To the untu­tored eye the sky seems two dimen­sion­al — a dome of glit­ter­ing dots just up there, like the light-flecked ceil­ing of the ter­res­tri­al ball­room. Only now and then, and only to the pre­pared imag­i­na­tion, does night’s third dimen­sion reveal itself, as dur­ing an eclipse of the sun or moon.

Some­times it’s fun to think about things that no one has thought about before.

This semes­ter I am teach­ing a course in intro­duc­to­ry astron­o­my. So far, of six nights sched­uled for obser­va­tion, five have been over­cast. On the sixth we caught glimpses of Ori­on, the Pleiades, Jupiter, and Mars through breaks in the clouds. That’s about par for the course.

For the past cou­ple of months I have had a New York­er cov­er tacked on the wall above my desk. The draw­ing on the cov­er, by Eugène Mihaesco, is sim­ple. A pen lays on a white table, its nib dark with ink. An ink bot­tle stands open. The ink in the bot­tle is a map of con­stel­la­tions of the north­ern sky — Ursa Major, Ursa Minor, and Dra­co — includ­ing the stars Dub­he, Mer­ak, and Mizar.