Sep 12, 2018
On Saturday morning at about 6 am in the Eastern time zone (10:00 GMT), the comet will glide directly through the open star cluster designated Messier 35 — a great sight in a telescope, and a photo opportunity!
Finally, between midnight and dawn sky on Sunday, September 16, the comet will approach the bright star Propus (Eta Geminorum) in Gemini (the Twins), passing within a full moon’s diameter to the upper right of the star just before dawn. The dim Jelly Fish Nebula (IC443) will be within a low magnification telescope’s field of view of the comet, too. Closest approach to Propus will occur at around 15:00 GMT, an event only visible for skywatchers in Asia and Oceania.
Comet 21P/Giacobini-Zinner returns to our vicinity every 6.5 years — hence the “P” for “periodic” in its designation. It has been gradually brightening because it will be closest to Earth and the sun tomorrow (Monday), then it will slowly begin to fade. This week will be an excellent time to see it because the moon will be out of the late evening sky. You should be able to see the faint fuzzy greenish object in binoculars or a small telescope, if you can escape city lights. A faint tail might be noted extending to the upper right of the comet (although your telescope will flip that direction around).
The comet is located in the northeastern sky on a track that carries it lower every night — so we’ll have to stay up after midnight, or get up before dawn, to see it. Tonight (Sunday), the comet will be positioned just below the imaginary line that joins the modest star Mahasim and the bright star Elnath — the lowest two stars in Auriga’s (the Charioteer) rough circlet. The comet will be left of their mid-point.