On two nights each month, the University of Maryland opens the doors to its Astronomy Observatory to the general public.

A professor from the Department of Astronomy, a student who has finished a research project or another guest speaker gives a half-hour presentation, and then visitors look at various celestial objects through its telescopes.

The open house at the observatory has been running continuously since 1969, with more than 50,000 visitors overall. During special occasions — such as Halley’s comet in 1986 or the opposition of Mars in 2003 — the lines can stretch down the street.

There are four mounted telescopes, including a 7-inch Astro-Physics Refractor, a discontinued vintage model that’s something of a crowd-pleaser, and the 20-inch Eichner Bent Cassegrain Reflector installed in 1964.

The open houses are held on the 5th and the 20th of each month, so they fall on different nights of the week, a decision made at the start to allow people to come who work at different times.

If you’re interested in going, the doors open at 7:45 p.m. in the winter. If you’re coming in a group of 10 or more, you need to make a reservation for a session that starts an hour earlier. The program is free, although they accept donations.

A few tips: There’s not much parking, so you might want to carpool. The telescopes are outside, so dress appropriately. And the observatory, which is just down Metzerott Road from the Wallace Presbyterian Church, has no street address, so there’s an entire page dedicated to how to find it.