In dealing with distance, there is one basic formula that is most essential, namely 6.1 \( \boxed{\boldsymbol{D}=\boldsymbol{v}\bullet \boldsymbol{t}} \) that was discussed in Chap. 1 regarding light-years. Let us do something with it more interesting regarding the pair of interacting galaxies Arp 147 (Fig. 6.1). These represent the aftermath of an interaction of two galaxies with the one on the left having passed through the one on the right leaving the circular ring of intense star formation remnant from a former spiral galaxy and a tidal ring remnant around the intruder elliptical galaxy.