Michael Mattiazzo, Andrew Pearce, and Colin Drescher -- all in Australia -- have now sighted a comet that was first suspected on low-resolution images taken with the SOHO spacecraft's SWAN ultraviolet camera.

Mattiazzo's CCD astrometry on May 16th, 21st, and 25th allowed the IAU Central Bureau for Astronomical Telegrams, http://cfa-www.harvard.edu/iau/cbat.html , to calculate a preliminary orbit. According to IAU Circulars 8346 and 8347, Comet SWAN (C/2004 H6) passed perihelion on May 12th at 0.776 astronomical unit from the Sun (between the orbits of Earth and Venus). It is heading on a high-inclination, parabolic trajectory through the inner solar system.

As Comet SWAN draws steadily away from the Sun, it will probably stay visible in telescopes as an 8th-magnitude glow for at least two months. During June, Southern Hemisphere observers can follow its trek in the predawn sky from Eridanus into Cetus. By July, Comet SWAN should be visible from anywhere in the world.

The following ephemeris, based on Daniel W. E. Green's preliminary orbit, gives the comet's right ascension and declination at 0 hours Universal Time on each date, followed by its distance from Earth (Delta) and the Sun (r) in astronomical units (one a.u. is about 149,600,000 kilometers). Included are the comet's elongation angle in degrees from the Sun, its predicted visual magnitude, and the constellation it is in. The last column, "optimum latitude," tells which geographical locations on Earth can view the comet highest in a dark sky.

Keep in mind that future observations will allow the comet's predicted path and magnitude to be improved. So it could drift slightly off the ephemeris below, especially after June.

Roger W. Sinnott
Senior Editor
SKY & TELESCOPE

This is turning out to be a good year for comet observers.