On the night of Sept. 10 UT, two amateur astronomers from Japan found what was thought to be a new comet. Koichi Itagaki and Hiroshi Kaneda used an 8″ telescope equipped with a CCD camera (CCDs are digital cameras optimized for astronomical observation) to find the 12th magnitude comet near the border of the constellations of Aquarius and Aqulia. Mr. Itagaki is no stranger to discovery having previously found Comet Tago-Honda-Yamamoto in 1968 (though credited with a discovery, the comet does not bear his name because comet names are limited to the first 3 discoverers) and with numerous supernovae to his credit.
Michael Meyer of Limburg, Germany suggested that the comet was not new but rather the return of a long-lost comet not seen since 1897. On 1896 Sept. 4, Michel Giacobini of Nice, France found Comet P/1896 R2 (Giacobini) the old fashioned way by looking through the eyepiece of a telescope. The comet was observed for 4 months before fading from view. Until last night the comet had been lost for 111 years.
You may be asking why 111 years is such a big deal since there are many long-period comets with orbital periods of hundreds to millions of years. For example, Comet Halley returns once every 76 years. Comet Giacobini, on the other hand, is a short-period comet and orbits the Sun every 6-7 years. In fact, the comet has orbited the Sun 17 times since 1896.
Why has this comet stayed hidden for so long? With a brightness of 11th magnitude in 1896 and 12th magnitude in 2008, Comet Giacobini is rather bright. No, it is not bright enough to see without a telescope and it is still too faint to see in the eyepiece of all but the largest backyard telescopes, but in the modern age of telescopes equipped with digital CCD cameras, this comet is an easy target.
Most likely the comet was brighter than usual in 1896 and the same may be true now. Comet outbursts happen from time to time and can be quite spectacular as we saw last year with the brightening of Comet Holmes to easy naked eye visibility. A comet that may be similar to Comet Giacobini is Comet Metcalf-Brewington which was originally seen in 1906 but lost until an outburst in 1991.
The new official name of Comet Giacobini is P/2008 R6 (Giacobini). The “P” means it has an orbital period shorter than 50 years. “2008” is the year of discovery, or re-discovery in this case. The ‘R’ means it was found during the first half of September and the ‘6’ means it is the 6th comet either discovered or recovered in the 1st half of September. Recovery, and technically the observations of Giacobini are a recovery, is when a comet is observed at a second close approach to the Sun. When comets are observed at two returns they are numbered. This comet will probably be numbered as 205P/Giacobini meaning it is the 205th comet observed at 2 or more returns.
Comet Giacobini currently orbits the Sun once every 6.7 years. It comes as close as 1.53 AU to the Sun (similar to the distance of Mars from the Sun) and travels as far as 5.55 AU from the Sun (slightly beyond the orbit of Jupiter). It is most likely at its brightest and will fade over the next few months.
Image of Comet Giacobini by Gustavo Muler can be found here.
Just Added: Image by Juan A. Henriquez can be found here.
– Carl Hergenrother
The Transient Sky - Comets, Asteroids, Meteors 
