Astronomical Society of South Australia
www.assa.org.au
The Society
Astronomy
Resources
A Tale of Discovery
Justin is shown here with the 8-inch f/6 Newtonian telescope he used to discover his first comet. The observatory was retrieved from a rubbish dump in exchange for a carton of beer.
Justin was observing the dwarf nova TV Corvi on the evening of July 22nd (local time), when he noticed a nearby 10th magnitude blob about 60-70 arcseconds in diameter. Not having seen it before, he took note of its position (1997 Jul 22 10:20 UT 12h18m23s -18d59' (2000)) using the VSS RASNZ chart for TV Crv. A second look at 11:54 UT showed the blob had moved (12h18m25s -18d55'), so Justin began phoning other observers for verification.
As luck would have it, everyone locally was clouded out; but we were able to confirm that no known galaxies, nebulae or comets were in the vicinity. An e-mail was then sent to the Central Bureau for Astronomical Telegrams.
IAU Circular 6705 announced the discovery on July 23. A preliminary orbit announced in IAUC 6707 showed the comet was close to perihelion at about 1.4AU from the Sun and a similar distance from the Earth. At the time of discovery the orbital plane was almost at 90d to our line of sight.
If the comet had been 3 months earlier, it would have been visible in 7x50 binoculars from the Earth.
Extrapolating the orbit backwards a few months helps explain why this comet wasn't found earlier. Although it has been over 60 degrees away from the Sun since March, it has spent most of the time cruising through Carina, Vela, and Centaurus - constellations with an abundance of stars, clusters, and nebulae.
Comet Tilbrook finally escaped the Milky Way in late June, but because it happened to be winter in the Southern Hemisphere the weather had been poor. In fact, the night of discovery was the first semi-clear night Justin had for two weeks.
This composite CCD image (3 x 120 second exposures) by Gordon Garradd was taken shortly after perihelion in late July 1997.
You can download a photograph of C/1997 O1 taken by Justin, using a 35mm camera with a 200mm zoom lens and 400ASA Fujicolor print film; on the evening of 1997 July 26 (10:45 UT). The camera was hand-guided for an exposure lasting 10 minutes, to capture this faint image of Comet Tilbrook. As can be seen from the included star chart, no known comets, galaxies, nebulae or other fuzzy-looking objects were nearby.
C/1997 O1 was Justin's first comet discovery. The majority of comets are found during deliberate searches by dedicated comet hunters - an activity Justin now pursues successfully. The first Comet Tilbrook (C/1997 O1) joins Comet Williams (C/1998 P1), Comet Kaho-Kozik-Lis (1936III) and Comet Jones (1946VI) as one of the rare and accidental discoveries of comets - by variable star observers - in a variable star's vicinity.