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.

Sweet.

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.

The Reclusive Ram

Aries the Ram is a bit of a recluse, hiding in plain sight in the late autumn sky. Although not terribly impressive to behold, Aries is historically an important constellation. In the astrological zodiacs of numerous ancient civilizations, it was considered the first zodiacal constellation, or first sign. This designation may have been made when it was noted by the ancients that the Sun was positioned in Aries at the spring equinox.

The spring equinox is the moment when the center of the Sun is directly above the Earth’s equator. This occurs only twice a year; the other occurrence is known as the fall equinox. Another name sometimes given for the spring equinox is first point of Aries, which, of course, derives from old astrological traditions.

Aries in Bayer's 1603 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

The “first sign” tradition continues today; Aries is usually listed first in horoscope columns found in newspapers and magazines. You’ll also note that the conventional date range given by astrologers for people “born in the sign of Aries” begins on March 21; spring equinox typically occurs on or near March 20.

By the way, the Sun is no longer in Aries at the spring equinox. Due to precession— sometimes called precession of the equinoxes— which is the wobbling-top motion of Earth’s axis, the equinoxes occur a few minutes earlier each year. As a result, at each spring equinox, the Sun is not in the same position it was the previous year against the background stars, as seen from Earth. If we track the Sun’s position over the years at a fixed point in time (such as the spring equinox), from our perspective on Earth the Sun would appear to move through all 12 zodiacal constellations in reverse. As a result of precession, the Sun is now in Pisces, the twelfth zodiacal constellation, at the spring equinox.

Aries in Middleton's 1842 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

In mythology, Aries represents the ram whose golden fleece was famously sought by Jason and the Argonauts. Let’s conduct our own search for the storied sheep.

1) Wait about two hours after sunset to begin observing, so that Aries is high enough in the sky.

2) 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 maps created with Your Sky

3) Find the Great Square of Pegasus, which is nearly overhead, and the Pleiades star cluster, which is low in the east. These are both easy to spot using the star map above. Midway between them is Aries. Look for a small, curved line of three stars. This is the most notable asterism (recognizable star pattern) in Aries. It always remind me of a comma, so I call this asterism the Comma. Unfortunately, there’s nothing in the scattering of stars in Aries that particularly resembles a ram, unless you want to think of the Comma as a curving ram’s horn.

4) The brightest star in Aries is Hamal (hah-MAHL), which comes from the Arabic for lamb. Other ancient names for this star had meanings like ram’s eye, horn star, and leading one (the ram that led the celestial flock). Hamal is an orange giant star, 66 light years away. One light year is the distance light travels in one Earth year, nearly six trillion miles.

5) The second brightest star in Aries is Sheratan (SHER-uh-tunn), from the Arabic for the pair, a reference to Sheratan and the final star in the Comma, which we’ll look at next. Sheratan is a white star. When you compare it to Hamal, can you see that orange giant Hamal has a more golden hue?

6) Mesarthim (mess-ahr-TEEM) is from the Hebrew for servants. Mesarthim, a white star, may only be Aries’ fourth brightest but it’s known as the first star of Aries. This is because in antiquity, when the spring equinox was in Aries, Mesarthim was the closest Ram star to the equinox. Physically, the equinox can be described as the intersection of the Sun’s path with the celestial equator, the imaginary line that represents the Earth’s equator extended out into space.

7) Although the third brightest star in Aries has no traditional name and is instead saddled with the uninspired moniker Flamsteed 41, it does manage to be part of a funky little asterism. The blue-white dwarf star is the brightest member of a four-star asterism known as the Northern Fly, riding on the ram’s back. To find Flamsteed 41 and the Northern Fly, look northeast of Hamal and east of the constellation Triangulum.

In its glory days, this asterism was part of a now-defunct constellation by the same name, which was introduced in the 17th century. Now it’s merely a curiosity for naked-eye stargazers like us and, I imagine, a constant irritant for our celestial ram.

The Ram and the Fly in Bode's 1801 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

A Crab, Two Jackasses, and 350 Bees

The constellation of Cancer the Crab holds a special place in my heart, because it’s my astrological sun sign. To be sure, I consider astrology a recreational pursuit rather than a scientific one. Nevertheless, I can’t help but feel somewhat possessive toward the little celestial crustacean as he scuttles across the spring sky.

Cancer is a constellation of the zodiac, following Gemini the Twins to its west and preceding Leo the Lion to its east. The zodiac is the band of twelve constellations that straddles the ecliptic, as seen from Earth. 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 also represents the plane of the solar system.

The name Cancer is, appropriately, from the Latin word for crab. In Greek myth, Cancer was the crab that pinched strongman Hercules’ foot as he fought the giant water snake Hydra, a monstrous creature with nine heads. During this skirmish, Hercules crushed the crab under his foot, and the gods placed the feisty but unlucky crab in the heavens.

Cancer the Crab in 18th century star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

How did a word that means crab come to refer to a group of cellular diseases? Early physicians noticed that tumors and the distended blood vessels around them formed a pattern that resembled a crab. Around the 17th century, they began applying the word “cancer” to the diseases that caused these tumors.

A dark sky--well away from city and suburban lights--is required to commune with the Crab. The faint constellation would be inconspicuous if it weren't for the renowned naked-eye star cluster at its heart: the Beehive. Even the brightest stars in Cancer are somewhat dim. If you see anything bright in Cancer, rest assured it’s a planet just passing through.

Grab your long-handled net, and let’s go crabbing!

1) You’ll need to face south, so 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) Wait at least one hour after sunset to begin observing, so that twilight’s finished and your sky’s good and dark. This observing week, you’ll have plenty of gazing time before the Moon floods the sky with light. It rises well after midnight on Friday the 25th and even later each night after that.

3) As you face south, tilt your head back slowly and move your gaze from the southern horizon up to the zenith, the spot directly above your head. Look for a fuzzy patch. It'll be almost as high as the zenith if you live in the southern latitudes of the continental U.S. If you live in the northern latitudes, it'll be slightly lower, up to a quarter of the way from the zenith down toward the southern horizon. It'll also be slightly west of due south. Find it? Excellent! This is the famous Beehive cluster.






If you’re having trouble finding it, first make sure you’re dark adapted. Avoid all white light for a minimum of 20 minutes. This will improve your night vision and help you see faint objects. Second, try to locate the Sickle, a prominent asterism (recognizable star pattern) in Leo. It looks like a big, backwards question mark. Right now the bright golden planet Saturn is hanging out at the base of the Sickle. The open end of the Sickle’s curving top points toward the Beehive. Also, if you can locate reddish planet Mars near the Gemini twin stars (Castor and Pollux) and draw a line between Saturn and Mars, it will cross the Beehive.

M44, the Beehive cluster

The Beehive is an open cluster, a loose grouping of stars that formed around the same time from the same gas cloud. It contains around 350 stars. It is alternately known as M44 and as Praesepe (pronounced pree SEE pee). The former is shorthand for Messier 44, the object’s designation in the catalog of famed 18th century French astronomer Charles Messier (pronounced MESS ee yay). The latter, Praesepe, is the oldest of these names and is Latin for manger. The origin of the whimsical Beehive name is unknown.

4) Once you’ve found the Beehive, look for two faint stars that bracket it on the left (east). The top star is Asellus Borealis and the bottom one, Asellus Australis. These are Latin for Northern Ass and Southern Ass. These two stars form the asterism known as the Donkeys or the Asses. In ancient mythology, the asses participated in a battle between the Greek gods and some giants. After helping the gods prevail by spooking the giants with their obnoxious braying, they were rewarded with a place in the sky next to a well-stocked feed trough, also known as a manger. Aha! Now we see why the Beehive originally had the name Praesepe.

5) The star Acubens, Arabic for the claw, marks the southernmost crab claw. Just south of Acubens (pronounced ak oo BENZ) is a circlet of stars--the asterism known as the Hydra’s Head, in the constellation Hydra. The water snake and the crab huddle together in the sky, sworn enemies of Hercules.

6) Al Tarf, the brightest star in Cancer, is an orange giant. Its name is from the Arabic for the end, and it marks one of the crab’s southern legs.

7) The only other star in Cancer with a traditional name is Tegmine (pronounced TEG-min-uh). It's Arabic for in the covering, referring to the crab’s shell. Tegmine is west of the Beehive.

8) The Beehive is a great binocular object. In fact, binoculars--rather than a telescope--usually give the most satisfying magnified view of a large object like this. If you have access to a pair of binoculars of any size, use them to peer at the fuzzy patch. The magnified patch will resolve, separating into distinct points of light.

Wander around in the cluster and notice how brilliant and sharp the stars are. I like to contrast this object with the Pleiades or "Seven Sisters" open cluster in Taurus. The Pleiades (pronounced PLEE uh deez) cluster is currently crossing paths with a nebula, a cloud of gas and dust. This makes its stars appear to be wrapped in cotton candy. In contrast, the sparkling Beehive is a visual 'palate cleanser,' not unlike a refreshing sorbet served between courses at a fancy banquet.

Moondance

This week, let’s watch the waxing Moon do-sa-do with planets and other objects as it sashays along the ecliptic. The waxing Moon is a growing moon. Each night, more of its face is illuminated by the Sun, from our changing vantage point on Earth. In actuality, half of the Moon’s sphere is illuminated by the Sun at all times. However, because of Earth’s position at the center of the Moon’s month-long promenade, we only ever see part of the Moon’s Sun-illuminated hemisphere, except during Full Moon and New Moon.

“Swing and circle, wheel around…”

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. As a result, we see the Moon and the planets sticking close to the ecliptic as they swing across the sky.

“Promenade family, chain down the line…”

Face south for all these observations, for which you will need no equipment but your eyes.

On Friday evening, April 11, the Moon’s face is 39% illuminated, as seen from Earth. Just to the left of the Moon is the bright planet Mars. Notice its reddish or coppery color; this distinct coloration is from its soil, which is rich in iron oxides. Mars is currently spending some quality time in the constellation of Gemini the Twins, and the Moon joins it there for the entire evening, until they both set in the west after midnight.

Slightly above the Moon and Mars are a pair of stars, the two brightest in Gemini: Pollux on the left and Castor on the right. In Greek mythology, Pollux and Castor were inseparable half-brothers with the same mother. When the mortal Castor was killed in a dispute, immortal Pollux was inconsolable. His father, supreme ruler Zeus, took pity on him and allowed him to live in the sky with his brother for half of the year.

Amazingly, the slightly dimmer Castor is actually a six-star system! With a telescope at high magnification, I have seen the three main component stars, but the three companion stars are too close to their partners to be visible in any telescope.

“Acey deucey, boys run right…”

On Saturday, April 12, the Moon reaches its First Quarter phase, when it is 50% illuminated as seen from Earth. This is sometimes referred to as a “half moon.” Note that the Moon is now to the left of Pollux and Castor. Tonight it straddles the boundary between the constellations Gemini the Twins and Cancer the Crab. Precisely-drawn boundaries between constellations were officially designated in 1930 by astronomers of the International Astronomical Union.

“Tag your neighbor, here comes the judge…”

On Sunday evening, April 13, the Moon is 61% illuminated as seen from Earth and is called a gibbous moon. The Moon is gibbous whenever more than half of its face is illuminated, the exception being a Full Moon, when its face is 100% illuminated. Tonight it is more than halfway across Cancer. Hidden in its glare is the Beehive, a wonderful naked-eye star cluster that has been known since antiquity. We will examine the Beehive in greater detail in a few weeks.

Every night at the same time, the Moon is farther to the east than it was the night before. This is because it orbits Earth counterclockwise, as viewed from above the North Pole. From our perspective in the Northern Hemisphere, it travels from right to left--or west to east--over the course of a month.

“Pass to the center, ride the tide…”

On Monday evening, April 14, 71% of the gibbous moon is illuminated, as seen from Earth. The Moon is now in the constellation Leo the Lion. Just to its left is the bright star Regulus, also known as “The Lion’s Heart.” Continuing a little farther to the left, we come to the even brighter Saturn, shining like a golden star. The iconic ringed planet is 750 million miles from Earth.

“Shoot the star, rollaway, weave the ring…”

On Tuesday and Wednesday, April 15 and 16, the Moon continues to travel east through Leo, following the gentle curve of the ecliptic. On the 16th, the Moon will glow--87% illuminated--below Denebola, the bright star that marks Leo the Lion’s tail.

Did you notice that the three constellations the Moon is traveling through this week--Gemini, Cancer, and Leo--are zodiacal constellations? This is because the zodiac is by definition the band of 12 constellations that straddles the ecliptic. As noted earlier, the Moon and the planets hug the ecliptic and, therefore, we see them moving through the zodiacal constellations over time.

“Flip the hourglass, sashay thru!”