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Peterson Field Guides #4: A Field Guide to Stars and Planets
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 planeets 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....
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