Delusions of Grandeur

I love that the naked-eye planets have unique colors. It's true that some beginning stargazers have actually accused me of being “delusional” when I’ve attempted to point out to them color differences among various planets and stars. And in the interest of full disclosure, I suppose I should mention that I’m pretty sensitive to the shades and subtleties of color, and am fully prepared to vigorously debate the relative merits of cornflower and periwinkle.

But three of the five naked-eye planets are visible in the night sky now, so you can put my claim to the test yourself.

Star maps created with Your Sky

Right after sunset, about one fist-width above the western horizon, look for the bright beacon of our neighboring planet Venus. (Never look directly at the Sun!) A fist-width is your fist held at arm’s length against the sky and measured across the knuckles. Venus is the third brightest celestial object in Earth’s sky, after the Sun and the Moon, and it blazes with a bluish-white hue.

About a half-hour after sunset, face east-southeast and look for bright planet Mars. It’s shining just below Castor and Pollux, the namesake stars of Gemini the Twins. You can also find it by drawing an imaginary line from Sirius, the brightest star in the sky, to Procyon, the bright star northeast of Sirius (that is, a little above it and to the left). Keep going past Procyon, a little less than the distance between Sirius and Procyon, and look for a reddish or orange-colored "star." It appears distinctively copper to me. This is the Red Planet, and its colorful name refers to the high concentration of iron oxide (aka rust) in the planet’s top layer of rock and dust.

About an hour after sunset, face east and look for a bright luminary coming up over the eastern horizon. It’s not quite as bright as Mars, but brighter than Regulus, the star at the bottom of the Sickle asterism (star pattern). The Sickle marks the head of the constellation Leo the Lion. The bright newcomer is the ringed planet Saturn, and it has a golden-yellow hue. This coloration is caused by sunlight (from our yellow Sun) reflecting off the ammonia clouds in Saturn’s upper atmosphere.

By the way, massive Jupiter and petite Mercury, the other two naked-eye planets, are aligned too close to the Sun right now— from our vantage point on Earth— to be visible. But yes, they too display unique hues.

That’s my story, and I’m sticking to it.

Shepherd Moon

Last week, I talked about starting an observing Life List and related how I’d recently checked off two items on my Life List. If you read that post, you know that both items are not for beginners. Rather they’re the elusive companions to a couple of familiar, naked-eye objects that are accessible to beginners.

The first tag-along object I checked off my Life List this year was Sirius B, the dim companion star to Sirius the Dog Star, brightest star in the night sky.

And now for the exciting conclusion…

Tag-along Object #2
On March 28 of this year, I gathered with fellow amateurs in my astronomy club for a much-anticipated Messier Marathon at a dark-sky site. Although I wasn’t “running” the Marathon this year, I was on a scavenger hunt of my own. I’d decided it was a good night to try for that most elusive moon of Saturn: Mimas. Since Saturn’s rings are nearly edge-on right now, it’s an excellent time to scout for the fainter moons. When Saturn’s rings are open, the planet reflects so much more light our way that faint moons get lost in the glare.

Diminutive Mimas (MY-muss) is only 243 miles in diameter—about the distance from Baltimore, Maryland to Norfolk, Virginia. Compare it to Saturn’s largest and brightest moon, Titan, which is 3,200 miles in diameter and which can easily be spotted when viewing Saturn through even a modest telescope. Titan looks like a little star, usually several ring diameters from the planet. If you view Saturn and see only one moon, chances are it’s Titan.

Saturn and the Cassini Division

Source: NASA, ESA and E. Karkoschka (University of Arizona)

Although small, Mimas has a powerhouse purpose. It’s a shepherd moon, so-called because its gravity tugs on the rock and ice particles in Saturn’s rubble-filled rings and herds them into formation. If you observe Saturn with a telescope when its rings are open, you may notice a thin black line inscribed on the surface of the ring plane, circling the planet. This is the Cassini Division, a gap in the rings that yawns nearly 3000 miles wide. The gravitational pull of Mimas is believed to be the force that keeps the Cassini Division clear.

Mimas the shepherd moon and Herschel the "Death Star" crater
Source: NASA

Close up, Mimas would look something like the Death Star from Star Wars, with its surface dominated by a huge impact crater, one third as wide as the moon itself. The crater is called Herschel, after Mimas’s discoverer, the eminent English astronomer William Herschel.

But on that cool spring evening, I was hoping for just a glimpse of a cagey little pinprick of light. Saturn was beckoning with its signature golden light, near the back leg of Leo the Lion. So I started my quest in our observatory dome, looking through a classic, 1950s Cave Astrola 16-inch reflector. The nearly edge-on Saturn looked like an olive on a toothpick.

Star map created with Your Sky

Bill, who was running the observatory that night, found Mimas’s position using the planetarium software on his laptop. A few more seasoned observers (I did tell you in my previous post that you have to be an instigator) climbed the tall ladder and yelled down that they thought they could see it, coming and going as it shimmered on the upper tip of the toothpick. I gave it a try and also thought I could see it dancing on the hairy edge of my vision.

We stumbled down onto the observing field and set upon Geoff, who was visiting from an astronomy club in northern New Mexico, but who had the most aperture on the field: a 24-inch-diameter reflector. Up the ladder to look through his graciously proffered cannon and…yes! There it was, a fleck of light, now separated from the toothpick and forming the bottom point of a diamond of moons. Comparing it to the other three moons in the diamond—Rhea, Dione, and Iapetus— I was shocked by how much dimmer Mimas appeared. It was an exhilarating revelation, because I realized how impossible it would be for me to spot such a faint fleck when Saturn’s rings are open. Sometimes in observational astronomy, timing is everything.

What’s next? Well, I’ve got a late spring/early summer rendezvous planned with astronomer buddy Dave (of Sirius B fame) to hunt down 3C 273 in the constellation Virgo.

3C 273 is the decidedly un-poetic name of the brightest quasar in the night sky. Although it appears dim to us and must be hunted with a telescope/eyepiece of substantial aperture and magnification, 3C 273 is believed to have a luminosity 100 times that of our entire Milky Way galaxy! Discovered in 1963, it was the first quasar found.

Quasars are distant energy sources that emit a tremendous amount of radiation. They are star-like in appearance due to their distance, but quasars are believed to be the bright, energetic cores of galaxies powered by supermassive black holes.

Note that 3C 273 and its quasar brethren are not in the Milky Way. In fact, at around two billion light years away, 3C 273 is considered one of the most distant objects an amateur astronomer can view. I simply can’t resist that challenge.

The quasar 3c 273
Source: NASA and J. Bahcall (IAS)

One final thought on the value of a Life List: there’s nothing to make you feel less like a novice than to find out that fellow observers with a lifetime of observing experience just saw the object on your Life List for the very first time also. Start your Life List…today!

Astronomy Essential: Sunspots are not spots.

Sunspots are irregularly-shaped dark regions on the Sun’s surface caused by intense magnetic activity. Sunspots vary in shape and size, and they’re often larger than the Earth. As the magnetic fields flux, existing sunspots may fade away and new ones may form elsewhere on the Sun.

The magnetic activity in those regions disrupts the flow of heat from the Sun’s core to its surface. Because of this, the sunspots are— although still intensely hot— much cooler than the solar surface that surrounds them.

Sunspots aren’t black either. The contrast in temperature with the blistering hot regions around them simply make them appear very dark in comparison.

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