"Let your soul stand cool and composed before a million universes" Walt Whitman
The Moon's current phase, courtesy of USNO.
Sierra NightSky for the period starting December 6, 2013 - by Jim Kaler
The next Skylights will appear Friday, December 20.
We begin the week with the Moon in its waxing crescent phase as it heads towards first quarter, which is passed on Monday, December 9, shortly before it rises in North America. As it rounds the Earth it then moves into the waxing gibbous, which ends with full Moon on Tuesday the 17th. During the remainder of the fortnight, the Moon fades in the waning gibbous phase. The evening of Friday the 6th, look for the waxing crescent well up and a bit to the left of Venus.
The night of Saturday the 14th, the rising Moon will shine to the right of the Pleiades, while the next evening finds it smack in the Hyades of Taurus, with Aldebaran below it. The full Moon then visits Jupiter, seen rising well to the west of it the night of Tuesday the 17th, then just to the south the following night.
The Moon passes apogee, farthest from Earth, on Thursday the 19th. Of more interest, because of the Earth's orbital eccentricity and axial tilt, we see the earliest sunset of the year on Saturday the 7th in spite of Winter Solstice passage not taking place until Friday the 21st.
Venus and Jupiter dominate the early evening, though Venus is starting to slip away. Reaching its greatest brilliance on Friday the 6th, the planet does not set in the southwest until an hour after the sky gets fully dark. By that time, Jupiter is well up in the northeast and quite unmistakable near the star Delta Geminorum (Wasat) more or less south of Castor and Pollux. Jupiter then crosses the meridian to the south one to two hours after midnight, getting earlier as the fortnight proceeds.
By that time, Mars is up in the east as it plods easterly against the stars of western Virgo, crossing the celestial equator into the southern hemisphere on Monday the 16th. Saturn then rises in the southeast in Libra shortly before dawn, giving us three planets to admire.
The highlight of the our two-week period (and one of the reasons for it) is the Geminid meteor shower, which runs roughly between December 11 and 15 and peaks the morning of Saturday the 14th, when in a dark sky you might see more than 100 meteors an hour that seem to come out of the constellation Gemini. Unfortunately, the bright gibbous Moon will get in the way. The Geminids are strange in that they are the debris of the "asteroid" Phaeton, which behaves more like a comet that takes but 1.4 years to orbit the Sun. Clearly the minor bodies of the Solar System are more complex than was once thought.
By mid evening, the autumn stars, featuring those of the Andromeda myth (including Cassiopeia, Pegasus, Perseus, and so on), are shining near the meridian, while those of Summer (Lyra, Cygnus) have dropped far into the northwest. We await then the constellations of true winter, which center on great Orion, with Taurus and Gemini above him, Lepus (the Hare) below, and Sirius, of Canis Major, his larger Hunting Dog and the brightest star of the sky, to the southeast.
STAR OF THE WEEK: EPS LYR (Epsilon Lyrae, a fascinating quadruple, updated from 2003)
Among the naked-eye sky's myriads of stars, few are more distinctive than the famed "Double-Double in Lyra," Epsilon Lyrae. While there are numerous other examples of multiple stars, even on the scale of Epsilon, this one may be the favorite. Writing from the nineteenth century, Smythe and Chambers tell us that "This system forms a very elegant object, and merits the closest attention."
Look just to the northeast of brilliant Vega, and if your eyes are very good -- or upon using binoculars -- you see that fourth magnitude (3.85) Epsilon is really two nearly identical stars close together, separated by 208 seconds of arc (3.5 minutes). The western of the naked-eye double is Epsilon-1, the eastern Epsilon-2. A decent telescope then shows each of these to be divided into two more closely similar class A stars, those of Eps-1 split by 2.8 seconds, those of Eps-2 by 2.2 seconds, all located 161 light years away (second Hipparcos reduction), give or take half a dozen.
From west to east, the four are labelled A through D as Eps-1 A, Eps-1 B, Eps-2 C, Eps-2 D. Much brighter Mizar and Castor are similar, but the individual pairs cannot be split by eye, and require the spectrograph or other sophisticated techniques.
Epsilon Lyrae is one you can see complete. Orbital motion is seen for each pair, but the errors are significant. In what seems to be the most likely solution of the limited data, the Eps-1 pair (Epsilon A and B) orbit with a period of around 1800 years at an average separation of 235 Astronomical Units, a high eccentricity bringing them as close as 73 AU and as far apart as 400. The Eps-2 pair (Eps C and D) take 724 years at a mean separation of 145 AU (95 to 195). Eps-1 and Eps-2 are vastly too far apart for any orbital motion to be noticed, as they are separated by at least 10,000 AU and from the masses below must take at least 400,000 years to make a complete circuit.
While they look similar, there really are differences among them all. From A through D, we find apparent magnitudes 5.1, 6.0, 5.1, 5.4; spectral classes A3, A7, A5, A5; temperatures 8000, 7700, 8200, 8200 K; luminosities 18, 8, 17, 14 times that of the Sun; and from the theory of stellar structure and evolution, masses of 1.9, 1.5, 1.9, 1.8 solar. The luminosities might be increased by at most 20 percent or so if some dimming by interstellar dust is taken into account.
The whole system is probably 800 or so million years old. All are fast spinners, with minimum equatorial rotation speeds of 200, 150, 177, and 212 kilometers per second, which precludes odd abundance anomalies caused by separation of elements. We still do not know just how such quadruples are formed, with pairs so reasonably close and the pairs of pairs so far apart.
From each of the pairs, an observer might see each of the other shining with the light of a quarter Moon perhaps a degree away from each other. The only way such a quadruple star can survive for any period of time is to be in a double-double hierarchy; if the four are mixed up close together, the combined gravitational influences will kick out first one, then another, leaving just a double behind.
Nevertheless, gravitational influences of passing stars and tides raised by the Galaxy will most likely separate Eps-1 and Eps-2 into doubles that go their separate ways. The easternmost of the Eps-2 pair may also be double with a faint companion 0.2 seconds or so away, yielding at least five stars in the system.
Do you have a favorite star or one you would like to see highlighted on the Star of the Week? Send a suggestion to Jim Kaler.
Sierra NightSky thanks to Jim Kaler.
Check out his site for more