Deep-space distances are so enormous that familiar measures are useless. That’s why astronomers reckon by light-years — the distance that light, racing at 186,282.4 miles per second, travels from one of your birthdays to the next.
The best way to link cosmic and human scales is to embrace that weirdness and think like a beam of light. The nearest bright star visible from northern locations (Sirius, blazing in the southwest after sunset this month) lies just 8.7 light-years away. The light from Sirius you see tonight matches the age of a third-grader. Capella, the yellow dazzler in the northwest, is the star of middle age, 43 light-years distant. Many people on Earth have outlived the 78-year-old light from Mizar (the middle star of the Big Dipper’s handle). None can match the age of the light from Dubhe (the upper front star in the Dipper’s bowl), 124 light-years away.
Then onward into history. Rigel, at the foot of Orion, is 860 light-years away; its light began heading Earthward a hundred years before Marco Polo was born. The Double Cluster, a clumping of stars between Cassiopeia and Perseus (lovely through binoculars), is 7,500 light-years away, from the dawn of the age of agriculture in ancient Egypt. For the ultimate trip, look low in the northwest right after dusk’s end for the Andromeda galaxy. This dim, fuzzy oval — one of the most distant objects visible to the naked eye — lies 2.54 million light-years away. Its light predates our entire species.