On December 21, we observe a mesmerizing astronomical phenomenon: Jupiter and Saturn will be so close to each other that they will look like a double planet. On this day, about an hour after sunset, the alignment between the planets will be similar to that seen in March 1226. The distance between the planets will be about one-fifth the diameter of the Moon. This is so rare event that we could observe it on March 15, 2080, and then by 2400.