Thursday, September 24, 2009

The Sea Goat

Last night the cats fought over who would sleep under the blankets against human warmth and in the sheepskin-lined cat bed. It was a frantic feline version of musical chairs, and when the music stopped, it was every cat for him-or-herself.

This was my first clue that the first nip of autumn was in the air and that flannel sheet season was nigh. My second clue was the “prime time” prominence of fall constellation Capricornus the Sea Goat, which can be seen approaching the meridian from the east about an hour after sunset, when the sky’s just gotten good and dark. It will soon dislodge Sagittarius the Archer— just leaving the meridian, heading west— from its summertime domination of our view to the south.

Capricornus (kapp-rih-KORN-uss) is one of the twelve constellations of the zodiac. This band of constellations is significant because it straddles the ecliptic, the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. Because the Earth, Moon, and planets all lie in roughly the same plane as they orbit the Sun, we see the Moon and the planets stick close to the ecliptic as they cross our sky.

You might say the ecliptic represents the plane of our platter-shaped solar system. But I like to think of it, and the zodiacal constellations that encompass it, as the celestial parade route upon which I’m sure to see a procession of planets float by.

Case in point: there are currently two planets passing through Capricornus, Jupiter and Neptune. Jupiter is easily seen with the naked eye; it’s the blazing “star” in the southeastern sky and the first luminary that pops out at you after sunset. Right now you can use Jupiter to help pinpoint the location of Capricornus, should you be unfamiliar with its star pattern.

Viewing Neptune (not a naked-eye object) requires a star map or software application that shows current planet positions, and large binoculars or a telescope.

Star map created with Your Sky

About an hour after your local sunset time, locate bright Jupiter. Using the map above and the relative position shown for Jupiter, trace out in the sky the star pattern shown. This is the asterism (recognizable star pattern) known as the Bandanna, and it contains the brightest stars in Capricornus.

Capricornus in Johannes Hevelius's 1690 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Trust me, you’re far more likely to learn to recognize a stellar bandanna than a sea goat. I like Mesopotamian and Greek mythology— and tales of animal metamorphosis— as much as the next person. But honestly, what sort of perverse storyteller would find it necessary to graft a fish’s tail onto the head and body of a goat? It's so grotesque as to defy understanding, so perhaps we should just wave the white bandanna of surrender.

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Here in the valley of the Rio Grande, the first nip of autumn means sweet relief from stifling summer heat along with the sweet smell of roasting New Mexican chiles. It also means I’ll be harvesting one of my favorite deep-sky objects from the telescopic depths of Capricornus. More on that later. Right now, I’ve got to air out my cozy flannel sheets and clean my high-power eyepiece.


Astronomy Essential: The four gas giant planets don’t have solid surfaces.

It’s hard to comprehend that no interplanetary traveler of the future will ever step out onto the surface of Jupiter or Saturn— even with the right protective space suit. Those gas giants are, as the name suggests, composed of gasses: primarily the elements hydrogen, helium, methane, ammonia, and oxygen.

On the two gas giants farthest from the Sun, Uranus and Neptune, the normally gaseous elements are believed to take liquid or semi-solid form. Gas-infused slush and frigid “seas” of liquid hydrogen, helium, methane, and ammonia make these two ports of call no less inhospitable than their vaporous neighbors.