Open clusters NGC 2158 (left) and M 35

Open clusters NGC 2158 (left) and M 35. M 35 is located at a distance of 2,800 light-years in the constellation Gemini. Its age is estimated to be 110–150 million years. M 35 is approaching us at a speed of 5 km/s. The best time for observation is winter. M 35 lies at the far western end of the constellation Gemini. Under good dark skies, the cluster can be glimpsed with some difficulty with the naked eye—it is situated at the northern vertex of a nearly isosceles triangle with the side formed by η Gem and 1 Gem. Just 15 arcminutes to the southwest lies the faint, compact cluster NGC 2158, which inexperienced observers sometimes mistake for a newly discovered comet. This open cluster is located at a distance of ~12,000 light-years—more than four times farther from us than M 35. It is an old open star cluster (age about 2 billion years), with low metal content, and therefore belongs to the old galactic population of the thin disk.