... Retrograde Mars . ... Most of the time, the apparent motion of Mars in Earth's sky is in one direction, slow but steady in front of the far distant stars. About every two years, however, the Earth passes Mars as they orbit around the Sun. Open Star Clusters M35 and NGC 2158 . ... Open clusters of stars can be near or far, young or old, and diffuse or compact. Open clusters may contain from 100 to 10,000 stars, all of which formed at nearly the same time. ... The Eskimo Nebula from Hubble . ...