As the wind sharpens with cold and the nights lengthen, Jupiter will again illuminate a dark part of the sky and accompany us through the winter. The Jovian year is 12 Earth years, so it moves one-twelth of the way around the sky each calendar year.
Since there are 12 signs of the Zodiac, it moves about one sign per year. Last year it was in Pisces, so this year it is moving through Aries, the ram.
The Greek root of the word "planet" is "wanderer" because the planets appear to wander among the stars. Most of the time, the planets move eastward against the background of stars due to their orbital motion, but they periodically stop and move westward for weeks or months.
This profoundly puzzled astronomers for millennia, giving rise to Ptolemy's complicated epicycles in a geocentric system and, finally, Kepler's neat elliptical orbits in Copernicus' heliocentric system. To understand it, imagine walking with other people on a track, all in the same direction but fastest in the inner lane and slowing toward the outer lane.
Walking in the inner lane, you generally see everyone else moving against the background in the same direction you are walking.
However, when you pass someone, for a few steps just as you pass, they appear to move backwards against the background. The planets, on the cosmic track of the solar system, also do this and the backward motion is called retrograde. Currently, Jupiter is in retrograde motion, moving westward with respect to the stars as Earth, on a smaller, faster orbit, passed it on Oct. 29. On that day (at 8 p.m. Eastern Daylight Time), Earth passed between Jupiter and the sun, putting Jupiter opposite the sun in a position known as "opposition." Not only did Jupiter rise and set opposite the sun that day, but Earth and the giant were at their closest approach, a mere 370 million miles. Though not a stone's throw, the proximity makes cloudbedecked Jupiter blaze in our sky. As Earth leaves Jupiter behind, it rises earlier each night as
the brightest object in the eastern sky, providing a guide to some of the more obscure stars and constellations.
As shown in Figure 1, Jupiter is south of Aries. I refer to the three westernmost stars of Aries (Mesarthim [mess-ARE-tim], Sheratan [SHARE-a-tawn], and Hamal [ham-ALL]) as the cosmic hockey stick and Jupiter has been moving westward (right) beneath it since Aug. 30.
As shown by the path in Figure 2, it will continue to move toward Omicron Piscium (PISS-i-um
a name the kids will love to say!) until our northern neighbors in Canada celebrate Boxing Day
after Christmas. Then it will begin moving eastward, in prograde motion, and get about half-way to the Pleiades before the sun catches up to hide this large planet from our view. The sun and Jupiter will be lined up, in "conjunction," on May 13 when Jupiter, on the far side of the sun, will be 558 million miles away.
As we admire Jupiter, we can use it as a guide not just to Aries, but to much less familiar Cetus the Sea monster with a ringlet of stars forming its head. I tend to see Cetus as a whale with the body to the west and the fluke of its tail high and to the east, but the historical figure is the opposite. We know this because Menkar (men-CAR) means "nostrils" (though, curiously, it's in the position of the monster's jaw). The star connecting the body to the neck,
Kaffalijidhma (cough-all-JID-ma) refers back to an (unknown) Arabic constellation, means "part of a hand" and actually refers to the entire circlet.
One astronomer refers to the area just below Jupiter's path as "The Land of Xi's" (xees) due to four stars (1 Ceti, 2 Ceti, Ari and Tau) being designated with the 14th letter of the Greek Alphabet. In his Uranomtria atlas of 1603, German astronomer Johann Bayer first used Greek letters and the possessive (genitive) form of their constellation to designate stars, generally in order of brightness with the Alpha being the brightest. Thus the brightest star in Aries, Hamal, is also referred to as Alpha Arietis, or Ari and Omicron Piscium is abbreviated Psc. Bayer didn't stick rigidly to this, the stars in the Big Dipper are lettered in order from the bowl to the handle. But the Land of Xi's does indicate a region of dim stars.
A reason other than chance for the lack of bright stars in this part of the sky is that we are looking toward the south pole of the Milky Way galaxy. Most stars, and thus most bright stars, are found along the thick disk of stars forming the Milky Way galaxy in which we live. When we look out of the disk there are simply fewer stars and less chance for any of them being bright (most stars are smaller, dimmer and redder than the sun).
The north galactic pole in Coma Berenices, tucked under the handle of the Big Dipper, is also lacking in bright stars. Another often overlooked constellation is lurking low in the southwest, soon to be lost in the
dusk. It is Capricornus, the Goat. It's usually represented as a mergoat (a mermaid with a goat's head) with outer stars Al Giedi (al gee-?dee, "the goat") and Deneb Algiedi (DEN-ebb al gee-?-dee, "tail of the goat"). These stars with their nearest neighbors indicate two nested V's. At the bottom of the outer V is Omega Capricorni which, in spite of its Omega status, is not the dimmest star in Capricornus. Between Capricornus and Pisces lies Aquarius.
There is no easy mnemonic stick figure, but the Alpha and Beta stars, Sadal Meliek (saw-DOLL MEL-ick, "lucky star of the King") and Sadal Suud (saw-DOLL sue-OOD, "luckiest of all") have wonderfully curious names and are fairly easy to spot. These, with Enif in Pegasus, are actually sibling stars slowly drifting away from each other on their cosmic journeys as many siblings do.
As the moon wanes and moves into the early morning sky, grab a mug of something warm and take some time to explore these darker parts of our sky before the blaze of Orion and the Winter Hexagon take over for the winter.
As always, anyone who has questions about astronomy is asked to visit the Adirondack Public Observatory website at apobservatory.org or email Aileen at firstname.lastname@example.org.