The discovery of Phoebe

When the 24-inch astrograph funded by Catherine Bruce (and therefore referred to as the Bruce Telescope) was installed at Arequipa in 1897, Pickering renewed his search again. In April 1899, he found indications of a new satellite on plates taken on 16, 17 and 18 August of the previous year. It was at the surprisingly large distance of 12.8 million kilometres from Saturn. He named it Phoebe, and estimated its orbital period as 490 days. When confirmation was not forthcoming, he ''began to wonder if the images on the plates of 1898 might not after all have been defects, or faint stars recurring by a curious coincidence in exactly the proper places to represent the motion of a satellite''. In 1900 he spotted it again, but it was even farther from the planet. By September 1902, Phoebe was found to be present on 42 of 60 plates which had been taken by the Bruce telescope of Saturn's vicinity, so it was possible to refine its orbit. However, on a series of plates exposed at Arequipa in April/May 1904, Phoebe was not at the predicted positions.

After re-analysing the data, Pickering announced in late 1904 that Phoebe orbits in a retrograde manner, doing so in an unusually eccentric orbit that varies between 9.8 and 15.6 million kilometres with a period of 546.5 days. P.J. Melotte took a series of photographs of Phoebe using the 30-inch reflector of the Royal Observatory in Greenwich over a four-month period during the ring-plane crossings in 1907 and tracked the moon as it moved clear of Saturn, out to apoapsis, and a little way back in again.

Significantly, not only is Phoebe's orbit retrograde, it is inclined. (By definition, an inclination exceeding 90 degrees is retrograde.) The inclination of Phoebe's orbit to the system's equatorial plane is 150 degrees. In effect, therefore, the divergence from the system's equatorial plane is 30 degrees. Even by this measure, Phoebe's orbit is the most inclined of all the satellites. However, Saturn's equatorial plane is inclined 26.75 degrees to its orbit, which is itself inclined some 2.5 degrees from the ecliptic, so Phoebe's orbit is actually within a few degrees of the ecliptic, and this strongly suggests that it is an interplanetary interloper that was captured when it strayed too close to the planet. This could also explain the retrograde motion.

Although Phoebe could be an asteroid, in light of Saturn's position in the Solar System it is more likely to be a cometary nucleus, and if this is the case it is a remarkably large one.

A.C.D. Crommelin said of Phoebe: ''There is no question that the discovery of Phoebe reflects the greatest credit on Professor Pickering. It was no mere accident, but the result of a deliberate search for additional satellites which he had been carrying on for many years. Even after the existence of the satellite is known, it is a tedious matter to identify it on a photograph, but to have discovered it in this way -one little grey dot among myriads of others - is indeed astonishing.'' In fact, the determination of Phoebe's orbit represented the transition from visual to photographic astronomy. Even when he knew precisely where to look, E.E. Barnard was only just able to see it visually in 1904 at 17th magnitude using the 40-inch refractor at Yerkes Observatory.

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