From the record
Real cases, written from the published record. Predict first; the record and its sources come after.
2026, any clear night · London, 51.5° N
The circle that never sets
Stars close enough to the celestial pole never dip below the horizon; they circle the pole all night, every night. How many stars do that depends on where you stand.
The rule: a star never sets if its declination is more than 90° minus your latitude. The Big Dipper's seven stars lie between about +49° and +62°.
Seen from London, is the Big Dipper ever below the horizon?
What happened. At London, 90° − 51.5° = 38.5°, and every star of the Big Dipper is north of that, so it never sets. It swings low in the north on autumn evenings and high overhead in spring. From Houston (about 30° N) the line is at 60°, so most of the Dipper does set for part of the year.
The circle that never sets grows as you walk north, and shrinks to nothing at the equator.
1923 · Jena
The wonder of Jena
In August 1923 the Zeiss works in Jena switched on a machine on its own roof: a projector that threw several thousand stars onto the inside of a white dome. The museum founder Oskar von Miller had asked for the sky indoors. Munich's Deutsches Museum opened its planetarium in 1925; Chicago's Adler followed in 1930.
Long before, at Gottorf in the 1650s, a dozen people could sit inside a turning globe painted with stars.
Why would people pay to see a made-up sky in 1925?
What happened. Crowds came in their thousands, and newspapers called it the wonder of Jena. Each of the three reasons is a fair guess: electric street light was already washing out city skies, the projector was a marvel, and a show could run at noon on a cloudy day.
What we don’t know. We know the attendance better than the reasons; why each visitor came is mostly guesswork.
Pause to think. What does a planetarium give you that the real sky can't, and the other way round?