Thursday, November 29, 2007

The Great Square of Pegasus

Once upon a time in ancient Greece, a handsome, high-spirited young man named Bellerophon, son of the sea god Poseidon, coveted a marvelous horse named Pegasus. Pegasus was a winged horse who had sprung from the blood of Medusa after the hero Perseus killed her. Medusa, you may recall, was a winged monster with a scale-covered body and hair of live snakes, so dreadful that anyone who looked upon her was instantly turned to stone.

The Pegasus Vase by the British Museum

With a golden bridle given him by the goddess Athena, Bellerophon tamed Pegasus. Together they soared through many thrilling adventures and feats of derring-do. Eventually hubris got the best of Bellerophon, when he tried to fly up to Olympus, the realm of the gods, to hobnob with the immortals. Pegasus knew better and threw his rider back down to earth where he wandered alone, abandoned and despised by the gods, until his death. Pegasus, however, was given shelter in the heavenly stable of the supreme ruler Zeus, where he served his master by bringing him thunder and lightning whenever “Father Sky” got the urge to wield his awful thunderbolt.

Pegasus the Winged Horse is a defining constellation of the autumn sky. When seasoned summer stargazers see the winged steed’s nose nuzzling the eastern horizon, they know that Pegasus is rising and autumn’s shiver is not far behind.

Amateur astronomers commonly talk about constellations, planets, and the Moon “rising” and/or “setting” during an observing session. What they really mean, of course, is that the Earth’s 24/7 rotation on its axis--toward the east--makes it appear as though objects are rising in the east, crossing the night sky, and setting in the west. To put another spin on it, our planet’s spin is the source of the night sky’s apparent motion.

Autumn sky chart by Hawaiian Astronomical Society

The easiest way to spot Pegasus is to look for its most prominent asterism, the “Great Square,” four stars that mark the corners of a large square. Asterism is a fancy word for any recognizable star pattern; think of an asterism as a sort of celestial landmark that tells you whose constellation ‘hood you’re in. For example, the Big Dipper is an asterism--probably the most famous--and when we find it, it tells us we are on the Big Bear’s turf. Provided the light pollution is not too bad and you don’t have unshielded light fixtures in your line of sight, you can even spot the Great Square of Pegasus in the city or suburban sky.

Saddle up, stargazers!

Pegasus star map by James B. Kaler

1) You will need to know where south is. Use a compass or make note of where the Sun sets at your viewing location. The spot where it touches the horizon is approximately west, and if you stand with your right shoulder to the west, you will be facing roughly south.

2) Wait at least one hour after sunset to begin observing, so that it’s good and dark. The Moon is currently waning (shrinking), so you will have several hours of dark before it rises and washes out the sky. This period of dark will increase each night over the next week as the Moon rises later and later.

3) Face south and then tilt your head all the way back until you are looking at the zenith, the point in the sky that is directly above your head. An hour after sunset, the Great Square is slightly to the left (east) of the zenith and may also be down a bit toward the southern horizon if you‘re in the upper half of the U.S. As the night goes on, the Square will progressively move to the right, toward the western horizon.

Each side of the Square is about two fists wide, if you hold your fist at arm’s length against the sky and measure across the widest part. Have you found it? Excellent! You have lassoed the torso of the legendary sky-horse. Sadly, when the constellations known to the ancients were reorganized by professional astronomers in 1930, Pegasus lost his hindquarters. He now prances through eternity with just a head, torso, and two front legs. Of course, he does still have his wings, so I suppose that is some consolation.

4) Now let’s find his head. From the southwestern corner of the Great Square, you should see a four-star asterism shaped like an “L” stretching out to the right (west). This is our noble steed’s neck and head. You can see from this old pictorial star atlas that the acrobatic Pegasus is flying upside down in the sky (if you’re facing south in the Northern Hemisphere, as we are).

The star at the end of the “L” is Enif (pronounced ENN if), which appropriately means nose in Arabic. Enif is the brightest star in Pegasus, an orange supergiant with a diameter 150 times that of our Sun. The star marking the corner of the Great Square where the “L” begins is called Markab (pronounced MARK ahb), Arabic for saddle. Markab is a hot, blue, rather average star that, like our Sun, is expected to end its life as a swollen red giant star ultimately dwindling to a white dwarf.

Next week we will explore a special use for the Great Square, as well as some binocular and telescope targets in Pegasus. Until then, happy gazing!

Thursday, November 22, 2007

My Own Private Lunacy

The 1987 video “A Private Universe” is a science education classic. In addition to revealing that most Harvard graduates don’t know the reason for the seasons (a topic for another day), the video explores how children latch onto erroneous ideas about the universe. These misconceptions persist, become central to their thinking, and make them resistant to new evidence that would lead them to the truth.

The first time I saw it, I found the video utterly fascinating, but I didn’t make a personal connection, at least not immediately. A few years later I thought about it again, and suddenly it struck me that I'd had a private-universe experience myself. When I was a child, I somehow got the notion that the Moon stored light from the Sun during the day and then released it back to us at night--not unlike a modern solar walkway light. I’m certain that at some point I was told the Moon reflected light from the Sun, but since I didn’t know (or understand) the relative motions and positions of the Sun-Earth-Moon system, I must have very creatively decided that “reflected” meant “stored it up and shone it back later.” Go figger.

Here’s the embarrassing part: I’m not sure precisely when I abandoned this notion for the astronomical truth, but it was well into my adulthood. And I can’t say for sure what triggered my epiphany, but I’m guessing it was seeing a diagram of the Sun-Earth-Moon system.

Since luna is the Latin word for moon (and the source of English words such as lunatic), I call my persistent erroneous thinking about the moon “my own private lunacy.” But I know I’m not alone. I see this at public astronomy events, when adults confidently tell me that the non-illuminated part of the crescent moon they’re looking at is the Earth’s shadow. They too are sorely in need of a diagram.

On this simple moon phase diagram,
you can see that the inner circle of moon icons depicts the moon’s orbit around the Earth, which takes about a month (29.5 days). The ABCD indicators show the direction of the Moon’s orbit. Only one hemisphere of the Moon is illuminated by the Sun at a given time, with the other hemisphere non-illuminated or dark (shine a flashlight at a baseball, and you‘ll get the idea). However, because of Earth’s position at the center of the lunar promenade, we only ever see part of the Moon’s Sun-illuminated hemisphere. The exceptions are New Moon and Full Moon.

The outer circle on the diagram depicts how each moon phase appears to us here on Earth. During New Moon, the Moon lines up between Earth and Sun and we can’t see any part of its illuminated hemisphere, so it vanishes from our view. During Full Moon, the Earth lines up between Moon and Sun, so we see all of the Moon’s illuminated hemisphere. Now, if the Earth and Moon were on the same orbital plane relative to the Sun, we would have a total solar eclipse and a total lunar eclipse once a month, respectively, when the Moon covered the Sun during New Moon and when the Sun cast the Earth‘s shadow onto the Moon during Full Moon. But that doesn’t happen, does it? This is because the Moon’s orbital plane around the Earth is tilted about five degrees from the Earth’s orbital plane around the Sun. These out-of-kilter orbits make those perfect line-ups needed for total eclipses a bit rare.

The Moon in Earth's shadow, during a total lunar eclipse

This week, go outside and look at the Moon naked eye. Then wait a week and do it again. Each time, notice what portion of the Moon’s face is illuminated and match it to the moon phase it most resembles on the diagram. Then think about where the Sun must be in relation to the Moon and you, to produce the phase you see. Know that you are looking at nearly-real-time reflection of sunlight from the Moon’s surface. I say nearly-real-time because it takes that reflected light about 1.25 seconds to traverse the quarter of a million miles separating Moon and Earth.

On Saturday, November 24, the Moon will be full. November’s Full Moon is called Beaver Moon by the Algonquin Indians, Moon of the Falling Leaves by the Lakota Sioux, and Freezing Moon by the Cheyenne. Full Moon is a great time to look at the Moon, not only because it is fully illuminated, but because it is in the sky all night long, from sunset to sunrise. If you can drag your bad self home before sunrise, you might just get to bask in a little of that reflected glory.

Thursday, November 15, 2007

Comet 17P Hunka Hunka Burnin' Love

OK, so its real name is Comet 17P/Holmes, but sky watchers everywhere--myself included--are in a frenzy over this rock star of a comet. It blazed onto the solar system scene in late October by jumping 14 magnitudes of brightness in under 24 hours and becoming visible to the naked eye. Now that’s climbing the charts.

Riding high against the backdrop of the constellation Perseus the Hero, the fuzzy-headed phenom is easy to spot if you know where to look. Let’s get started.

1. You’ll need to be away from city lights to see the comet naked eye. Even if you have binoculars or a telescope, a dark sky is still best to see the star patterns that will lead you to the comet.
2. Plan to look for the comet after moonset, as the bright Moon washes out the sky and makes it difficult to see faint objects and star patterns. The Moon is currently increasing in illumination and setting later each night, as it advances toward Full Moon on November 24. If you’re not a night owl and can‘t wait up, it might be easier for you to get up a couple hours before sunrise to comet watch. Get the sunrise/set and moonrise/set times for your location at the U.S. Naval Observatory website.
3. You’ll need to know where north is. If you don’t have a compass, aren’t that familiar with your viewing location, or are just directionally challenged, determine north by watching where the Sun sets at your viewing location. The spot where it touches the horizon is approximately west, and if you stand with your left shoulder to the west, you will be facing roughly north. Take note of a natural or manmade landmark that lines up with the direction you’re facing, so you can re-orient later when you begin your comet hunt.
4. To begin, face north and look for a distinctive W-shaped star pattern, which is the “Lazy W” of the constellation Cassiopeia the Queen. The rather shallow W will be found about halfway between the horizon and the zenith (the point directly above your head). It may be a bit east or west of north, depending on what time you look for it. The W is about a fist and a half wide, if you hold your fist at arm’s length against the sky and measure along the widest part. The Lazy W will begin the night standing on end, balancing on the leftmost star of the W, as shown on this helpful star map from Throughout the night--because the Earth is always spinning--this view will change. By morning, the Lazy W will appear to have flipped over and will be balancing on its rightmost star. To simulate this apparent movement of the constellations throughout the night, simply print out the star map and slowly rotate it counterclockwise until you have flipped the Lazy W and the top of the map is now at the bottom. If you start observing in the middle of the night, about halfway between sunset and sunrise, you should see the Lazy W upside-down and looking more like an “M.”
5. After you’ve found the Lazy W in the sky, continue to face north and match its orientation in the sky on the star map, by rotating it counterclockwise. Now check the rotated map to see where Perseus should be in relation to the Lazy W. Then try to find Perseus in the sky.
6. If you’re having trouble finding Perseus, try this trick. Number the five stars that make up the Lazy W (not the upside-down M) from left to right. Draw an imaginary line between star #3 (the middle star) and star #2, extend that line below the W, and keep going straight until you reach a fairly faint star. This will be the first star in the four-star chain that makes up the ‘backbone’ of Perseus. The third star in the chain, noticeably brighter and more golden-colored than the others, is Mirfak, the brightest in Perseus (pronounced MURF ock). Right next to Mirfak is a faint, fuzzy looking patch and--you did it! You found the comet!

Even in my petite 10x24 binoculars, the comet looms into view as a large fuzzy blob next to Mirfak’s brilliant golden point. The ‘fuzz’ is the comet’s coma, the expanding cloud of dust and gas that envelops the comet when it is near the Sun. A comet is essentially a hunk of dirty ice, and the Sun’s heat vaporizes the outer layers of the ice. As the ice becomes gas, trapped dust is released. In small binoculars, you can see that the inner coma is denser and brighter than the outer coma.

In beefier binoculars like 10x50‘s--or in a small telescope--you begin to see the elongated shape of the bright inner coma, as well as some background stars coyly shimmering through the gossamer outer coma. See if you can discern any color in the coma; many observers are reporting a dull gold or pale green hue. You may also notice the sharp leading edge of the comet, which indicates its heading, as well as the fuzzier trailing edge, where the comet’s tail originates. Currently, there is no clearly visible tail because we are viewing the comet nearly head on and can’t see much of the tail that streams out behind it.

If you don’t have observing equipment but are hankering for a magnified view, check with your local astronomy club, planetarium, science museum, or nature preserve to see if any telescope observing events are scheduled.

Whether gazing at the comet naked eye, through binoculars, or with a telescope, everyone can appreciate its magic. Here are a few things to ponder while you commune with the comet:

- The word “comet” comes from the Greek word for hair. In ancient times, comets were known as “hairy stars.”
- All of the comet’s illumination is light from the Sun reflecting off dust and gas in the coma. The comet has no light source of its own.
- The comet’s nucleus, that is, the hurtling hunk of ice inside the expanding coma, is around two miles in diameter.
- All the stars you see near the comet--and in fact all the stars you see in the night sky--are in the Milky Way, our home galaxy. The comet is much much closer to us than any of those stars, which are all light years beyond our solar system. The comet is a solar system body, currently about 150 million miles away, in the asteroid belt between the orbits of Mars and Jupiter.
- Comets begin their long, strange trip around the Sun from the outer reaches of our solar system, in either the Kuiper Belt (pronounced KIGH purr) or the Oort Cloud. The Kuiper Belt is a huge ring of small bodies beyond the orbit of Neptune. The Oort Cloud is a vast sphere of comets that encases the entire planetary solar system.
- There are, conservatively, one trillion comets on the move in the Oort Cloud. That‘s right, conservatively.
- A comet’s orbit around the Sun can take a few years or a few million years. It will spend all or most of that time as a homely hunk of brown ice drifting through silent space. Only if and when its orbit takes it near our Sun does it make a spectacle of itself, like the effervescent Comet Holmes.

Yes, it dances for us. Can you feel the love?

Monday, November 5, 2007

Your First Telescope: A Cautionary Tale for the Holidays

When you are ready to acquire your first telescope, or a first telescope for a child, please please please think twice before buying it in a department store, grocery store, or big box store. Do not be seduced by the misleading hype on the packaging, like “high power!” and “400 times magnification!” Magnification (power) is only one part of the equation for telescope performance, and you are probably going to be disappointed. Instead, get something that is a real astronomical instrument, not a hard-to-use piece of junk with bad optics that will end up collecting dust in a closet.

In my opinion, the best first telescope money can buy is the StarBlast Astro by Orion Telescopes & Binoculars. It is compact and easy to store. It is easy to set up and use. You can set it up on a tabletop. Younger children can set it up on a stool or chair, or even the ground. It has quality optics. It costs less than $200. In my opinion, it is the most inexpensive GOOD telescope around.

The StarBlast Astro will give you great views of the Moon, the planets, and some of the brighter deep-sky objects such as the Pleiades, the Beehive Cluster, and the Andromeda Galaxy—even in the city where you must contend with light pollution.

The Pleiades aka The Seven Sisters

If you have more money to spend, you are certain that you are committed to telescope observing, and you want something with more light-gathering power for deep-sky observing of galaxies, nebulae, etc., please please please think twice before buying a “go-to” scope. A go-to scope is one of those motorized scopes that automatically moves to an object in its catalog when you enter a command in an attached computer unit or hand paddle. I’m not saying that there is anything inherently wrong with go-to scopes; they have their uses for seasoned observers. However, I believe that they do not serve the beginner well. You won’t learn the night sky at all if you have a computer doing all the work for you. This is lazy astronomy! Do not deny yourself the deeply satisfying sense of accomplishment you will get from locating deep-sky objects using your brain, your eyes, and a star map or star chart.

If you want the next step up from the StarBlast, I suggest you take a look at Orion’s SkyQuest Classic line of reflector telescopes, available in a variety of sizes. They are Dobsonian design, same as the StarBlast, and you simply can’t beat a Dob for ease of use. The first telescope I used was a six-inch Dob, and I found all 110 Messier objects with it (the Messier list of deep-sky objects is a staple of amateur observers--pronounced MESS ee yay).

By the way, I have no affiliation whatsoever with Orion Telescopes, except as a satisfied customer. I find their products to perform well and to be reasonably priced. I can find the accessories I need in their catalog, and they too are reasonably priced. Their customer service and order fulfillment is very good. I also like the way they do business; their company doesn’t smack of unbridled greed, like certain other telescope manufacturers who rush to market with substandard products and mislead consumers with shamelessly over-the-top hype.

So, now you have had the guesswork taken out of your first telescope purchase. Step away from the department store telescope! You have been warned.