Many years ago I took a bird-watching tour in the Peruvian Amazon. We hiked through the rainforest at dawn, while our guide spotted birds. All I could see was trees, but he could always find a colorful parakeet or macaw hidden in the lush green canopy. “Look there! Just below that crooked branch,” he would say. Once he pointed it out, it was obvious. But without him, we would have seen nothing.
I asked him how he did it. Was his eyesight unusually sharp? No, he said, it was just experience. After spending years in the rainforest, he could guess where birds were likely to be. He taught me a few tricks, such as always looking at the tallest tree around because birds like to perch there. A few simple guidelines allowed me to see the invisible.
THE MILKY WAY
Observing the many beautiful objects of the night sky is both easier and harder than bird-watching. On the one hand, most sights follow a predictable pattern: the Orion Nebula is always in the sky in winter (at least in the Northern Hemisphere). On the other hand, seeing detail in a faint patch of light will challenge your skills even if you know where to look.
Only experience—hundreds of nights spent under the stars—will turn you into an expert. And while this book is not designed to be a complete field guide, I hope to give you a few tricks to get you started.
If you could stand perfectly still out in space, the stars would not move at all—not in less than a human lifespan, anyway. But of course, we’re not standing still. We’re standing on a planet that spins on its axis once a day and orbits around its sun once a year. We’re constantly moving, and therefore the stars appear to move.
Imagine that you’re standing on a spinning carousel, looking up at a sky dotted with clouds. As you spin around, you might notice that all the clouds seem to be spinning around a single cloud in the center of your view. There’s nothing special about that cloud: it just happens to be directly above the carousel’s spinning axis.
STAR TRAILS AND THE NORTH STAR
In the same way, the axis of the world’s rotation points toward Polaris—the North Star. As the Earth turns, all the stars appear to circle the North Star.
The Earth takes approximately 24 hours to make a full rotation. Since there are 360 degrees in a full circle, the stars around Polaris move 15 degrees per hour. Each hour, some stars will set in the west while others will rise in the east. You’ll see a different set of stars at 9 p.m. than at 3 a.m.
At the same time, the Earth is orbiting the sun. In winter we’re on the other side of the sun when compared to summer. That means the stars of winter are different from the stars of summer. There are 12 months in the year, therefore the stars shift by 30 degrees every month.
The stars in your sky therefore depend on the date, the time, and your position on Earth (specifically, your latitude). The sky charts in this book (and all printed books) are designed for a specific date, time, and latitude.
Planetarium programs such as Stellarium, TheSky, and Starry Night can accurately plot the skies for any date, time, and location you choose.
Long before TV beamed sitcoms and crime dramas to our living rooms at night, people told each other stories under the stars. And it doesn’t take much imagination to convert the random pattern of stars in the sky into familiar animals, heroic figures, and even gruesome monsters.
ORION, ONE OF THE MOST FAMOUS CONSTELLATIONS
More than 2,000 years ago, Ptolemy codified 48 constellations in his book, The Almagest. Many of the most well-known constellations, like Orion and Leo, were described by Ptolemy, though they probably predate him by at least a millennium.
Though the ancients did not know it, the stars that make up a constellation are generally unrelated. While they appear close together from Earth, in reality they could be thousands of light-years apart. For example, the three stars in Orion’s belt appear to be right next to each other, but in reality, one is twice as far away as the other two.
In 1603, Johann Bayer assigned Greek letters to the brightest stars in each constellation in descending order of brightness. The brightest star was named Alpha, the second brightest was named Beta, and so on. Thus stars could be named by letter and constellation name: Alpha Centauri is the brightest star in the Centaurus constellation.
In later years, astronomers devised constellations for the stars of the southern skies, and named them after scientific objects that were then top of mind: Microscopium (the microscope), Horologium (the clock), and Fornax (the chemical furnace).
Today, the International Astronomical Union (IAU) defines a constellation as a region of the sky, not just an arrangement of stars. Every part of the sky belongs to one (and only one) constellation. The rectilinear boundaries of the constellations look like the boundaries of the Western US. Every star (and even every galaxy) belongs to one of the 88 modern constellations defined by the IAU.
The easiest way to navigate the night sky is, of course, with a (properly aligned) computerized go-to telescope. There is no shame in using technology to make your life easier. But there are also many reasons to learn how to find objects in the sky on your own, not least being the joy of feeling at home among the stars. When you can look up in the night sky and recognize the stars and constellations, you will feel like a native in your home city, not like a lost tourist.
Start by learning the major star patterns and constellations for each season. Use the seasonal sky charts starting to orient yourself at night. There are also many good books and apps for learning the night sky. I can recommend Jonathan Poppele’s Night Sky, and Turn Left at Orion by Consolmagno and Davis.
Once you can recognize the major stars, you will be able to find objects by “star hopping.” In this book, I’ll often direct you to the star that is closest to the sight; in astronomy, stars typically don’t have proper names, so I reference their “official” names, which consist of Alpha, Beta, and so on, and these are labeled on the star charts on each page. Of course, if a star has an actual name, like “Alcor,” for instance, I’ll reference that instead.
THE BIG DIPPER, PART OF THE CONSTELLATION URSA MAJOR
For example, suppose you’re trying to find the Ring Nebula (see the chart). You can start by aiming your telescope or binoculars at Vega—one of the brightest stars in the sky and one of the stars you should learn to recognize.
By consulting the sky chart you should be able to move from Vega to Zeta Lyrae, the star in the nearest corner of the quadrilateral in Lyra. From there you can move to Beta Lyrae, another corner star. Finally, you can move between Beta Lyrae and Gamma Lyrae to find the Ring Nebula.
Unlike the stars, the planets and moons of our solar system move in their own cycles. They are not in the same place at the same time each year.
THE MOON The nearest celestial object is the moon; it is tens of millions of times closer than even the nearest star, and yet it took more than three days at hypersonic speeds for the Apollo astronauts to reach it.
THE PHASES OF THE MOON
The moon changes phase as it moves in its orbit because the sun hits it from a different angle. When the sun hits it face on—as seen from Earth—it’s full. When it hits at a right angle, it is half full.
The best time to see the features of the moon is when it is around half full. Craters and mountains at the boundary between light and dark (known as the terminator) will have long shadows, which make them easy to see. Use binoculars or a telescope, and focus on the terminator.
VENUS IN THE EVENING SKY
THE INNER PLANETS Mercury and Venus are closer to the sun than Earth, and from our vantage point, they don’t wander too far away from it. When they are closest to Earth, they are in front of the sun, which means we can’t see them. But when they are “full”—that is, when sunlight is hitting them face on, they are farthest from Earth and behind the sun.
The best time to see them is when they are at right angles to the sun with respect to Earth. At those times they are visible after sunset or right before sunrise. This is why Venus is known as the Evening (or Morning) Star.
Use the charts on 157 and 99, respectively, to figure out the best times to see Mercury and Venus.
MARS Every two years and two months (or so) Mars lines up such that it is directly opposite the sun as seen from Earth. This is known as opposition, and it is the best time to see the Red Planet.
MARS AND ITS RUDDY GLOW
With the naked eye, Mars is easily recognizable by its reddish-orange tint. Through a telescope, high magnification is required to see any detail. To give yourself the best possible view, observe Mars during opposition, when it’s visible all night (and closest to Earth); consult the chart for upcoming opposition dates.
JUPITER AND SATURN These gas giants rule the outer solar system. Both are visible through at least part of each year. Jupiter is often brighter than any star. While Saturn is slightly dimmer, it too is unmistakable: if you see a steady, unblinking glow, then you’ve likely spotted a planet instead of a bright star.
AN IMAGE OF JUPITER THROUGH A TELESCOPE
The chart lists the location in the sky to see Jupiter in the years ahead.
Saturn’s famous rings change over time. Sometimes we see them as a wide oval circling the planet; other times they are a thin line crossing the equator. The chart lists the position of Saturn and where to look.
URANUS AND NEPTUNE Though large compared to Earth, both Uranus and Neptune are too far away to be visible with the naked eye. Neither was known before the age of telescopes, and even today Neptune is difficult to find.
Use a good sky chart to locate these giants. With sufficient magnification, you should be able to see their pale disks, confirming them as planets.
Many of the sights in this book are deep-sky objects: nebulae, star clusters, and galaxies. Each type of object requires a slightly different observation technique.
DIFFUSE NEBULAE Our galaxy is filled with diffuse nebulae—vast clouds of interstellar gas—large enough to swallow a dozen star systems. Some of these nebulae are stellar nurseries. Newborn stars have condensed out of the gas and their fusion light now illuminates their birth cocoon. From Earth we see these kinds of nebulae as faintly luminous clouds, sometimes embedded with a clutch of brilliant stars. The Lagoon Nebula is a beautiful example.
THE LAGOON NEBULA
To observe a diffuse nebula you need dark, moonless skies away from light pollution. Most of the nebulae in this book are large—sometimes as large as the full moon—so they don’t require much magnification. Instead, maximize your light-gathering power: Use short (fast) focal ratios and larger aperture telescopes. For details, see the section on rich-field telescopes.
Like other faint objects, nebulae benefit from using averted vision. Look at the nebula out of the corner of your eye, which is much more light-sensitive than the center.
PLANETARY NEBULAE When a sun-like star dies and collapses into a white dwarf, it leaves behind an expanding shell of gas—the last vestiges of its red giant phase. This shell of gas, lit by the cold fires of the white dwarf, appears from Earth as a small, faint spherical cloud. It looks like a ghostly planet wandering the depths of interstellar space.
THE RING NEBULA
Planetary nebulae are relatively bright, but very small. At low magnification they may seem no more than a slightly fuzzy star. Higher magnification brings out more detail, but you’ll need steady skies. These objects are often bright enough to punch through moderate light pollution.
The Dumbbell Nebula and the Ring Nebula are two of the most famous examples of planetary nebulae.
OPEN CLUSTERS Stars are often born in large groups. The diffuse nebulae that serve as stellar nurseries are large enough to give birth to hundreds of stars at a time.
THE PLEIADES
The intense light of these newborn stars pushes out their dusty cocoon, dissipating the nebula and leaving behind a loose cluster of stars: an open cluster.
From Earth, many of these clusters are small and require a telescope to see. But some of the best are visible even to the naked eye, and they are spectacular in binoculars. The Pleiades is the most famous example; Messier 44 is another beautiful one.
The best way to observe a cluster depends on its size. For those larger than the full moon, binoculars or a rich-field telescope are best. Smaller clusters require more magnification.
Unlike nebulae, star clusters are mostly unaffected by light pollution.
GLOBULAR CLUSTERS Whereas open clusters are groups of a few hundred stars, globular clusters pack thousands of stars in a tight spherical (hence, “globular”) area.
Globular clusters were formed billions of years ago, probably at the same time the galaxy was born, and they consist mostly of ancient stars.
The sight of a globular cluster is unmistakable. They are a tight, cluster of stars, so close together that the core is often difficult to resolve. In smaller telescopes, the core of a globular cluster looks like a fuzzy glow, almost like the core of a galaxy. Larger telescopes and higher magnification are required to resolve the center into individual stars.
THE HERCULES CLUSTER
Globular clusters are best seen with medium or large telescopes. A high focal-ratio Schmidt- or Maksutov-Cassegrain telescope is ideal.
GALAXIES Galaxies are the most distant objects visible through amateur instruments. Most are millions of light-years away—a statistic that is nearly incomprehensible. They are visible to us on Earth only because they are so large: 50 to 100 thousand light-years across—more than a thousand times larger than diffuse nebulae.
Nevertheless, galaxies are usually difficult to see. Dark skies, free from light pollution, are a must. Even then, do not expect to see much more than a faint smudge. Use a rich-field telescope with a large aperture for best results.
THE ANDROMEDA GALAXY
