Cambridge University Press 978-0-521-72170-7 - Stephen James O’Meara’s Observing the Night Sky with Binoculars: A Simple Guide to the Heavens Stephen James O’Meara Excerpt More information CHAPTER 1 A Dipper full of wonder here is no better way to start our journey than by in horror as Juno, red with rage, transformed her into a T admiring the Big Dipper (also known as the Plough giant bear. First Callisto’s arms turned hairy, then her legs. or Wagon), the most famous star pattern in the world. Her sensitive jaw became crooked. Her gentle lips peeled Spending your first nights with this easily seen star pat- back to reveal sharp teeth. When the deed was done, Cal- tern is also an excellent way to learn the basics of obser- listo fled into the woods on all fours. She became terrified vational astronomy. Doing so will teach you how to use of her new situation, because, although Callisto looked your binoculars to full advantage; besides, if you’re just like a beast, she had retained her human thoughts and beginning in this hobby, you’ll need to know the basics, heart. which are the building blocks of a strong foundation. I’ve One day, many years later, a young hunter encountered always been a proponent of learning by doing. So let’s a foraging bear and startled it. Out of surprise, the mighty get started. But before we do, understand that there’s no bear stood on its hind legs, opened wide its paws, and let need to rush. The stars, just like our friends, will always out a terrifying roar. The hunter jumped back. The bear be there for you, night after night, month after month, charged. But just as the hunter raised his spear to lance year after year. It’s the beauty of longevity. the beast, Jupiter appeared on the scene in time to stop Let’s start our journey with some understanding. First, the killing. Yes, the bear was Callisto, who, forgetting the Big Dipper is not a constellation. It is an asterism in the her appearance, was racing toward her long-lost son to constellation Ursa Major, the Great Bear. An asterism is a embrace him! familiar pattern of stars that forms part of one or more of Toprevent any future tragedy,Jupiter turned Arcas into a the 88 officially recognized constellations (see Appendix A). bear, so he could forever enjoy the company of his mother. The bowl of the Big Dipper lies in the body of the Bear, The great god then grabbed the two bears by their tails while the Dipper’s handle is the Bear’s tail – at least that’s and swirled them high over his head. As he swirled the how the early Greek and Roman stargazers saw it; other massive beasts, their tails stretched to great length before cultures, such as the Cherokee Indians of the southern Jupiter finally flung them into the heavens. Today we see Appalachian Mountains, saw the three handle stars as three Callisto and Arcas, as Ursa Major and Ursa Minor, the hunters in pursuit of a bear. Great and Little Bear, respectively. But the story is not over. When Juno discovered what Jupiter had done, she was outraged. How could her hus- How the Bear got its tail band place that adulteress and her spawn in the night sky Anyone who has seen a bear knows that its tail is nothing but a short stub. So why is Ursa Major’s tail so long? The answer, though a bit of a stretch, comes from the mytho- logical tale of Callisto – one of several chaste maidens who cared for Diana (Artemis),1 the virgin goddess of the Moon. As fate would have it, Callisto caught the ever- roaming eye of Jupiter (Zeus), the thunderbolt-wielding King of the Gods. After a roll in the hay, the maiden bore Jupiter a son, who she named Arcas (Arctos). Jupiter never could keep a secret from Juno (Hera), his wife and twin sister, who became enraged with jealousy. When Juno sought out and found Callisto, she grabbed the nymph by her long golden locks and threw her to the ground, screaming, “Curse your beauty! I’ll make sure that no one will desire you ever again!” With those words, Juno raised her arms. Lightning flashed, and thun- der pealed. Powerless to defend herself, Callisto watched 1 The names in parentheses are the Greek counterparts of the Roman or Latin names. © Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-72170-7 - Stephen James O’Meara’s Observing the Night Sky with Binoculars: A Simple Guide to the Heavens Stephen James O’Meara Excerpt More information 2 Observing the Night Sky with Binoculars so near the heavenly pole? Seeking comfort, she went to If you lived on the equator (latitude 0◦), the North Star her foster parents who ruled the seas and complained: would sit on the north horizon (altitude 0◦). Likewise, if, “Why should anyone fear offending Jupiter when he you live in New York City (40◦ latitude), the North Star rewards them for causing me displeasure?” She beseeched is 40◦ above the north horizon (40◦ altitude). How high her relatives to grant her one request: that the bears never is 40◦? Draw an imaginary line from the north horizon set foot in her parents’ sacred waters, as do the other stars to the point overhead (the sky’s zenith). That line spans an on their nightly courses; instead, she wanted the bears angular distance of 90◦. Forty degrees is almost one-half to circle the pole, night after night, without time to rest. of the way from the horizon to the zenith. Now hold an And so it is today that observers from mid-northern lati- upright fist at arm’s length and look at it with one eye tudes never see Ursa Major or Ursa Minor set below the closed. The amount of sky covered by the fist is about horizon; they are circumpolar constellations – ones that cir- 10◦. cle the north celestial pole without ever setting, which is why finding the Big Dipper is always a good way to start your skywatching experience. Given a low horizon, you can see it at any time of the year or night. How to find the Big Dipper Big Dipper To find the Big Dipper, you’ll need to get your bearings. Go outside when twilight ends and allow at least 15 min- utes for your eyes to adapt to the growing darkness. As Little Dipper you wait, determine which way is north; use a compass or the compass setting on your Global Positioning Ser- North Star EastWest vice (GPS), if necessary. When you face north, your left arm will point west, your right arm will point east, and 40˚ south will lie directly behind you. Find a tree, house, or other landmark at these cardinal points, and record them 30˚ in your observing log. When outside observing, be sure to use a red flashlight to record your notes; the eye’s rod 20˚ cells, which are responsible for our keen night vision, are insensitive to red light. 10˚ The landmarks you select will serve as guides to cer- tain stars or constellations mentioned later in this book. For instance, if I were to tell you to look for a bright star June 20th Looking North 9:00 p.m. halfway up the northeastern sky, you will know how to position your body (say, halfway between an oak tree in the north and a neighbor’s house in the east) before look- For an observer living in New York City to find the ing up. That said, your first challenge is to find Polaris, North Star, he or she would face north, place the base the North Star, which will serve as your faithful guiding of their upright fist on the horizon line, make a fist light. Be warned, despite popular myth, the North Star is with his or her other hand, and place it on top of the not the brightest star in the night sky; it ranks 48th. But it first fist (like one potato, two potato). Four fists equals is solitary and obvious; it also shines with a yellow light, about 40◦. The North Star should be sitting on the top so it’s not difficult to detect. fist. The height of the North Star above your horizon (its Now let’s look for the Big Dipper. Since the Dipper is altitude) is the same as your latitude on Earth. (Latitude circumpolar, you can go out at any time of the night, ◦ ◦ is measured from 0 at the equator to 90 at the pole.) at any time of the year to find it. If you go outside ◦ If you lived on the North Pole (latitude 90 ), the North and observe the Big Dipper and the North Star at, say, Star would be at the zenith (the point in the sky directly 9:00 pm on April 1, then observe them again on the same ◦ overhead; altitude 90 ). The reason for this coincidence night three hours later, you will see that the Dipper has ◦ is that the North Star happens to be within 1 of the rotated counterclockwise around the north celestial pole point where Earth’s imaginary axis of rotation intersects 2 the dome of the sky. wobble takes 26,000 years to complete, Earth’s imaginary axis slowly precesses over the years, transcribing an invisible circular path in the northern sky.