Mt. Redoubt bellows steam and ash during it eruption on March 26. For up to date information on the volcano, check out the Alaska Volcano Observatory website. Credit: Al Grillo / AP

Alaska’s 10,000 foot high Mt. Redoubt (READ-out) erupted multiple times last week, sending plumes of volcanic ash and sulfur dioxide up to 65,000 feet high. The volcano is located about 110 miles southwest of Anchorage and its effects are already widespread. Not only has it dumped tons of razor-sharp ash particles across parts of Alaska, but the Global Ozone Monitoring Experiment sensor aboard Europe’s MetOp-A satellite has been tracking the movement of huge ash and gas clouds across Canada, the U.S. and now Europe.

This sunset, featuring the planet Venus, was one of the more colorful from late last summer. Volcanic aerosols from Mt. Kasatochi was largely the reason why. Photo: Bob King

Do you recall the colorful sunsets of last summer? They even hung around into the fall, a gift of another Alaskan volcano named Kasatochi . Eruptions send lots of gases and particles, called aerosols, into the stratosphere, where they scatter the blue portion of sunlight. Blue mixes with the red light from the low sun at sunset and sunrise to create vivid purples. Yellows and oranges are enhanced as well. Mix it all together and the western sky after sundown is transformed into a scene awash in rich color.

Scientists predict similar sunsets from Mt. Redoubt’s eruptions, so be on the lookout this week when another ash-gas cloud is predicted to cross over parts of the U.S.

Not far from the prominent lunar crater Copernicus is a cluster of volcanic domes just north of the 10-mile-diameter crater Hortensius. The domes are about 4-5 miles in diameter and visible in a 6-inch telescope. Moonwatchers can spy them this weekend when the moon is just past first quarter phase. Credit: Consolidated Lunar Atlas

The moon’s been up all week in the evening sky. Although it’s a very quiet place now, the moon was once volcanically active. No tall, conical volcanoes like the iconic ones on Earth are present on the moon, but it does have its share of the blister variety called domes.

Domes are circular in form and typically about 4-8 miles in diameter. Many are topped by a tiny craterlet, which once served as a vent for thick, syrupy lava to pour out onto the moon’s surface. Repeated outflows of lava gradually built up a large, gently-sloped hill of volcanic rock called a shield volcano. Nothing violent here, just a steady and repeated oozing of hot lava. You’ll find many examples of shield volcanoes on Earth — all the Hawaiian islands are shield volcanoes, with Mauna Kea and Mauna Loa as standout examples.

A closeup of Hortensius and the domes, each of which is named after a letter in the Grek alphabet. Notice the summit craterlets. These are the holes where the lava poured out onto the surface over three billion years ago. Credit: NASA

While Hawaii’s volcanoes are still active, those on the moon expired long ago. The great age of lunar volcanism occured 3.8-3.1 billion years ago. This was during a period of intense meteorite bombardment that fractured the crust, allowing lava to well up to the surface. Things have quieted down since then except for the occasional meteorite hit or moonquake. No so with Earth, a planet packed with surprises, where a new eruption could mean a pretty sunset picture for your photo album.

Last night was clear and skywatchers enjoyed the crescent moon, a space station pass and wherever else their eyes took them. Spring is such a great time to be out. Around here that means temperatures in the 20s at night, barely sweater weather after a winter of minuses.

Here are a few photos, some my own and others submitted, for you to enjoy. All but one were taken Sunday night. 

The crescent moon with Earthshine during twilight. If you look closely, you can just make out the Seven Sisters (Pleiades) star cluster touching the top of the horizontal beam. Details: 35mm lens at f/2.8, 2-second exposure at ISO 400. Photo: Bob King

I observed from a rural location but by mid-evening a few clouds edged in and glowed yellowish-pink from lights in Duluth, some 20 miles away. Orion is off to the far right while brilliant Sirius shines just to the right of the silhouetted spruce tree. Details: 16mm lens at f/2.8, 30-second exposure at ISO 800. Photo: Bob King

William Wiethoff photographed these supernovas in a variety of galaxies last night from his home in Port Wing, Wisconsin. When a star dies, it collapses and rebounds in a titantic explosion that blows it to bits. Through a telescope a supernova looks like a faint star embedded within its host galaxy. There are probably a dozen different supernovas visible in galaxies beyond the Milky Way any particular week of the year. Most are faint like these, and require long time exposures through a telescope to record. "SN" stands for supernova and the double letter tells you the order in which it was discovered.

Jim Schaff of Hermantown succeeded in getting a nice, sharp photo of Saturn on March 19 using a webcamera attached to his 6-inch reflecting telescope. You can see subtle clouds bands on the disk and the slight tip of the rings.

When the space station flew over around 8:53 p.m. I tried to frame it with the northern half of Orion the Hunter. The three stars of his belt are at bottom left. The station makes a great pass tonight (Monday) across the top of the sky beginning at 7:44 p.m.

A few days ago we got acquainted with how much distance one light year measures and learned it was about six trillion miles. That only takes us a quarter the way to Alpha Centauri, the nearest star system beyond the sun. How do astronomers measure such enormous distances? Well, it all starts with our own two eyes.

Stand in front of a window and do a "thumb’s up" while sticking your arm straight out in front of you. Now open and close your right and then your left eye in a back and forth blinking pattern. As you do, you’ll see your thumb appear to jump back and forth across the more distant background. Because your eyes are separated by about three inches, you see your thumb from a slightly different perspective as you blink. This makes it appear to shift against the distant scene. The shift is called parallax (PAIR-uh-lacks), and astronomers use the very same concept to measure star distances.

Because our eyes are separated by several inches, we see things along two different lines of sight, allowing us to sense depth and distance. If you know the distance between your eyes and then measure how much an object shifts in front of you by blinking, you can find its distance. Astronomers apply the same concept to measuring star distances using the diameter of Earth’s orbit — the equivalent of two eyes separated by 180 million miles. Photo: photos.com

If you measured the distance between your eyes and calculated the shift in the angle of your dancing thumb, you could determine exactly how far away your thumb was from your head using simple trigonometry.

Now place your thumb right in front of your face and do the same thing. Notice how it jumps even more? The closer an object is to you, the larger its parallax. You can also make your thumb jump through a bigger angle if you could somehow increase the distance between your eyes. Either way works.

We use parallax for depth perception and seeing the world in 3-D. Unfortunately, stars are much too far away to see even the closest ones shift against the background of those more distant by blinking. Even distant objects right here on Earth show no shift when we "blink" them.

Astronomers uses the "eyes" of the Earth in January and then again in July to measure a nearby star’s parallax or shift against the more distant background stars. See below. For an excellent, interactive animation of parallax, click here. Illustration: Bob King

Astronomers need a set of eyes that are many miles apart to measure the shift of nearby stars against those in the remote distance. Luckily, Earth’s orbit provides just the ticket. Our planet is about 93 million miles from the sun, which adds up to about 180 million miles from one side of our orbit to the other. The brightest stars are generally the closest. To find the distance to one, astronomers photograph a star through a powerful telescope in, say January, and carefully measure its position in relation to the fainter stars around it. Then they wait six months and photograph the star again in July. During that time, our planet has traveled all the way around to the other side of its orbit, providing us with two different lines of sight. Careful measurement of the the second photo will reveal a tiny shift in position, enough to calculate a distance. 

In this demonstration, you can see how our star has shifted against the background ones when photographed through a high powered, professional telescope on opposite sides of Earth’s orbit. Illustration: Bob King

Tiny is an understatement. Alpha Centauri moves 0.77 arc seconds against the background sky over six months — that’s the size of a quarter seen from over three miles away. The bright star Sirius, below and east of Orion, shifts only half as much or about the size of a dime at the same distance. The Earth’s orbit allow us to measure star distances to about 160 light years. After that, the atmosphere blurs the stars too much to measure any tinier angles.
To escape the effects of our tubulent atmosphere, the European Space Agency launched the Hipparcos satellite in 1989. During its four years in orbit, Hipparcos measured parallaxes of 118,000 stars to 20 times more precisely than possible from the ground and one million stars to good precision. We now have good estimates of star distances out to about 800 light years.

That’s still a small area when you consider the 100,000 light-year-wide diameter of the Milky Way galaxy alone. There are other methods that take us out farther, but they’re all ultimately based on the parallaxes we’ve gleaned from our yearly orbit around the sun and the work accomplished by the Hipparcos Mission.

The moon will be near two star clusters Sunday night. Take a look in the west around 9 o’clock. Created with Stellarium.

Tonight we’ll enjoy a banana moon lined up with the Seven Sisters (Pleiades) and Hyades star clusters. The International Space Station will pass from west to east across the southern sky beginning at 8:53 p.m. Watch for it to first glide beneath the moon and then above Orion.

A pillar of light, created by ice crystals floating in the air, stands above the sun early Saturday morning. Photo: Bob King

Outdoors writer Sam Cook and I headed over to the Brule River in northern Wisconsin today for the opening day of steelhead trout fishing. While traveling through Superior, we saw this awesome sun pillar for a few brief minutes just as the sun broke through the clouds around 7 o’clock. I picked up the camera and took a few pictures, making sure the exposure was fast enough to quell the bounce induced by a succession of potholes.

Sun pillars can happen anytime, but seem more common in the colder months. That might be because they’re formed by sunlight reflecting off tiny, hexagon-shaped plates of ice. The plates are floating horizontally like a million frisbees stopped in mid-air. Sunlight reflects off their mirror-like bottom surfaces, creating a beam or shaft of light. You can read more about pillars here.

The space shuttle Discovery landed this afternoon at Kennedy Space Center in Florida. That leaves the International Space Station (ISS) all by itself in the night sky. Watch tonight (Saturday) as the ISS cruises almost overhead after its initial appearance in the northwestern sky starting at 8:26 p.m. At best, it will shine brighter than the planet Venus, now departed from the evening sky.

The glowing trail of 2008 TC3 lingered long enough in the atmosphere over Sudan to be recorded on a cellphone. Photo: Mohamed Elhassan Abdelatif Mahir (Noub NGO), Dr. Muawia H. Shaddad / NASA handout

Do you recall the car-sized asteroid 2008 TC3 that was spotted last October 6 by an automated sky survey? Scientists quickly computed an orbit for the space rock and predicted it would hit the Earth some 19 hours later in the Nubian Desert of Northern Sudan. Sure enough, on October 7, a spectacular fireball was seen over Sudan and recorded on at least one cellphone camera.

Peter Jenniskens, meteor astronomer, finds another fragment of 2008 TC3 (right) in the desert of Sudan. The black crust of a meteorite, called fusion crust, forms as the outer millimeter or two of the rock melts and blackens during its speedy passage through the atmosphere. Photo: Peter Jenniskens / NASA handout

Later, a recovery team led by Peter Jenniskens of the SETI Institute in
California and Muawia Shaddad of the University of Khartoum, searched
for meteorites along the projected approach path in northern Sudan. They
recovered 47 fragments of the meteorite which turned out to be a very rare type called a urelite (YOUR-uh-lite).

A broken fragment of one of the recovered meteorites. It looks like a hunk of asphalt, but 2008 TC3 is one of the rarest of meteorites. The black color of the interior is due to carbon and other minerals. For more photos, click here. Photo: Peter Jenniskens / NASA handout

Urelites are meteorites that were subjected to tremendous pressures on their "asteroid of residence", enough to turn the carbon in them into graphite and microscopic diamonds. When 2008 TC3 slammed into our atmosphere at over 25,000 miles per hour, the air pressure then fractured it into many smaller stones that rained down over a broad area of the desert.

Scientists are naturally excited about recovering such rare material. It’s even more amazing because the meteorites came from the first-ever asteroid predicted to hit Earth. Astronomers are busy studying 2008 TC3′s orbit and the black stones it left behind, hoping to nail down where in the solar system it originated.

Almost all meteorites are at least 4.6 billion years old, older than any Earth rock. Scientists study them to try and figure out how rocks formed in the solar system in the first place.

Every tiny fragment promises to reveal a clue in a story otherwise lost in the vault of time.

A delicate crescent moon will be visible tonight (Friday) in the west shortly after sunset. Created with Stellarium.

How would you like a crescent with your evening? You can see one low in the west tonight if you start looking about 15 minutes after sunset or around 7:45 p.m. The striking sickle will float just an outstretched fist high above the sunset point.

A crescent is a familiar and pleasing shape and shows up frequently in comics and cartoons, where it’s used to symbolize evening or night. You couldn’t miss it if you watched the Late Night with Conan O’Brien, where it’s incorporated in the show’s logo. In these comedic situations, the bright crescent is always illuminated on the left side, not on the right, as it normally appears during evening hours.

Take a look at our map and you’ll see what I mean. A left-lit crescent only appears before dawn near the end of the night. So how do cartoonists get away with a backwards crescent when they draw scenes with people walking their dogs or watching TV at night? I wish I knew. Perhaps it’s artistic license gone too far and then copied forever after. I challenge you to find an evening crescent in a cartoon anywhere.

The crescent and star(s) motif is a popular symbol depicted on a variety of Asian countries’ flags. Top row from left: Tunisia, Algeria and Turkey. Middle: Malaysia, Uzbekistan and Azerbaijan. Bottom: Mauritania, Pakistan and Comoros.

The crescent — accompanied by a star or stars — also shows up on more than a dozen countries’ national flags. Most of these are Muslim nations, and the crescent-star is an internationally-recognized symbol of the Islamic faith. Historians trace the flag crescents back to 1453 when the Turks took Constantinople and adopted the city’s flag, which featured a crescent moon.

The origin of the crescent symbol goes back much further to ancient times when the sun and moon were worshipped as deities. But what about that star next to the moon? Do its five points represent the five pillars of Islam? I’d like to think that the moon-and-star emblem harkens back to either a remarkably close moon-Venus conjunction or even a pairing of the banana moon with the brilliant daylight supernova that appeared in 1054 AD. No one can deny these alignments were seen as special omens. Even today, people see personal meaning in unusual astronomical events. Still, it’s just speculation.

Crescent moons as well as the sun’s likeness turn up on old Roman coins. The photo at left shows a denarius from 76 BC with a crescent moon accompanied by seven stars thought to represent the Big Dipper. The Romans knew the moon goddess as Diana and worshipped her at a festival on August 13.

Then of course there are those delicioius, buttery croissants, an edible version of the moon, which you can pick up at the local grocery. Crescents are seemingly everywhere, including the one baked up special for our eyes tonight.

While you’re out, don’t forget there will be two passes of the International Space Station (ISS) and the Discovery space shuttle tonight, both starting in the west and moving eastward across the northern sky. Discovery will appear one minute earlier on both passes, close enough so you’ll see the two of them cross the sky at the same time.

* Discovery shuttle first pass: 7:58 p.m., second at 9:33 p.m.
* ISS first pass: 7:59 p.m., second at 9:34 p.m.

A croissant roll. The sight of it makes me very hungry. Photo: photos.com

How Vega looked to me last night shortly before 11 ‘clock. It held its own low in the northeastern sky. Created with Stellarium.

I saw only one star last night. No kidding. The sky was overcast but way down in the northeast I saw a single trembling light. During the entire half hour I was out, it stubbornly refused to be quenched by clouds. After so many starless night, this was my star of hope — Vega. We visited with Vega in several blogs last summer and fall. Its fame lies not only with its brilliance, but also because it forms the Summer Triangle with help from its mates Deneb and Altair.

I looked hard for another star to bust through the grey but no more were to be found. That was OK. Vega was enough to put a twinkle in my eye.

Alphard is easy to find in the blank area of sky to the left of the Winter Triangle. If you look one fist above and a little right of Alphard, you’ll spot the head of the snake. Keep going and you’ll run into the smoky-looking Beehive star cluster, a binocular treat. The map shows the sky as you face south around 9 p.m. in late March. Created with Stellarium.

Seeing one solitary star reminded me of another star, the name of which literally means "solitary one" in Arabic. Alphard (AL-fard) is the brightest star in Hydra the Water Snake . Look for it two outstretched fists to the left of the Winter Triangle below the five star asterism that forms the head of Hydra. Alphard is only as bright as one of the Big Dipper stars, but stands out in a singular way because it’s all by its lonesome, like a hermit on 40 acres.

Alphard is 175 light years from us and shows a pale orange hue to the attentive eye. Binoculars will bring out the color better. Like brighter Arcturus, Alphard is an orange giant star, only not as bright because it’s further away. The constellation Hydra is very long and faint, and trails off to the east for a long way. Alphard and the little bunch of gems above it are the easiest part of the snake to grab hold of as it slips through the night.

The space shuttle Discovery undocked yesterday from the International Space Station, and the two will be flying in tandem until Discovery lands this Saturday. Viewers with clear skies can watch the them "chase" each other during passes across the sky. See yesterday’s blog for times.

Venus is quickly slipping away. Later this week you have a rare chance to see it both in the evening sky and in the morning before sunrise. For northern Minnesota, sunset occurs at 7:30 p.m., sunrise at 7 a.m. Created with Stellarium.

Time is running out to see Venus after sunset. It’s been too cloudy here to know for sure whether there’s still stars in the sky, but I hope your weather’s different. If you can find a place with a completely open horizon to the west, Venus will make a brief appearance to the upper right of the sun for the next three evenings. To see it, put your index and middle fingers together, tip them horizontal and extend your arm all the way out to the west. Venus will be just "two fingers" above the horizon — very low. The sky will also be bright so bring binoculars to help you find it.

As Venus swings past Earth this week it will move from the evening sky (left of the sun) to the morning sky. Conjunction occurs on Friday. Illustration: Bob King

On Friday, Venus will be in inferior conjunction, a term you might think is a tad disrespectful for the goddess of beauty. Not to worry. In astronomy, "inferior" refers to any planet that’s between the Earth and sun. A conjunction occurs when two celestial bodies appear closest together in the sky.

The opposite of an inferior conjunction is a superior one. That will happen next summer when Venus lines up with the sun again, but on the opposite or farthest side of its orbit. Because the planet is between Earth and sun, it’s also closest to us for the year. If you can find Venus, your binoculars should have no problem resolving its delicate crescent phase.

Because Venus’ orbit is tipped about 3.4 degrees relative to Earth’s, it passes north or south of the sun during a typical conjunction. Credit: Theresa Knott

Venus cruises north of the sun during its upcoming conjunction. These views show the planet and sun at 1 p.m. each day from outside the Earth’s atmosphere. The space station astronauts would see such a view. No doubt they’d want to block the brilliant sun with their hand to get a better look at the planet. Created with Stellarium.

This inferior conjunction is a bit unusual in that Venus passes a full eight degrees (almost one fist) north of the sun at closest approach. Because Venus’ orbit is tipped relative to Earth’s, it swings north or south of the sun during most conjunctions. Only very rarely does it cross directly in front. The last time this happened was in 2004, and the next will be 2012. After that we’ll have to wait until 2117.

As Venus passes between us and the sun in the next few days, we can watch its wire-thin crescent rotate around from left to right as its angle between us and the sun changes rapidly. The yellow line marks the center of the crescent. Created with Stellarium.

Because it’s so far north of the sun this time around, enterprising Venus gazers can see the planet in BOTH the evening and the morning sky. It’ll be low down and you’ll probably need binoculars but it’s certainly an unusual sight.

While you’re out this week, keep your eye open for the International Space Station and the Discovery shuttle. They’ll be crossing the northern sky from west to east during the early evening hours. Here are some viewing times.

Weds. March 25 at 8:40 p.m.
Thurs. March 26 at 9:07 p.m. — especially bright pass; ends at the Big Dipper
Fri. March 27 at 7:59 p.m.
Fri. March 27 at 9:34 p.m.

March brought many clear nights for our region up until last weekend. Nothing but rain and clouds since. In writing this blog, I frequently toss out vast distances with little numbers like 25 light years. I’ve even emphasized that distances like there are in the "neighborhood" astronomically speaking. While that’s accurate, maybe it’s time to examine just how big a distance a light year really is.

The distances to some of the March night sky’s familar stars are shown in light years except for Saturn, which is only "minutes" away. Created with Stellarium.

The Earth seems vast enough for most humans but even the moon, the closest celestial body beyond our planet, is almost 10 times as far as the Earth’s is round. If you pointed a really bright flashlight at the moon, the light beam would arrive at the lunar surface in a mere 1.2 seconds. That seems like almost no time at all, but then you’ve got to remember that light travels at 186,000 miles per second. That’s as fast as anything can go in this universe.

Saturn is now easily visible in the evening sky in Leo the Lion. How long will our light saber take to reach it? Figuring a distance around 880 million miles, it would arrive 79 minutes later. Now that we’re moving into the realm of big numbers, we could just as easily describe Saturn’s distance using time instead miles — 79 light minutes.

Keep in mind that radio waves, X-rays, gamma rays and infrared light also travel at the same speed as light since they’re all forms of electromagnetic radiation. That means that when mission controllers send a command to a spacecraft at Saturn, it takes 79 minutes to get there and another 79 minutes to receive a reply. That’s an over 2 1/2 hours round trip – talk about slow communication.

The boundary of the solar system, called the heliopause, is the place where the wind blowing from the sun meets the winds from other stars. Scientist estimate its distance at 17.6 billion miles. Light would take more than 26 hours to get there. Not far enough for you?

Distances quickly add up in our universe. Even in an X-15, the fastest manned plane ever made, it would take many thousands of years to travel to the nearest star beyond the sun. Illustration: Bob King

Let’s make a big leap. The distance light travels in one full year is about 6 trillion miles. That’s well beyond the bounds of the solar system, yet only a quarter of the way to the nearest star, Alpha Centauri, in the southern constellation of the Centaur. Our shaky little flashlight beam requires another three-plus years before it flashes by that star’s burning surface. Alpha Centauri is more than 26 trillion miles or 4.3 light years away, and we’re still practically in our backyard!

A view of Alpha Centauri and its bright companion star, Alpha Centauri B, as seen from the surface of a hypothetical planet in orbit about the two. Alpha Centauri is a little larger and hotter than our own sun. Credit: Wiki Commons

If we reverse the trip, the light leaving Alpha Centauri for Earth takes four years and four months to get here. The light we see tonight left the star that long ago. We’re talking 4.3-year-old light here. Because distances in the universe are so vast, we’re never able to see how anything looks RIGHT NOW, only how things looked in the past. The further away they are, the further back in time we see them. Take a look at the distance map to get an idea of time’s depth among the early spring stars.

The experimental X-15 rocket powered plane during a test in 1959. The X-15 could travel at 3600 mph. Credit: NASA

4.3 years is a long time for a trip, but I’m afraid it’s the best we can do. We’ll never travel at the speed of light, but even if we could zoom there at the respectable speed of a mile a second, the fastest speed ever reached by a manned aircraft, our journed would last 760,000 years. Since most of us do our flying in a 747 jetliner, which has an average cruising speed of 565 mph, the trip would require 4,842,477 years. Give or take.

I say the best way to get there is in your imagination. Free and easy trips depart the very next clear night.

The pyramids at Gizah in Egypt stand against the ancient sky when Thuban in Draco was the polestar. Our era’s polestar, Polaris, is the yellow star at left. Photo: Ricardo Liberato; background from Chris Marriott’s SkyMap.com.

Back in the good old days, Thuban (THU-ben) in the constellation Draco the Dragon was the polestar. It occupied the spot where Polaris the North Star sits now. Ah, but that was almost 5000 years ago. Much has changed since then.

Keeping track of the stars is bad enough. Earth’s rotation and revolution around the sun are infamous for sowing confusion among beginning skywatchers about what’s up when. Over a longer span of time we also have to contend with precession.

While our axis maintains its tip of 23 1/2 degrees, over a period of 25,800 years it changes the direction of its rotation like the gyroscope does in the animation. Since the "north star" is the star that Earth’s north polar axis happens to point to in space, that means we see a series of different north stars during a full cycle. Think of the Earth’s axis as describing a slow circle in the sky like a children’s top running down. Whatever star lies on that circle will eventually be a pole star.

Right now, Polaris in the Little Dipper holds that title but not forever. The role will slide to the star Gamma in Cepheus the King around the year 4000 A.D. and finally return to Thuban in 23,000 A.D. That’s so much time to think about it hurts my head.

This map shows the sky around 9 o’clock in late March as you face north. Thuban is easy to find between the two Dippers. Created with Stellarium.

Thuban, while not bright, is very easy to find. Just look midway between the two end stars of the Little Dipper and the star Mizar, in the bend of the Big Dipper’s Handle. In the days of the Great Pyramids, all the other stars in the northern sky circled around Thuban, hub star of the north, just like they do around Polaris in our era. Since Thuban’s is on the dim side, you wonder if anyone really took notice of it. I mean, it’s not like our North Star, which is as bright as one of the Dipper stars.

This is the circle described in the sky by Earth’s wobbling axis. Thuban was the polestar around 3000 B.C. and closest to the pole in the year 2787 B.C. Polaris has replaced it in our era. The minus numbers refer to B.C. years, the plus to A.D. Credit: Tao’lunga

If you’ve studied pyramids at all, you’ll come across frequent references to the astronomical orientation of Khufu’s Great Pyramid at Ghizah, constructed around 2560 B.C. There’s a shaft built into one face that some say is aligned with the Thuban. Others think it has more to do with building construction than astronomy but no one knows for sure. One thing’s for certain: the shaft does point north, nearly in the star’s direction. Perhaps it was the escape route for the pharoah’s soul. Whatever the truth is, who can deny the poetry of a star shining all night down a stoney hole into the heart of an ancient tomb?

Precession (left) is caused by the gravitational attraction of the sun and moon on the Earth’s slightly bulgy equator, which makes our axis wobble in rhythm. Didn’t know Earth was a little heavy around the middle? Photos may imply otherwise, but the Earth’s not a perfect sphere. Our planet’s rotation makes it bulge slightly at the equator, increasing the diameter there by 27 miles compared to the poles. Good thing my equatorial bulge is considerably smaller. It keeps me out of trouble.
 

Two views of the northern sky on a March evening separated by nearly 5000 years. Illustration: Bob King with Chris Marriott’s SkyMap.

Cassiopeia lies on the opposite side of the North Star from the Big Dipper. This map shows the sky in late March around 9 o’clock. During spring, Cassiopeia resembles the Greek letter Sigma. Two outstretched fists separate the Dipper Bowl from the North Star. Created with Stellarium.

The other night while watching the space station quietly pass through the constellation Cassiopeia the Queen, it occured to me how many guises this familiar group of stars wears. We usually refer to Cassiopeia as the W-shaped constellation, but in early spring, as it descends in the northwestern sky, those five stars remind me more of the the Greek letter Sigma (right). That recalls a story my older brother Mike told me about the requirements for acceptance in his college fraternity. For one of them, he had to recite the Greek alphabet three times — very quickly — while holding a burning match. Seeing the Sigma of Cassiopeia is far less nerve-wracking. The symbol is also widely used in math, where it represents the summation of groups of numbers.

Cassiopeia circles around the North Star during the year, showing us different "sides" of its personality. Illustrations: Bob King, Stellarium

Only in summer, when the queen skirts the northern horizon, does the constellation resemble the letter W. By fall, she morphs into a zigzag and then finally transforms into a letter of the alphabet again — this time an M — in early winter. All of this shape shifting happens because Cassiopeia is close enough to the North Star that it never sets for northern latitudes. Instead, we watch it describe a circle around steady-still Polaris, changing its orientation just as the seasons change.

Some of Cassiopeia’s several guises.