February Sky from Keeble Observatory
Is one of our planets missing? Not exactly, but the number of planets officially recognized by the IAU may be about to go down from nine to eight. Pluto, the most distant from the Sun (on average) and the one with the most eccentric orbit, is about to be demoted from "planet" to "trans-Neptunian object." And, since February is a month with lots of Pluto "anniversaries" in it, we should pay due attention before it's no longer a planet!
What is a planet, anyway? The name comes from the Greek, and it means "wanderer" - so called
because the planets shift their positions in the sky relative to the "fixed" background stars. The
ancients counted the Sun and Moon as planets, but also included Mercury, Venus, Mars, Jupiter, and
Saturn. These objects are all visible to the unaided eye. Venus, now visible in the early evening sky,
is the closest to us, and also the brightest. It shifts from evening to morning "star" with predictable
regularity. Mercury is harder to see, since it's often lost in the glare of the rising or setting Sun.
Mars can be seen in the early morning right now, and Jupiter and Saturn are currently seen in the
Uranus was discovered in the 18th century, and is only visible with a pretty good telescope. Neptune is similarly a target for telescopes - it's existence was predicted based on anomalies in the orbital path of Uranus. Finding it was considered a triumph of theoretical astronomy in the 19th Century. Pluto's tale is both very similar, and very different!
The orbit of Uranus still seemed not quite right. Since Neptune's discovery was based on anomalies in the orbit of Uranus, astronomers naturally believed that yet another planet further out must be responsible for the remaining discrepancies. In 1930, Clyde Tombaugh found Pluto on the night of February 18th, where it had been predicted to be. Yet, even smaller than Earth's moon, Pluto could not possibly be the cause of a noticeable perturbation in Neptune's orbit. Further, there were errors in the calculation that Tombaugh was following, so he was looking in the wrong place, anyway!
Pluto's orbit has carried it closer than Neptune to the Sun since the 7th of February in 1979 - a distinction it will lose on the 10th of February this year. Because it is closer to the Sun than its average distance, we observe that it has formed a thin atmosphere as ice sublimates upon heating. This is similar to the process by which comets form their tails, and some advocate reclassifying Pluto as a large comet because of this. Also, the region beyond Neptune harbors a reservoir of comets and planetessimals left over from the formation of the solar system. This region is known as the Kuiper Belt, and some advocate reclassifying Pluto as merely the largest yet-discovered Kuiper Belt Object. A poll is being conducted of members of the International Astronomical Union, which has the authority to name celestial objects. They are being asked if Pluto should retain its status as a planet and, if not, what should be its new classification.
I'll keep you posted!
Lunar phases for February: No Full Moons, because we experienced a "blue moon" in January, and will again in March. Last quarter on the 8th, at 6:58 am. New moon (with an annular solar eclipse visible from the Indian Ocean and Australia) at 1:39 am on the the 16th. First quarter occurs ont he 22nd, at 9:43 pm.
At dusk, you can see Venus, low on the west-southwest horizon, with Jupiter and Saturn following across the sky. Mercury emerges from behind the Sun at mid-month, appearing below and to the right of Venus By months end, Jupiter will pass Venus, appearing in the sky lower on the horizon at sunset.. Mars is visible in the morning sky to the southwest. (Don't confuse it with Antares, in the constellation Scorpio, which appears more to the south.)
An overhead look at mid-month finds the zenith with four bright stars nearby. Castor and Pollux, in Gemini, are to the east, while Capella and Elnath, in Auriga, are just to the west. Orion and Taurus lie below these markers, yet high above the southern horizon. Following the familiar shape of Orion, we find the brightest star visible in the sky, Sirius. To the west of Aldebaran, still in Taurus, we find the bright cluster of the Pleiades. (The Japanese name this asterism "Subaru" - which is why you see a cluster of stars on the grill of that particular brand of automobile.) To the east, below Castor and Pollux, we see Leo rising. The bright star at the "heart" of the lion is Regulus, which figured in our narrative last month of the Star of Bethlehem. To the northeast we can now pick out the distinctive "dipper" of Ursa Major. Following the imaginary line drawn through the two highest stars in the "bowl" will lead your eye to Polaris, which is almost directly over Earth's north pole.
The Milky Way bisects the sky, running from just north of northeast to just south of southeast. Low on the northwest horizon, it may be still possible to find the Andromeda Galaxy on a clear, cold evening.