The fire in Griffith Park burned 25 acres northwest of the observatory Tuesday and was 90 percent contained by Wednesday morning. About 2,000 people were evacuated from the observatory, and some had to be shuttled aboard buses to pick up their vehicles, Los Angeles Fire Department Capt. Erik Scott said.
No injuries were reported.
About 600 to 700 visitors and staff were in the building at the time of the evacuation, observatory museum guide Juan Gutierrez told Reuters by telephone.
The observatory's grounds, roads, parking lots and hiking trails and the areas around the Greek Theatre and Western Canyon will also be closed, according to a statement on the observatory's website.
Humphrey said the cause of the fire was under investigation.
It's not clear whether the observatory will reopen Thursday.
The 4,300-acre park and observatory are both named for Griffith J. Griffith, an investor and developer who donated much of the land to the city of Los Angeles in the late 1890s before he was tried, convicted and sent to prison for shooting his wife in the eye.