Deep-space distances are so enormous that familiar measures are useless. That is why astronomers use light-years, the distance that a beam of light (racing at 186,282.4 miles each second) travels from one of your birthdays to the next. The best way to link cosmic and human scales is to embrace the weirdness and think like a beam of light — and this month is the perfect time to start.
The December sky neatly frames the nearest bright star visible from northern locations (Sirius, 8.7 light years away, rising in the southeast after sunset) and the most distant (Deneb, roughly 2,500 light years away, setting in the northeast). The light from Sirius you see tonight matches the age of a third grader, whereas Deneb’s light set out when the Pharaohs were building pyramids in Egypt.
Capella, the yellow dazzler now nearly overhead, is the star of middle age: 43 light years away. The stars of the Big Dipper, low in the north, speak to the human lifespan. Many people on Earth were alive 78 years ago when the light from Dubhe (the upper front star in the dipper’s bowl) started its journey. None can match the age of the light from Mizar (the middle star of the handle), 124 light-years away.
For the ultimate trip, look almost straight up under dark skies for the faint, fuzzy oval of the Andromeda Galaxy. The most distant object readily visible to the naked eye, it lies 2.54 million light-years away. The bits of Andromeda’s light hitting your retina started their journey when Homo habilis, the earliest member of our genus, first scratched his brow in Africa’s Olduvai Gorge.