Celestial Events

Natural History, Oct, 1999 by Richard Panek

Most of the discussion about Pluto this year has focused on how exactly to categorize it. Is it a planet? An asteroid? Or perhaps that recent addition to the astronomy lexicon, a trans-Neptunian object? (See "Pluto's Honor," February 1999.) For sky watchers, however, this controversy has prompted a different question: Can we actually see Pluto?

The answer, as is often the case with Pluto, involves qualification: yes, we can, but with difficulty. Still, this month offers the last best chance to see Pluto for some time to come--that is, for anyone with a detailed star map and at least an eight-inch reflecting telescope.

For about 20 years of its 248-year orbit, Pluto cuts across the path of Neptune to become the eighth planet from the Sun, a position it occupied until this past February 11, at 11:29 A.M. eastern standard time. Now Pluto is once again the ninth planet, heading away from the Sun and from us. Pluto's position at aphelion (its farthest distance from the Sun) is considerable: 4.6 billion miles, compared with 2.8 billion miles at perihelion (its closest approach to the Sun). For earthbound observers, the slog outward movement of Pluto is hardly noticeable from one month to the next, or even from one year to the next. But with every decrease in magnitude--from 13.7 to 13.9 this year--the object leaves some sky watchers behind.

Fortunately, Pluto still resides in the vicinity of an easily identifiable frame of reference, a star in the constellation Ophiuchus. Known in English as the Serpent Holder, Ophiuchus is said to represent the Greek god of medicine, Asclepius, who is identified with serpents. Indeed, the constellation Serpens appears to wrap around him. In October, Serpens Caput (the head) and Serpens Cauda (the tail) are low in the west-southwest sky just after nightfall, and if you can find these two, you can see the bell-shaped Ophiuchus between them.

The first step in finding Pluto is to locate the 2.6 magnitude star Zeta Ophiuchus, the middle star in the base of the Serpent Holder. During October, Pluto will be swinging south of the star, moving on a roughly northwest-southeast diagonal. By November 1, it will lie half a degree, or what appears to be about one moon's width, from the star.

How will you know at a glance which faint dot is Pluto? You won't--at least not at first. The best method for locating Pluto is still the one used by the late Clyde Tombaugh, of the Lowell Observatory in Arizona, when he discovered the so-called Planet X in 1930. Tombaugh was looking for a planet that would account for supposed perturbations in Neptune's orbit, and he focused his search on a section of the sky where, according to calculations, such a mystery object might exist. Patiently comparing thousands of photographs taken several days apart, Tombaugh looked for the one object that moved, and in two photographs taken on January 23 and 29 of that year, he found it.

Never mind that the perturbations of Neptune proved to be a mathematical anomaly. Never mind that the mass of what Tombaugh found (1/500 of Earth's) wouldn't have been remotely sufficient to account for Neptune's perturbations, even if they had existed. And never mind that as of 1992, astronomers have been finding Pluto-like objects by the dozens, leading to a new addition to the solar system bestiary, the trans-Neptunian object (part of the Kuiper belt). If it's Pluto you want, here's how to get it: For several nights, observe the section of the sky described above. Sketch the points of light that you see. Over the course of the observations, one point should be showing movement. Bingo: Pluto.

The last time Pluto was this close to Earth, a couple of centuries ago, it wasn't a planet. And maybe the next time it's this close, a couple of centuries from now, it won't be a planet. But for now, whatever it is, there it is--just not for long.

Richard Panek is the author of Seeing and Believing: How the Telescope Opened Our Eyes and Minds to the Heavens (Viking, 1998).