Planetariums are wonderful places where many of us first discovered a sense of wonder for the cosmos. On an enormous domed screen stars and comets and nebulae and quasars are projected. Now the job of projecting these celestial objects is taken by digital cameras. But once upon a time, the projectors were analogue. At the Morrison Planetarium in San Francisco, for instance, you’d sit down in comfy chair, angled up toward the dome. The lights would dim, and then from beneath the floor the whine of a machine would grind to life. Doors would open, and a strange object would rise from the floor. Two spheres connected by a cylinder, every inch covered in lenses that look like mechanical eye-stalks. Nothing in the world looks like a planetarium projector. Here’s more from B&H:
The traditional OM planetarium projector, nicknamed “The Ant” due to its outward resemblance to the insect, consists of two large globes with a central light source in each dome. External to the main spheres are sun, moon, and planet “cages” that housed independent projectors for “the wanderers.” The Zeiss Model II, introduced in 1926, is the first dual-sphere ant-style projector, projecting almost 9,000 stars. For this model, tiny holes were drilled into the spheres in the precise positions of the stars above. Larger stars or groups of stars would get projection lenses to increase their brightness. Single-sphere projectors showed either the Northern or Southern Hemisphere, whereas dual-sphere ant-type projectors could show the sky in both hemispheres. To keep the spheres from projecting into the eyes of spectators, shutters mask the sky below the dome’s “horizon.”
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