Thursday, December 25, 2008

Ringing in the New

For a sky-watcher, ringing in the New Year can best be accomplished by gazing rapturously at the rings of Saturn. This year, however, as we say goodbye to 2008 and hello to 2009, we can gleefully gawk at the absence of rings.

Planet Saturn is tilted in relation to its orbital path around the Sun. As it chugs around the Sun— in an orbit that lasts 30 Earth years— sometimes its rings are tilted open from our vantage point, showing us either the “top” surface of the rings or the “bottom.” And twice during its orbit, that is, every 15 years, we experience a ring plane crossing, when our view of Saturn is side-on or edge-on, and the remarkably thin rings disappear from view.

The next ring plane crossing is coming up quickly and will occur in September of 2009. Because of this, the rings have been gradually closing from our perspective. By the end of this month, they’ll be less than one degree open. Imagine an angle measuring one degree: that’s really small! Since Saturn will be quite close to the Sun in September and therefore not well placed for us to view the vanishing of the rings, this is an excellent time to sneak a peek. After all, what’s one silly little degree among friends?

Grab your telescope, beg or borrow a telescope, or look for telescope observing opportunities through your local observatory, planetarium, science museum, or astronomy club. This is no time to be shy!

1) To spot Saturn, you’ll need to face east. 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 back to the west, and you’ll be facing approximately east.

Star map created with Your Sky

2) About an hour or so before midnight, Saturn will rise over the eastern horizon, behind Regulus, the bright star that marks the Lion’s Heart in the constellation Leo. You may also recognize Regulus as the star marking the bottom of the asterism that looks like a backwards question mark, the one called the Sickle. An asterism is a recognizable star pattern. Saturn will be brighter than Regulus, and it will have a golden color.

3) The later in the evening you can wait to observe Saturn, the better. The higher Saturn is in the sky, the less atmosphere you’ll be looking through. Generally speaking, this translates to a clearer, steadier image in your eyepiece.

4) Once you have the golden planet in view, you’ll notice the nearly edge-on rings. When I observed Saturn recently, this is what I saw:

It was quite startling to see no ring plane surface. Rather, there was the effect of a straight line, white on the ends outside the planet’s disc and black where it crossed the disc. The black line is the shadow of the rings cast onto the surface of the planet by sunlight.

5) What I also observed, which you can see quite readily, was Saturn’s slightly squashed shape. Because the planet is a rapidly spinning sphere of gasses, it has developed an equatorial bulge, that is, it bulges along its equator. Having just indulged in Christmas dinner, I can certainly relate.

With the incredibly bright rings out of the way and a sideways view, now we can enjoy what is often overlooked: the oval shape that resembles a sat-upon beach ball.

6) To finish up your communion with the ringed one, look for the usual sprinkling of moons just beyond the edges of the rings They’ll look like little stars, and it’s a fun challenge to see how many you can spy.

See you next year!

Credit: NASA/JPL

Thursday, December 18, 2008

A Lizard's Tale

The minor constellations are often overlooked because their stars tend to be dim and are best spotted in dark skies relatively free of light pollution. For a change of pace, it can be fun to stalk these secretive creatures when you’re away from city lights.

This is a good time of year to search the crevices of the sky for Lacerta the Lizard (luh-SIRR-tuh). Although at various times and places its unassuming star pattern was seen as a newt, a weasel, a winged serpent, and a scepter, the Lizard designation finally took hold.

The small, reptilian constellation was established in 1690 by Johannes Hevelius, a Polish astronomer and star atlas maker. It is, therefore, considered a modern constellation, rather than one of the classical constellations, which have roots in ancient cultures and are usually associated with myths and legends.

Lacerta in Bode’s 1801 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Lacerta is one of the ten smallest constellations of the Northern Hemisphere sky and fairly dim. Let’s dark adapt and see if we can tease our wriggly friend into view.

1) Face south. 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) About an hour after sunset you should be able to spot the Great Square of Pegasus nearly overhead or, if you’re observing from a far north latitude, about a quarter of the way from the zenith (the spot directly overhead) down toward the southern horizon.

Star maps created with Your Sky

3) If you draw an imaginary line between the center of the Great Square and Deneb, the brightest star in Cygnus the Swan, you’ll cross the tip of the lizard’s tail. His body extends up towards the House of Cepheus the King.

4) Because of Lacerta’s modern origin, it has no stars with traditional names. The brightest star, which we call Alpha for its star catalog designation, is a white dwarf star.

How many of the lizard’s six zigzagging segments can you spot?

Thursday, December 11, 2008

Biggest Moon, Smallest See

This week, for your observing pleasure, I give you the biggest Moon to moon over and the smallest sea to see.

Just after sunset on Friday, December 12, look east to watch the Full Moon rising. This month, the Full Moon occurs only a few hours after it reaches perigee, which makes it the biggest Moon of the year. Perigee is the point in the Moon’s orbit where it’s closest to Earth. The Moon’s orbit is an oval-shaped ellipse, not a circle. At one narrow end of its elliptical orbit, the Moon is at perigee. At the other end, it’s at apogee, the point in its orbit where it’s farthest from Earth.

Image source: NASA

As with any object, the closer the Moon is to us, the larger it appears. In particular, Friday’s Full Moon will appear about 14% bigger and 30% brighter than it did when it was near apogee, back in June. Of course, you won’t have the apogee Moon hanging in the sky next to the perigee Moon to aid you in size comparison. You’ll just have to take my word for it and enjoy the show as the super-sized Moon climbs in the sky.

Appearance of Moon at perigee and apogee in 2007

Take advantage of the jumbo Moon to look for the smallest naked-eye lunar feature that most people can see: the Sea of Crises or Mare Crisium (MAH-ray CRISS-ee-yum). A mare (MAH-ray) is a crater that filled with lava that afterwards cooled and solidified into basalt. Although Mare Crisium is not the smallest sea on the Moon, it is a mere 260 miles in diameter. Spotting it is quite an accomplishment.

Image from Wikipedia

Look for Mare Crisium soon after moonrise on the 12th, while the sky is still bright with twilight. I don’t have the sharpest eyesight, and I find it easiest to pick out that feature when the Full Moon is not yet blindingly bright, as it will certainly become after twilight fades and the sky darkens. Mare Crisium will be near the upper limb of the rising Moon; the limb is the outer edge of the Moon’s disc. Look for a small, dark dot just above the “ears” of the Rabbit in the Moon.

If you can’t look on Friday evening, watch for the moonrise on Saturday evening, about an hour after sunset, and try it then. After Saturday, you won’t be able to spot Mare Crisium, as that edge of the waning (shrinking) Moon will be in shadow. You’ll have to wait until a couple days after the next New Moon on December 27 to try to spot the little sea in the first illuminated slice of the waxing (growing) crescent Moon.

If you observe the biggest Moon and/or if you spot Mare Crisium, post a comment on this page and share your success!

Thursday, December 4, 2008

The Way

There are an estimated 100 billion galaxies in the universe, and we occupy just one of them, the Milky Way.

It was the great 20th century American astronomer Edwin Hubble who determined that many mysterious telescopic objects thought to be emerging solar systems were in fact galaxies— immense star systems lying beyond the Milky Way. The Milky Way was not, it seemed, the whole enchilada. This discovery expanded our cosmic horizons and gave us our first inkling of just how immense the universe was.

Since Hubble’s landmark discovery, we’ve learned quite a bit about our cosmic 'hood. We now know that the Milky Way is a barred spiral type of galaxy. A spiral galaxy is shaped somewhat like a pinwheel, with curved arms radiating out from a center that is densely packed with stars. In a barred spiral, the center has an elongated shape.

We also know that there’s a supermassive black hole lurking at our galaxy's center. A black hole is an object so dense that nothing— not even light— can escape from its gravitational field.

Courtesy NASA/JPL-Caltech

In addition, we’ve learned that the Milky Way contains around 400 billion stars. We know it’s around 200,000 light years in diameter; one light year is the distance light travels in one Earth year, nearly six trillion miles. Knowing these numbers doesn’t necessarily mean we mere mortals can fully grasp the size and distance involved. The implication for a short-lived species such as Homo sapiens is that space travel to other parts of our galaxy would require thousands of successive generations of people on each pioneering space ship.

However, like all the generations of our species that came before us, we can see the Milky Way on most any night. If we look skyward from a dark site away from urban light pollution, we can enjoy an edge-on view of the star-packed arms of our platter-shaped galaxy. Because the Milky Way is a horde of stars too numerous and faint to be resolved (separated into distinct points of light) with the naked eye, we see it as a hazy band of light arching across the sky. Some folks seeing the Milky Way for the first time mistake it for a long, lingering cloud.

The poetic name for our home galaxy comes to us from the Latin Via Lactea (Milky Way), which in turn derived from the Greek word for milk. A number of other cultures also saw the band of light as a stream of milk. But around the world, there have been numerous names given to this distinctive celestial object. Here are just a few:

- Silver River
- Winter Street
- Path of White Ashes
- Birds’ Way
- Straw Road
- Pilgrims’ Road
- Great Serpent
- River of Heaven
- Heavenly Girdle
- Road of Souls
and my personal favorite, from the Polynesians:
- Long Blue Cloud-Eating Shark

My pet name for the Milky Way is simply “the Way,” because it represents for me a way of being and a way of seeing. I learned an important lesson a number of years ago on a camping and observing trip in southern Arizona. I had spent the better part of a night glued to the eyepiece of my telescope, hunting various galaxies, globular clusters, and nebulas on an observing list and methodically ticking off those “faint fuzzies” as I found them. Straightening up from the eyepiece to stretch my back, I faced south and audibly gasped. The woods south of the clearing where I and my partner were observing were on fire!

After a few heart-pounding seconds, I noticed that the raging firestorm licking the sky above the trees was black-and-white. Black-and-white flames? Then it dawned on me that what I thought was a forest fire was the Summer Milky Way, which had risen in all its magnificence while I was attached like a barnacle to my telescope.

Because I am fortunate to live and observe in New Mexico, which has excellent observing conditions, I thought I had seen the Milky Way. But I had never seen it like I saw it that night. The combination of superb transparency (atmospheric clarity) and a deep, dark, black-as-the-ace-of-spades sky background made every tendril and wisp of the Way pop out in blazing, 3D relief.

I almost missed the finest view of the night, a deeply satisfying naked-eye spectacle that flickered across my retinas and seared itself into my memory. And here’s the strange lesson: sometimes even serious sky observers have to remember to just look up.