The plaque reads:
Here in Volcano in 1860, miner and amateur astronomer George Madeira built California's first astronomical observatory of record and operated it for two years. Using a three inch refractor telescope he studied the sun and moon, stars and planets, and other glories of the heavens, including the great comet of 1861.
The marker was moved in 2004. Orginally, it was located on Shake Ridge Road at
38° 27' 57.31", -120° 38' 25.43". The plaque on this marker read:
On the knoll behind this marker, George Madeira built the first amateur astronomical observatory of record in California and there discovered the great comet of 1861 with a three-ich refractor telescope.
Although the marker is still there, the plaque has been removed from this site sometime in the last two years.
It appears that there is some debate about where the Observatory was actually located.