Thursday, April 24, 2008

A Crab, Two Jackasses, and 350 Bees

The constellation of Cancer the Crab holds a special place in my heart, because it’s my astrological sun sign. To be sure, I consider astrology a recreational pursuit rather than a scientific one. Nevertheless, I can’t help but feel somewhat possessive toward the little celestial crustacean as he scuttles across the spring sky.

Cancer is a constellation of the zodiac, following Gemini the Twins to its west and preceding Leo the Lion to its east. The zodiac is the band of twelve constellations that straddles the ecliptic, as seen from Earth. The ecliptic is the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. Because the Earth, the Moon, and the planets all lie in roughly the same plane as they orbit the Sun, the ecliptic also represents the plane of the solar system.

The name Cancer is, appropriately, from the Latin word for crab. In Greek myth, Cancer was the crab that pinched strongman Hercules’ foot as he fought the giant water snake Hydra, a monstrous creature with nine heads. During this skirmish, Hercules crushed the crab under his foot, and the gods placed the feisty but unlucky crab in the heavens.

Cancer the Crab in 18th century star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

How did a word that means crab come to refer to a group of cellular diseases? Early physicians noticed that tumors and the distended blood vessels around them formed a pattern that resembled a crab. Around the 17th century, they began applying the word “cancer” to the diseases that caused these tumors.

A dark sky--well away from city and suburban lights--is required to commune with the Crab. The faint constellation would be inconspicuous if it weren't for the renowned naked-eye star cluster at its heart: the Beehive. Even the brightest stars in Cancer are somewhat dim. If you see anything bright in Cancer, rest assured it’s a planet just passing through.

Grab your long-handled net, and let’s go crabbing!

1) You’ll need to face south, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

2) Wait at least one hour after sunset to begin observing, so that twilight’s finished and your sky’s good and dark. This observing week, you’ll have plenty of gazing time before the Moon floods the sky with light. It rises well after midnight on Friday the 25th and even later each night after that.

3) As you face south, tilt your head back slowly and move your gaze from the southern horizon up to the zenith, the spot directly above your head. Look for a fuzzy patch. It'll be almost as high as the zenith if you live in the southern latitudes of the continental U.S. If you live in the northern latitudes, it'll be slightly lower, up to a quarter of the way from the zenith down toward the southern horizon. It'll also be slightly west of due south. Find it? Excellent! This is the famous Beehive cluster.

If you’re having trouble finding it, first make sure you’re dark adapted. Avoid all white light for a minimum of 20 minutes. This will improve your night vision and help you see faint objects. Second, try to locate the Sickle, a prominent asterism (recognizable star pattern) in Leo. It looks like a big, backwards question mark. Right now the bright golden planet Saturn is hanging out at the base of the Sickle. The open end of the Sickle’s curving top points toward the Beehive. Also, if you can locate reddish planet Mars near the Gemini twin stars (Castor and Pollux) and draw a line between Saturn and Mars, it will cross the Beehive.

M44, the Beehive cluster

The Beehive is an open cluster, a loose grouping of stars that formed around the same time from the same gas cloud. It contains around 350 stars. It is alternately known as M44 and as Praesepe (pronounced pree SEE pee). The former is shorthand for Messier 44, the object’s designation in the catalog of famed 18th century French astronomer Charles Messier (pronounced MESS ee yay). The latter, Praesepe, is the oldest of these names and is Latin for manger. The origin of the whimsical Beehive name is unknown.

4) Once you’ve found the Beehive, look for two faint stars that bracket it on the left (east). The top star is Asellus Borealis and the bottom one, Asellus Australis. These are Latin for Northern Ass and Southern Ass. These two stars form the asterism known as the Donkeys or the Asses. In ancient mythology, the asses participated in a battle between the Greek gods and some giants. After helping the gods prevail by spooking the giants with their obnoxious braying, they were rewarded with a place in the sky next to a well-stocked feed trough, also known as a manger. Aha! Now we see why the Beehive originally had the name Praesepe.

5) The star Acubens, Arabic for the claw, marks the southernmost crab claw. Just south of Acubens (pronounced ak oo BENZ) is a circlet of stars--the asterism known as the Hydra’s Head, in the constellation Hydra. The water snake and the crab huddle together in the sky, sworn enemies of Hercules.

6) Al Tarf, the brightest star in Cancer, is an orange giant. Its name is from the Arabic for the end, and it marks one of the crab’s southern legs.

7) The only other star in Cancer with a traditional name is Tegmine (pronounced TEG-min-uh). It's Arabic for in the covering, referring to the crab’s shell. Tegmine is west of the Beehive.

8) The Beehive is a great binocular object. In fact, binoculars--rather than a telescope--usually give the most satisfying magnified view of a large object like this. If you have access to a pair of binoculars of any size, use them to peer at the fuzzy patch. The magnified patch will resolve, separating into distinct points of light.

Wander around in the cluster and notice how brilliant and sharp the stars are. I like to contrast this object with the Pleiades or "Seven Sisters" open cluster in Taurus. The Pleiades (pronounced PLEE uh deez) cluster is currently crossing paths with a nebula, a cloud of gas and dust. This makes its stars appear to be wrapped in cotton candy. In contrast, the sparkling Beehive is a visual 'palate cleanser,' not unlike a refreshing sorbet served between courses at a fancy banquet.

Thursday, April 17, 2008

Top Ten Moon Myths

As you gaze at this week’s big moon--and frolic in the light of the Full Moon on Sunday, April 20--ponder my picks for Numbers 10 through 6 of the top ten moon myths of all time:

10) The Moon’s surface is completely smooth. Early science through the time of the Renaissance was based upon the teachings of the influential Greek philosopher Aristotle. According to his worldview, the Moon was a perfect sphere with a perfectly smooth surface. The invention of the telescope in the 17th century blasted that myth into oblivion. When the pioneering Italian astronomer Galileo turned his homemade telescope skyward in 1609, he discovered that the Moon was pockmarked with craters and crisscrossed with mountains and ridges.

Unfortunately, Galileo misinterpreted one type of lunar feature he saw: the dark, basalt-filled craters. He thought they were water features similar to Earth’s seas. Consequently, lunar mapmakers of the time gave them names beginning with “mare,” the Latin word for sea, and those names persist to this day.

9) The Moon does not rotate. Many think that, because the same side of the Moon always faces the Earth, it doesn't spin. In fact, if it did not rotate on its axis, we'd eventually see all parts of the Moon parade by us over the course of its month-long orbit around Earth. But we don’t, do we?

Millions of years ago, the strong pull of Earth’s gravity put the brakes on the Moon’s spin, slowing it down until its rotation became synchronous with its orbital motion. Now the Moon makes one complete rotation on its axis in the same time it takes to complete one orbit around the Earth. You can picture this slow rotation best if you imagine observing the Earth/Moon system from the Sun. From your vantage point on the Sun, you could watch Moon circling Earth, turning just enough on its axis to keep the same face toward Earth as it went around. And from the Sun you’d notice that, over the course of a month, your view of the Moon’s surface continually changed until you'd seen it all, proving that it was rotating.

8) Dogs and wolves howl at the moon. This is a common belief--perpetuated in popular art, music, and film--but there's no scientific evidence that it is so. Dogs and wolves howl to communicate with one another. A couple theories as to why humans have imagined this causal relationship between the Moon and howling animals are: a) Animals, particularly predators, may be more active on moonlit nights when they can see better, and b) When we hear animals howl at night, we're more likely to notice a big moon in the sky if it’s plainly visible. However, we may not recall all the times we heard animals howl when there was a crescent moon or no moon in the sky. This is called selective memory, and we use it to perpetuate our already-held beliefs.

7) The Full Moon is larger when it’s near the horizon than when it’s overhead. This effect, known as the Moon illusion, has been recognized since antiquity. How many times have you seen the rising Full Moon hanging just above the eastern horizon and thought to yourself, “The Moon looks huge!”? In fact, the Moon is a little nearer to us and therefore slightly larger in apparent size when it’s high in the sky. Think about it: as the Earth spins east, we are brought toward the Moon.

So what causes us to believe otherwise? The second century mathematician and astronomer Ptolemy first postulated that foreground objects near the horizon, such as trees and buildings, give us something with which to compare the low Moon, making it seem larger. Modern scientific researchers call these distance cues. They are still considered in current theories about the Moon illusion, along with the physiology of our eye muscles and the neurological processes in our brains. Some believe evolutionary selections were made for useful human survival skills like seeing eye-level objects better than high-in-the-sky objects. This skill might improve the chances of finding shelter and food, as well as detecting danger. But there is still no one definitive answer as to why we experience this optical illusion.

6) The Moon has a “dark side.” The Moon has no side that is always dark. The entire surface of the Moon is illuminated by the Sun at one time or another during its month-long orbit around the Earth. However, only half of the Moon’s sphere can be illuminated by the Sun at any given time (shine a flashlight at a baseball, and you'll get the idea).

Although the Moon has no dark side, it does have a far side that always faces away from Earth. Because the Moon’s rotation (spin around its axis) is in lockstep with its revolution (orbit around the Earth), the same lunar hemisphere always faces us.

Stay tuned for next month’s Full Moon to find out which moon myths made my top ten list in the Numbers 5 through 1 slots. Until then, happy howling!

Thursday, April 10, 2008


This week, let’s watch the waxing Moon do-sa-do with planets and other objects as it sashays along the ecliptic. The waxing Moon is a growing moon. Each night, more of its face is illuminated by the Sun, from our changing vantage point on Earth. In actuality, half of the Moon’s sphere is illuminated by the Sun at all times. However, because of Earth’s position at the center of the Moon’s month-long promenade, we only ever see part of the Moon’s Sun-illuminated hemisphere, except during Full Moon and New Moon.

“Swing and circle, wheel around…”

The ecliptic is the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. Because the Earth, the Moon, and the planets all lie in roughly the same plane as they orbit the Sun, the ecliptic can also be said to represent the plane of the solar system. As a result, we see the Moon and the planets sticking close to the ecliptic as they swing across the sky.

“Promenade family, chain down the line…”

Face south for all these observations, for which you will need no equipment but your eyes.

On Friday evening, April 11, the Moon’s face is 39% illuminated, as seen from Earth. Just to the left of the Moon is the bright planet Mars. Notice its reddish or coppery color; this distinct coloration is from its soil, which is rich in iron oxides. Mars is currently spending some quality time in the constellation of Gemini the Twins, and the Moon joins it there for the entire evening, until they both set in the west after midnight.

Slightly above the Moon and Mars are a pair of stars, the two brightest in Gemini: Pollux on the left and Castor on the right. In Greek mythology, Pollux and Castor were inseparable half-brothers with the same mother. When the mortal Castor was killed in a dispute, immortal Pollux was inconsolable. His father, supreme ruler Zeus, took pity on him and allowed him to live in the sky with his brother for half of the year.

Amazingly, the slightly dimmer Castor is actually a six-star system! With a telescope at high magnification, I have seen the three main component stars, but the three companion stars are too close to their partners to be visible in any telescope.

“Acey deucey, boys run right…”

On Saturday, April 12, the Moon reaches its First Quarter phase, when it is 50% illuminated as seen from Earth. This is sometimes referred to as a “half moon.” Note that the Moon is now to the left of Pollux and Castor. Tonight it straddles the boundary between the constellations Gemini the Twins and Cancer the Crab. Precisely-drawn boundaries between constellations were officially designated in 1930 by astronomers of the International Astronomical Union.

“Tag your neighbor, here comes the judge…”

On Sunday evening, April 13, the Moon is 61% illuminated as seen from Earth and is called a gibbous moon. The Moon is gibbous whenever more than half of its face is illuminated, the exception being a Full Moon, when its face is 100% illuminated. Tonight it is more than halfway across Cancer. Hidden in its glare is the Beehive, a wonderful naked-eye star cluster that has been known since antiquity. We will examine the Beehive in greater detail in a few weeks.

Every night at the same time, the Moon is farther to the east than it was the night before. This is because it orbits Earth counterclockwise, as viewed from above the North Pole. From our perspective in the Northern Hemisphere, it travels from right to left--or west to east--over the course of a month.

“Pass to the center, ride the tide…”

On Monday evening, April 14, 71% of the gibbous moon is illuminated, as seen from Earth. The Moon is now in the constellation Leo the Lion. Just to its left is the bright star Regulus, also known as “The Lion’s Heart.” Continuing a little farther to the left, we come to the even brighter Saturn, shining like a golden star. The iconic ringed planet is 750 million miles from Earth.

“Shoot the star, rollaway, weave the ring…”

On Tuesday and Wednesday, April 15 and 16, the Moon continues to travel east through Leo, following the gentle curve of the ecliptic. On the 16th, the Moon will glow--87% illuminated--below Denebola, the bright star that marks Leo the Lion’s tail.

Did you notice that the three constellations the Moon is traveling through this week--Gemini, Cancer, and Leo--are zodiacal constellations? This is because the zodiac is by definition the band of 12 constellations that straddles the ecliptic. As noted earlier, the Moon and the planets hug the ecliptic and, therefore, we see them moving through the zodiacal constellations over time.

“Flip the hourglass, sashay thru!”

Thursday, April 3, 2008

Green Universe

With Earth Day 2008 fast approaching on April 22, we Earthlings are busy congratulating ourselves on our sustainability practices and our green lifestyles. To hear us tell it, you’d think we had invented recycling. But recycling is nearly as old as time itself. And thereby hangs a tale.

Once upon a time in the early universe (and when I say “early,” I mean around 200 million years after the Big Bang), a prolific period of star formation began.

Prior to that time, the expanding universe had been a dark wasteland. Only hydrogen and helium atoms--the two simplest chemical elements--were abundant. But now, in the vast primordial clouds of those simple gasses, gravity was about to liven things up.

The gasses began to condense under the force of gravity, and the clouds fragmented. The cloud fragments collapsed further and heated up, eventually becoming giant spinning spheres of hot gas. When a sphere’s internal pressure and temperature grew high enough, its primary fuel--hydrogen--began burning into helium in a nuclear reaction. The sphere began to glow, heat and light were released, and a star was born.

These first-generation stars were massive and short lived, quickly using up their hydrogen fuel and then exploding. However, their remains enriched the universe with the extra helium from their nuclear reactions and may also have seeded the universe with some heavier elements. The next-generation stars, created from this enriched matter, were a bit smaller and more stable. In their nuclear furnaces, helium could now burn into significant amounts of carbon, an element essential to life. In turn, carbon could burn into heavier elements, and those elements into progressively heavier and more complex elements, and so on.

It is in the death throes of the element-rich, next-generation stars that our story--the human story--begins. Modest-sized stars reached the end when internal pressure from nuclear burning exceeded the gravity that kept them intact. They gently disintegrated, puffing off layers of life-supporting elements like carbon, oxygen, and nitrogen. Massive stars, however, met with a violent end.

The Dumbbell Nebula - a disintegrating star

These stellar giants burned increasingly heavier elements in their cores until their cores burned into iron. The iron cores collapsed and rebounded violently in cataclysmic explosions called supernovas. These explosions blasted the complex elements created in the stars into space. In addition, new elements were forged in the extreme environments of the supernova blasts themselves.

The Crab Nebula - a supernova remnant

The peculiar life forms known as human beings are able to exist because of stars that died billions of years ago. Those stars seeded the cosmic cloud that became our solar system with the elements needed for life--not to mention the elements needed for rocky planets like Earth to form. Most of the elements found today in the universe formed from nuclear reactions in stars and during supernova explosions. Many of these elements are found in trace quantities in our bodies.

Today, in galaxies (gravitationally-bound star systems) like our own Milky Way, giant clouds of gas and dust called nebulas are collapsing, fragmenting, and heating. These nebulas contain the complex chemical legacy of the early stellar generations. With the ignition of nuclear burning in ultra-dense, ultra-hot clumps inside these hydrogen-rich nebulas, new stars are forming--just as stars have formed throughout the history of the universe. Some of these stars are Sun-like, some may have rocky planets in tow, and some may one day support life forms. And like our Sun and its attendant planets and inhabitants, one day these new star systems will complete their life cycles and, unraveling, will return all their raw materials to the cosmos for reclamation.

The cycle continues, because the universe is the ultimate recycler. Happy Earth Day.