Tilbrook Strikes Again!

by Fraser Farrell

This time, Justin Tilbrook was actually comet hunting when he found a 10th magnitude arcminute-sized blob heading southwards; on the evening of 1999 January 12 (Jan 12.486UT). He found it at 23h38m -28.1d (2000), in a barren piece of sky on the Aquarius-Sculptor border.

Justin - armed with ephemerides of all visible comets - was sure it wasn't a known comet; and he could not recall seeing any uncharted galaxies (or similar fuzzies) during previous searches of this area. Two Australian astrometrists (Gordon Garradd and Frank Zoltowski) were then contacted to provide confirmation and precise positions. Then the observations were e-mailed to the Central Bureau for Astronomical Telegrams for checking.

At this stage, it was still possible that this object was a very faint known comet (not in Justin's ephemerides) that had brightened dramatically - or that it had just been discovered and reported by someone else. Fortunately, Justin's anxious wait was ended on January 13 by IAU Circular 7084, announcing the discovery of C/1999 A1 (Comet Tilbrook).

The revised parabolic orbit from MPC 33651 is:

T = 1999 Jan 29.5750 (the time of perihelion)
q = 0.731240 AU (the perihelion distance, 109.4 million km)
peri = 232.5353 degrees
node = 259.0986 degrees
i = 89.5607 degrees

The comet is heading south in an orbit at right angles to the Ecliptic and reaches perihelion almost directly "below" the Sun, so the comet will be an exclusively Southern Hemisphere object. It is currently 10th magnitude, and about 1-1.5 arcminutes diameter (in a large telescope). Because it is receding from the Earth it is expected to fade (and shrink) rapidly.

Below is a composite of 3 unfiltered CCD images taken at Stockport Observatory, shortly before midnight (local time) on January 14. The observers were Jim Costello, Trish Ellin and Rob Purvinskis.

Although each exposure was only 15 seconds, they span about 5 minutes of time in total - so the comet image is slightly trailed. No tail was seen.