Gibbous Moon

Note: There will be no December 3 post. Enjoy the November 26 post or browse my older posts. I'll be back with a new post on December 10.

The night sky is dominated right now by a waxing gibbous Moon, that is, a Moon that is more than half illuminated and that is growing in percent illuminated. Put another way, it is the phase of the Moon that occurs between First Quarter (also known as “Half Moon”) and Full Moon.

Once Full Moon occurs, our nearest celestial neighbor will enter its waning gibbous phase, when it is more than half illuminated and shrinking in percent illuminated. In other words, the waning gibbous Moon is the phase that occurs between Full Moon and Last Quarter (the other “Half Moon” phase).

If you’re curious about the etymology of these terms, as I was, you’ll find the following of interest.

“Wax” means to increase in size or intensity, and comes from the Old English weaxan, meaning to increase. “Wane” means to diminish in size or intensity, and comes down to us from the Old English wan, meaning deficient, and the Latin vanus, meaning empty. Finally, “gibbous” means marked by swelling, or humpbacked, and comes from the Latin gibbus, meaning hump.

Phases of the Moon
Credit: http://starchild.gsfc.nasa.gov/

So if the Moon appears humpbacked, it is gibbous. If the western--or right--side of the humpbacked Moon is illuminated, it’s waxing gibbous. If the eastern--or left--side of the swollen Moon is illuminated, it’s waning gibbous.

And if the slithy toves gyre and gimble in the wabe, it's plain gibberish.

Astronomy Essential: Stars do not twinkle.

Stars sometimes appear to blink on and off rapidly, dim and brighten wildly, and scintillate in a variety of colors. This “twinkling” is merely an optical distortion caused by the turbulence of Earth’s atmosphere, through which starlight must travel to reach our eyes. The starlight is “bent” in many random directions as it travels through atmospheric layers and pockets of different density and temperature. Our eyes interpret this bending or refraction of the light as twinkling.

This effect is usually most pronounced near the horizon, because the light from a star near the horizon must travel a longer path through the atmosphere to reach our position than it would if the star were positioned directly above us.

The next time you notice stars near the horizon twinkling, check the stars directly above you to see if they are in fact twinkling less or not twinkling at all.

Tropical Fish

There’s a third fish in the autumn sky, less heralded than the Pisces duo and--in the Northern Hemisphere--best seen from warmer latitudes such as the southern U.S. and Mexico. Considered a Southern Hemisphere constellation, this tropical creature is Piscis Austrinus the Southern Fish.

One would have to travel well south of the equator to, say, Santiago, Chile or Sydney, Australia in order to see Piscis Austrinus at the zenith, directly overhead. However, many northerners with a clear view of the southern horizon should be able to locate it easily because its brightest star is among the top twenty brightest stars in the night sky. Folks in far northern latitudes, such as Fargo, North Dakota, will have a bit more of a challenge because the star sits only about a fist-width above the southern horizon. A fist-width is the width of your fist, held at arm’s length against the sky, and measured across the knuckles.

Piscis Austrinus (PIE-siss aw-STRY-nuss) swims solo, south of Aquarius. In fact, the stream of water from the Water Bearer’s Water Jar has traditionally been depicted as pouring into the open mouth of Piscis Austrinus.

Piscis Austrinus (under Aquarius) in John Flamsteed’s 1729 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

I’m ready for a dip in warm, tropical waters. How about you?

1) About an hour after your local sunset time, 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.

Looking south to Fomalhaut in Piscis Austrinus
Star maps created with Your Sky

2) Look south to find the Southern Fish, floating just east of the meridian. You can use the Bandanna of Capricornus to locate it, especially this year while the Bandanna is occupied by brilliant Jupiter. Jupiter is the brightest “star” you’ll see in the southern sky and the first celestial landmark to become visible after sunset.

3) The luminary you’re looking for is the only bright star in the Northern Fish and the only one with a traditional name: Fomalhaut. Fomalhaut (FOAM-uh-lott) is from the Arabic for mouth of the fish, and it marks the gaping maw of the oddly thirsty fish.

Only 25 light years away, this seemingly ordinary white star is surrounded by an emerging solar system four times the diameter of ours. A planet several times larger than Jupiter has been imaged in orbit around Fomalhaut.

It’s difficult to mistake Fomalhaut for any other star, since there are no other bright stars near it. In fact, Fomalhaut has been characterized by a number of writers as “lonely.” Once you spot it and take in the oceanic expanse of dark sky and dim stars around it, you’ll understand why its alternate classical name was Piscis Solitarius--the Solitary Fish.

Astronomy Essential: The zodiac is the band of 12 constellations that lie along the ecliptic.

The ecliptic is the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. Because the Earth, the Moon, and the planets all lie in roughly the same plane as they orbit the Sun, the ecliptic can also be said to represent the plane of the solar system. This is why the Sun, Moon, and planets all appear to move along the ecliptic and through the constellations of the zodiac.

Although conventionally we say the Sun, Moon, and planets move “through” the zodiac, we need to remember that the stars of the zodiacal constellations are much farther away than the Sun, Moon, and planets. In essence, they form the backdrop for those solar system bodies.

The constellations of the zodiac are: Aries the Ram, Taurus the Bull, Gemini the Twins, Cancer the Crab, Leo the Lion, Virgo the Maiden, Libra the Scales, Scorpius the Scorpion, Sagittarius the Archer, Capricornus the Sea Goat, Aquarius the Water Bearer, and Pisces the Fishes.

The Ties That Bind

Last week, we located the most recognizable asterism (star pattern) in the constellation Pisces the Fishes: the Circlet. The Circlet marks the body of the Western Fish, the fish that points westward.

The other fish in the Pisces (PIE-seez) pair is the Northern Fish, the fish that points northward. Starting at the Circlet, let’s see if we can reel in the other fish on our line.

The Northern Fish (left) and the Western Fish (right) of Pisces
Courtesy of Linda Hall Library of Science, Engineering and Technology

1) About an hour after your local sunset time, 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) First, locate the Circlet. If you need help, consult last week’s post.

Looking south to Pisces

Star map created with Your Sky

3) Heading east from the Circlet is a streamer of eight naked-eye stars. This is the string binding the Western Fish. The terminal star--and the brightest in the string-- is called Alrescha. Alrescha (ahl-RESH-uh) is Arabic for the cord. It is also known as the Knot Star, because it marks the knot that binds the two strings--and the two fishes--together.

Alrescha is a binary star, that is, a system of two stars in orbit around each other. In Alrescha’s case, both are white dwarf stars. Because of their close proximity from our vantage point, we see their combined light as one star.

4) Heading northwest from Alrescha is the binding cord for the Northern Fish. From Alrescha, look for two bright stars spaced an equal distance apart. The second one, and the brighter of the two, is Kullat Nunu. It’s a little brighter than Alrescha. Kullat Nunu is Babylonian for cord of the fish. It’s a yellow giant and the brightest star in Pisces.

Beyond Kullat Nunu, the Northern Fish disintegrates into a jumble of not very bright stars with no readily apparent pattern to pick out. Suffice it to say that the ancients imagined a second succulent fish tethered there, ready for the fire.

Pass the tartar sauce, please.

Astronomy Essential: There are stars in the daytime sky.

If you could turn off the Sun during daylight hours--like you would turn off a light bulb--you would see a star-filled sky appear.

As the Earth spins, we are always looking out at the Milky Way and its legions of stars. During the day, those stars form a backdrop to the Sun. Consequently, they are--for the most part--not visible due to the Sun’s glaring brightness.

Ambitious amateur astronomers challenge themselves by trying to spot the brighter stars during the day, using a telescope as well as precise coordinates so they know where to point it. Popular daytime targets include: Castor and Pollux, the Gemini Twins; Rigel and Betelgeuse, brightest stars in Orion; Sirius, brightest star in our sky (after the Sun); and even not-so-bright Polaris, the North Star.

Circling the Circlet

Like Aquarius, Pisces the Fishes is another faint constellation that’s a little difficult to spot. But if you can find the Great Square of Pegasus, you can locate an asterism (recognizable star pattern) in Pisces and thereby orient yourself to the fishy constellation.

On classical star atlas maps, Pisces is typically represented as two fishes, each tied with a string at the tail, and the two strings joined with a knot. One fish heads west, and the other fish heads north.

Pisces is a constellation of the zodiac. The zodiac is a band of twelve constellations that straddles the ecliptic. The ecliptic is the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. Because the Earth, the Moon, and the planets all lie in roughly the same plane as they orbit the Sun, the ecliptic can also be said to represent the plane of the solar system. This is why the Sun, Moon, and planets all appear to move along the ecliptic and through the constellations of the zodiac.

Pisces in Johann Bode’s 1782 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

Let’s circle our quarry.

1) About an hour after your local sunset time, 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.

Looking south to the Circlet
Star maps created with Your Sky

2) First, locate the Great Square asterism in Pegasus, high in the southeastern sky.

3) Now look under the Great Square for a circular arrangement of five naked-eye stars. If you don’t see it, you’ll need to try again at a darker site, away from urban and suburban light pollution. This is the Circlet asterism, the most recognizable star pattern in Pisces. It lies a little east of the Water Jar asterism in Aquarius. The Circlet marks the body of what is known as the Western Fish, the fish that points westward.

The Circlet of Pisces

4) The stars in the Circlet do not have traditional names, but just west of the Circlet is Fum al Samakah, which is Arabic for fish’s mouth, and which marks the gaping mouth of the Western Fish. Fum al Samakah is a blue-white dwarf star, nearly 500 light years away (one light year is nearly six trillion miles).

Next time, we’ll locate the Northern Fish. Then you’re all invited to the fish fry.







Astronomy Essential: There are planets around other stars.

Thus far, astronomers have discovered over 350 planets orbiting stars other than our Sun. These are known as exoplanets. It seems the familiar eight planets of our solar system are not unique in the universe.

Astronomers use a number of methods to detect the presence of planets around distant stars. Orbiting planets can make stars wobble a bit, and that wobble can be measured to confirm the existence of a planet that can’t be directly observed. The periodic dimming of a star when an orbiting planet passes in front of it--that is, transits it from our perspective on Earth--can be measured. And in rare cases, exoplanets can be directly imaged.

The exoplanets found thus far are primarily gas giants and ice giants. The holy grail of current exoplanet detection research is to find terrestrial planets--rocky planets like Earth--orbiting in the habitable zone of their suns where, potentially, liquid water and life might exist.