CHICAGO | Officials at the Adler Planetarium in Chicago are breaking out the birthday cake to mark the 100th anniversary of the facility's historic Atwood Sphere.
The 15-foot-diameter large-scale mechanical planetarium opened in 1913, and museum officials say it's the oldest one in the U.S.
It shows the positions of the sky's brightest stars by allowing light to come through 692 holes on its metal surface. The sphere was first used to help teach pilots how to navigate the nighttime sky.
To mark the sphere's anniversary, Adler officials waived admission on Wednesday for Illinois residents, gave away birthday cake and held a series of other events.