Thursday, December 27, 2007

Queen of the Night

There are a handful of constellations that beginning stargazers of all ages can learn to locate right away. These constellations are easy to spot because they contain a distinctive asterism--a recognizable star grouping. One of these constellations is Cassiopeia the Queen, with its defining asterism, the “Lazy W,” visible even in the city.

In legend, the beautiful but beleaguered Cassiopeia (pronounced kass ee oh PEE yuh) was the queen of ancient Ethiopia. She was the wife of King Cepheus (pronounced SEE fee yuss) and the mother of Andromeda. The queen’s vanity was her undoing. She rashly boasted that she was more beautiful than the sea nymphs. The miffed ocean-dwelling goddesses, fifty in number, appealed to their protector, the sea god Poseidon. He retaliated by sending the sea monster Cetus to ravage Ethiopia’s coast. Consulting an oracle for guidance, King Cepheus learned that he had to sacrifice daughter Andromeda to the monster in order to appease the angry god. That is how Andromeda came to be chained to a rock by the sea, waiting for her nick-of-time deliverance by the hero Perseus.

Andromeda and sea monster by Domenico Guidi
in the Metropolitan Museum of Art, NYC

The Lazy W’s bold, connect-the-dots pattern makes it easy for us to see why Cassiopeia is one of the oldest constellations, known to numerous ancient cultures and mentioned in Greek literature of the 5th century BCE. Known to the early Arabs as “The Lady in the Chair,” she is usually depicted sitting on a throne. But the gods had the last laugh. They placed the self-absorbed royal in the sky such that she spends half of her time upside down, a most undignified position for a queen.

Cassiopeia, still primping, in an 18th century star atlas
Image courtesy of G.M. Caglieris

The astronomical explanation for Cassiopeia’s topsy-turvy existence has to do with it being a circumpolar constellation. In the Northern Hemisphere, a circumpolar constellation is a constellation that circles the North Celestial Pole, the imaginary fixed point in the sky that the Earth's axis would intersect, were it extended northward. This imaginary point just happens to be extremely close to the star Polaris, which is why we call Polaris the North Star.

In latitudes above 35 degrees north, circumpolar constellations never set; they are always above the horizon as they endlessly circle the North Star counterclockwise, making one revolution per day. But although we commonly say they are "circling," constellations like Cassiopeia are not really moving around the Pole. Their apparent motion--or the way they appear to move in our sky--is actually caused by our rotation, as planet Earth reliably spins 24/7 on its axis.

Let’s go outside and take a gander at the Queen Mum, shall we?

1) You’ll need to know where north is. If you don’t have a compass, take note of where the sun sets, which is approximately west. Stand with your left shoulder to the west, and you will be facing approximately north.

2) Wait at least one hour after sunset to begin observing. The waning (shrinking) moon won’t rise until at least three to four hours after sunset on Thursday the 27th, and then later each night as the week progresses. This late-rising moon will give you plenty of time to observe under dark skies.

3) Face north and 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. Then slowly lower your head around one-quarter to one-third of the way down toward the northern horizon, until you spot a sprawling “M” pattern in the sky. This is the Lazy W of Cassiopeia, but it will be upside down because it is in mid-revolution around the North Star. If you were to go out just before dawn and look at it, you would notice that it had swung down closer to the horizon and had flipped over to its W orientation.

4) The five stars of the Lazy W asterism are the five brightest stars in Cassiopeia, so let’s take a closer look at them. Moving from left to right in the M orientation, the first star in the pattern is Caph (pronounced KAFF), which means the stained hand in Arabic, a reference to the ancient practice of body painting with henna, a plant dye. The second star is the orange giant Shedar or Schedar (pronounced SHEDD er), Arabic for breast, marking the position of the queen’s heart. The middle star of the asterism, a rapidly spinning blue star, is commonly known as Navi (pronounced NAHV ee), a modern name given it by Apollo astronaut Gus Grissom and the reverse spelling of Ivan, his middle name. The fourth star in the pattern is Ruchbah (pronounced ROOK buh), which means knee in Arabic. As it is nearly 100 light years away, we are looking at century-old light when we gaze at Ruchbah. The fifth star, Segin (pronounced segg EEN), is about seven times the size of our Sun and much older. Its name is of unknown origin.

5) Naked-Eye Challenge for a dark-sky site: Can you see the winter Milky Way winding behind Cassiopeia? It looks like a long, filmy cloud stretching across the sky from the northwestern to the southeastern horizons. On the star map above, it is shown as a gray swirling band. This is the Perseus arm of the Milky Way. At 6000 light years away, it is the closest spiral arm of the Milky Way galaxy to Earth, which itself lies in the Orion arm. The diagram below shows the named arms of our platter-shaped galaxy, technically a barred spiral type.

God’s-eye view of the Milky Way
An Atlas of the Universe by Richard Powell

6) The Lazy W of Cassiopeia was easy to spot, wasn‘t it? Why not point it out to someone else--a friend, family member, or neighbor--the next time you are out under the stars? And once you have their attention, why not entertain them with the story of:
…that starr’d Ethiop queen that strove
To set her beauty's praise above
The sea-nymphs, and their powers offended…

~ John Milton

Thursday, December 20, 2007

The Sun Stands Still

Here in the Northern Hemisphere, Saturday, December 22 marks the 2007 winter solstice, a time of revelry for vampires, werewolves, and amateur astronomers. Why this motley party crowd, you ask? Because the winter solstice offers something irresistible to denizens of the dark: the longest night of the year, not to mention the shortest day.

Speaking of revelry, the marking of the winter solstice has been ritualized by many cultures throughout history, and some practices persist into modern times. A number of ceremonies involve the lighting of fires and candles to encourage--and then celebrate--the return of the Sun after that longest night.

A Scandinavian/European solstice tradition
Image from
The Book of Days

The longest night/shortest day phenomenon occurs because the Sun is at its lowest--its most southern--position in our sky. You can see from the following diagram that the Sun's low arc (shown in blue) shortens the distance it has to travel across the sky, thereby shortening the day length.

Diagram by the YPOP Solar Classroom

The winter solstice marks the official first day of winter in the Northern Hemisphere, after which the Sun begins to arc ever higher in the sky and the days grow ever longer. The word solstice, derived from Latin, means “sun stands still,” a reference to the halt of the Sun’s movement southward. By the way, the apparent movement of the Sun north and south in our sky over the course of a year is caused by the tilt of the Earth’s axis; I will cover this in depth in a future post.

Lest you think that contemporary folk don’t take their solstice rituals seriously, I call your attention to Newgrange in Ireland. Built around 3200 BCE, the mound-like stone structure was apparently oriented so as to be a winter solstice marker. For five days surrounding the solstice, sunlight enters a slot in the roof and fully illuminates an inner chamber for 17 minutes. Just ten visitors per day may witness the lighting of the chamber, and admission is obtained solely by lottery. Over 28,000 applications were received for the 2007 lottery! Only 50 lucky people will experience this unique event, provided the weather in Ireland cooperates.

Aerial image of Newgrange from

Here’s some exciting news for those of you who didn’t win the lottery. Because it is the 40th anniversary of the discovery of the solstice phenomenon at Newgrange, the illumination of the chamber will be broadcast for the first time this year via webcast and satellite transmission, on both Friday, December 21 and Saturday the 22nd. Of course, the harsh reality of the time difference is that the event will occur in the wee hours of the morning for those of us in North America. No guts, no glory, so lay in a store of your favorite high-test java.

Here’s another way to celebrate the winter solstice, in your own time zone. You can participate in the 2007 Cassini Favorite Image Contest by voting for your favorite images of Saturn and its moons. The voting deadline is December 30, and you’ll have a chance to win a poster of the top color image.

Saturn image by CICLOPS

But what, you ask, does that have to do with the winter solstice? Well, the winter solstice festival in ancient Rome was the rather raucous Saturnalia, dedicated to the god of agriculture, none other than, yup you guessed it, Saturn. Historians believe that the Catholic Church selected December 25 as the date of the birth of Jesus in order to absorb the established traditions of the month-long pagan Saturnalia festival, which already encompassed that date.


Astronomically speaking, the winter solstice represents a turning point in the Earth’s year: the ‘return’ of the Sun from its southern decline. So too we bounce back, with all the frantic optimism of our year-end resolutions, moving again toward spring, following the Sun.

Thursday, December 13, 2007

Twin Treats from Gemini

On the evening of December 13th into the morning of December 14th, cold-hardy sky watchers in North America will be rewarded with a generous sprinkling of cosmic fairy dust. The top-billed meteor shower of December--the Geminids--returns once more to fill the sky with “shooting stars.” The 13th/14th is the peak of the shower, when observers can expect to see 50-plus meteors per hour streak across the sky. But don’t worry, if you can’t get out under the stars on Thursday night or if clouds thwart you, try again on Friday night into Saturday morning, December 14th/15th, because rates should still be high enough for a good show.

A meteor, aka shooting star, is the streak of light we see in the sky when a bit of dust or space debris hits Earth’s atmosphere at high velocity. Typically, the meteoroid--the bit of debris--vaporizes upon impact with the atmosphere. But occasionally, a large chunk may survive its screaming 75,000-plus mph entry and hit the ground, at which point we refer to it as a meteorite.

The Willamette Meteorite at the Rose Center for Earth and Space, NYC

You can venture out under a dark sky any night of the year and see a handful of meteors. These are usually sporadics, meteors not associated with a particular shower. But when Earth encounters one of the streams of debris left behind by a comet’s close approach to the Sun, we experience a meteor shower.

3200 Phaethon’s orbit - image from NASA/JPL

In the early 1980s, astronomers were stunned to discover that the source body for the Geminid meteor stream was not a comet, but rather what appeared to be an asteroid, 3200 Phaethon (pronounced FAY uh thahn). It is now suspected that 3200 Phaethon may in fact be the burned-out shell of a comet, charred by too many close encounters with the Sun. The rocky skeleton that remains would naturally resemble an asteroid.

This map shows the Geminid shower radiant, that is, the point in the sky from which the meteors appear to emanate, as the Earth slams into 3200 Phaethon‘s debris. In the case of the Geminids, the radiant is a point within the constellation of Gemini, hence the shower’s name.

Bundle up now, and put on the teakettle for a thermos of hot chocolate!

1) Go to as dark of a site as you can, with no line-of-sight outdoor lights. Dress in many layers because you won’t be moving around much and you’ll get cold fast.

2) Take a lounge chair and a pile of blankets. Reclining is really the best way to watch a meteor shower, because it allows you to see the most sky at one time. Although the meteors originate from the radiant, you can see them fizzling anywhere in the sky. I normally lie with the top of my head pointed toward the rising radiant (which will be in the northeast) so that I can be scanning the rest of the sky.

3) The radiant will rise about two hours after sunset. You won’t see any meteors before then. The moon will set about four hours after sunset, but it will be a small crescent hanging low in the west and shouldn’t be bright enough to impact your viewing.

If you can stand the cold and can wait until midnight to go out and begin observing, the radiant will be better placed in the sky--nearly overhead. The hourly rate will be greater and you should see many more meteors. Be brave! The experience of seeing a terrific meteor shower--even with some physical discomfort--is a memory that will stay with you always.

Another alternative is to get up a couple hours before sunrise on Friday to watch the sparks fly before day breaks.

4) As you watch the fireworks, keep in mind that meteors are typically no bigger than a grain of sand. The really bright ones that go off like Roman candles may be no bigger than a grain of rice. It is their high entry velocity as they slam into our atmosphere that causes the wonderful light show. Amazing, isn’t it? Before I knew any better, I always imagined that the shooting stars I saw were each about the size of a basketball!

Meteor storm - 1889 engraving by Adolf Vollmy

Just as the Geminids peter out on December 18, another gift from the Gemini Twins--planet Mars--takes center stage. Mars will be great to look at all week. But on the night of the 18th, Mars will be at its closest to Earth since 2005, a mere 55 million miles away. It will not come closer to Earth until 2016.

Mars is unmistakable in the night sky now, shining like a drop of molten copper, brazen, in your face, the dude with a bad ‘tude. It is not much of a stretch to see why the planet is the namesake of the Roman god of war.

NASA and the Hubble Heritage Team (STScI/AURA)

You will find Mars rising in the east about one hour after sunset, to the left (north) of the easy-to-spot Orion constellation. He will be the brightest object in the sky, after the Moon, which unfortunately will be 65% illuminated on the 18th. But even the placid-faced Moon can’t keep the warmonger at bay. Mars marches her across the sky until she sets, and then he dominates the night, sunlight reflecting off his terrible shield.

Peace on Earth, good skies to all.

Thursday, December 6, 2007

My God, It's Full of Stars

Let’s continue the exploration of the constellation Pegasus that we began last week. If you’re just tuning in to this blog, please read last week’s post to come up to speed. Hopefully you were all able to spot the Great Square of Pegasus without difficulty. Now let’s look at a special use for the Square.

Pegasus star map by James B. Kaler

One of the ways astronomers gauge sky quality and extent of light pollution at a given location is by counting how many naked-eye stars they can see inside the Great Square, the area bounded by the stars Alpheratz, Algenib, Markab, and Scheat.

Let’s try it. With a group of people, it can be fun to compare all the different star counts. Even at the same location, naked-eye counts may vary a bit due to each individual’s visual acuity, age, fatigue level, and observing experience. Not to mention vivid imagination and propensity for cheating.

1) To see the maximum number of stars you can, you should dark adapt before beginning your count. Dark adapting involves avoiding exposure to all white light for a minimum of 20 minutes before an observing session. No lamps, no TV, no house lights, no headlights, no street lights. You may have to live with some diffuse white light in the general vicinity (hard to avoid it in most neighborhoods), but do not look directly at unshielded bulbs. Use a red light flashlight if you need light to safely walk around.

The reason for this strange preparatory ritual is that your night vision is greatly enhanced by the restoration of chemicals in your eyes that were ‘bleached’ by light. This restoration occurs in the dark, and the longer you linger in the dark, the easier it will be to see faint stars. Trust me, dark adapting does make a difference.
Example of red light flashlight from

2) Start your observing at least one hour after sunset. It will be dark by then, and winged Pegasus will be well positioned, flying high in the sky. Generally speaking, the transparency of the night sky is better up near the zenith than down near the horizon because near the zenith we are looking through less atmosphere, aka “gunk.” Transparency means atmospheric clarity, and good transparency can mean the difference between seeing objects clearly and not seeing them at all. Be aware that transparency can be negatively impacted by clouds, haze, dust, or humidity.

3) At first glance, the Great Square is a blank slate. In fact, in the city or other light-polluted environment, you may not be able to see any stars inside the Square. Observing from a darker location is best. From my semi-rural backyard, I can see five stars inside the Square. From a rural location, it is possible for a person with good eyesight to see 12 or more stars. From an isolated location where the sky appears carpeted with stars, a person with keen eyesight may see 20 or more stars.

How many can you see? (Note: don’t count the four corner stars in your total.) Please share your count in the Comments section below, and be sure to tell us your location (city/state/province/country).

Johann Schmidt, Director of the Athens Observatory during the late 1800s, is reported to have counted 102 naked-eye stars inside the Great Square. Can you imagine?! This was before the proliferation of electric streetlights, so the nighttime environment world-wide must have been magnificently black-cat-in-a-coalbin dark.

4) If you have binoculars, train them on the Square’s interior after you’ve finished your naked-eye count. Now you can see that it’s packed full of stars--dozens and dozens of twinkle lights of the Milky Way galaxy. Some are nearer to Earth than the corner stars of the Square and others sparkle hundreds of light years beyond them. We may not see the three-dimensionality of the night sky, but we can certainly imagine it. Sweep your binoculars around in there, and savor the moment.

5) To continue our bino-viewing, let’s goggle at one of the showpiece globular clusters of the Milky Way, which resides just off the horse’s nose. M15--short for Messier 15--is a dense ball of gravitationally bound stars.

There are 150 known globular clusters in our Milky Way galaxy. Globulars contain thousands to perhaps millions of stars and are among the oldest objects in the universe. Locate M15 by extending the imaginary line between Biham (pronounced bih HAHM) and Enif, half again as long past Enif. Then train your binoculars on that spot. If you spy a round, fuzzy object, you’ve found M15. Feel free to let loose with a celebratory whinny! In a six-inch or larger reflector telescope, you should begin to resolve the stars in the cluster, that is, they should begin to separate into distinct points of light.

6) The Great Square is a window not only on the Milky Way, but also on the universe beyond. Skilled amateurs with large telescopes are able to view over 100 distant galaxies through the giant, floating window frame of the Square. But the brightest galaxy in Pegasus, the unromantically-named NGC 7331, lies outside the Square. You’ll need at least a six-inch reflector telescope to see it. Look northwest of the star Matar (pronounced MAH tahr) to find this spiral galaxy, sometimes referred to as the ‘twin’ of our galaxy, also a spiral.

Although NGC 7331 is the centerpiece of a noted galaxy group called the “Deer Lick Group,” you won’t see the other four faint galaxies without a large telescope. They are far more distant than the whorled beauty NGC 7331, itself an incomprehensible 49 million light years from Earth.