When astronomers say some object is so many million light years away, how do they know that? Astronomer Steve Ruhl will climb the cosmic distance ladder and show how astronomers, past and present, determine distances to objects at the planetarium show “Climbing the Cosmic Distance Ladder” at 7 p.m. Saturday, Dec. 14 at Ritchie Observatory.

If the sky is clear, astronomers will be on hand with telescopes for public star-gazing.

The show is free to members of the Battle Point Astronomical Association; $2 donation suggested for nonmembers, $5 for families.

The Battle Point Astronomical Association is a nonprofit amateur astronomy organization that operates the Edwin E. Ritchie Observatory and John H. Rudolph Planetarium in the Helix House at Battle Point Park.

For more information, call 206-842-9152 or visit www.bpastro.org.