

If you continue along the arc from the Pointers through Polaris, you
will come to the Great Square of Pegasus. This pathway and others
that you can follow from one constellation to another are marked with
dotted lines on Figs. 1-4 and 1-5. For example, instead of following
the Pointers to Polaris, you can follow the curve of the Big Dipper's
handle over about 300 (three fists' width, thumb included) of sky to
the bright star Arcturus. If you can follow the same arc for another
30 degrees without hitting the horizon, you will come to the bright
star Spica. To remember this, think of "arc to Arcturus," and
then "spike to Spica."

If, instead of finding Polaris, you follow the Pointers or the two
stars that form the rear of the Big Dipper's bowl in the opposite
direction, you will come to the constellation Leo, the Lion, about 35
degrees away. Leo contains the bright star Regulus, which is located
at the base of the "sickle" in figure 1-4. You can find other stars
and constellations using the pathways marked on figures 1-4 and 1-5;
the angles between some of the stars and constellations are listed on
page 20....

About this edition

• Publisher: Houghton Mifflin
• Publication date: 1999
• ISBN 10: 039593432X
• ISBN 13: 9780395934326
• Binding: Hardcover
• Number of pages: 578