Moondark for March: Wrong Way Snowball

Here’s an observing treat for late winter: a newly discovered comet, easily visible in binoculars or a small telescope, is racing across the sky. To many observers, it looks like a bright globular cluster, but bigger, blue-green tinged and not quite round. So far, comet watchers have had to set an early alarm as the comet slides across the morning sky from Libra into Virgo. And this week, the comet is at its closest and brightest. There’s an interesting story behind Comet Lulin’s discovery. In July, 2007, a university student in mainland China, Quanzhi Ye, was the first to recognize the comet on photographs taken at the Lulin Observatory in Taiwan. In both China and Taiwan, this comet is known as the Comet of Cooperation, so its apparition has geopolitical as well as astrophysical dimensions. If you’ve been following comet Lulin you’ve no doubt taken note of its track near Libra’s Zubenelgenubi, past Spica, and next toward Saturn in Leo and on to Regulus. This line is amazingly close to the ecliptic, the path of the Sun, the planets and most of the other solar system bodies. As comets come from the spherical Oort cloud in the far reaches of the solar system, there is no particular reason why a first-time comet should follow the ecliptic and most clearly don’t. An even stranger fact is that Lulin is going from east to west, counter to most things in the solar system (except in retrograde motion and around inferior conjunctions): this is the wrong way! Coincidence? Conspiracy? Hardly: predicted positions of a comet Lulin are tabulated in an ephemeris, and these coordinates are derived from special numbers, known as elements. You need to specify six quantities: positions in three dimensions as well as three velocities in those directions. Gravitational accelerations derived from Newton’s Law F=ma then completely describe the orbit. Although such position and velocity vectors are often used, elements for planetarium programs are frequently given as classical parameters that describe the orbit in geometrical terms (to be precise, elements are strictly valid for a particular point in time, the epoch). Two elements describe the size and shape of the orbit: the semi-major axis a (or perihelion distance q) and eccentricity e. Elliptical orbits have e<1, and parabolic and hyperbolic orbits have e=1 or e>1 , respectively. In addition, you also need to know when it’s at a particular location on the orbit, say the date of perihelion, T for comets (or more frequently for asteroids, the mean anomaly). Three angles define the orientation of the orbit: its inclination to the ecliptic (denoted i), the longitude of the ascending node (Ω) and argument of perihelion (ω). What the latter two angles mean is easier to see than to explain in words, and fortunately there are several interactive applets for just for this purpose, including one for Comet Lulin. You can find these six numbers for the asteroids and many comets on the web. For Comet Lulin, the inclination (angle to the ecliptic) explains its wrong-way zodiacal track. Lulin’s i is 178°, less than 2° from 180° opposite the ecliptic: it is almost exactly along it in the wrong direction. Visually, from above (i.e., north of the Sun), planets' orbit counterclockwise, Lulin goes clockwise, backwards, in essentially the same plane. It’s in a nearly parabolic orbit since e is very close to 1, so the orbit is open: this is likely the comet’s first, and maybe its only time round the Sun in this orbit. The perihelion date of January 10th and its distance q of 1.2 AU (1 AU is approximately the Earth’s distance from the Sun) means it passes near the Earth’s orbit. Fortunately for us, we swing past it near its closest approach to the Sun, and thus it will appear large and relatively bright. We are closest, only 0.4 AU, on the 24th, and Lulin is at opposition in Leo on the 25th. Since we are travelling in opposite directions, the geometry changes quickly, its apparent motion reaches 5° per day westward near opposition, and the comet will fade rapidly thereafter as Lulin passes through Cancer and Gemini.   By the time you receive this newsletter, Comet Lulin will be near its best, visible all night long and easy to find below Leo’s reclined feline figure. It may be visible to the unaided eye from a dark location, but binoculars or any telescope will provide a much better view of both the tail and anti-tail, both oriented along the ecliptic (the elements and geometry of the orbit explain this as well). Through March, watch it cruise away to the depths of the solar system, this is your one chance: the elements say it won’t return for thousands of years, if ever. Hopefully the departure of this dirty snowball will herald the departure of winter, and bring us luck with clear skies and warmer nights. Spring begins on the 20th of March at 6:44 am EST. At right the track of Lulin (curved solid line) almost exactly matches the ecliptic (curved dotted line). Lulin's position from 1 February through 13 March is indicated every five days, and Saturn, the Moon and Sun are plotted for the 24th, the date of the comet's closest approach to Saturn.  Moondark is written by Douglas C. Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 22 February 2009. Text and graphics on this web page are free for non-commercial use with attribution under a Creative Commons Attribution-Non-commercial 3.0 License. Ask Doug about other uses.