Even though I am a private pilot, I spend every hour of jet flight viewing the earth below. COvered window allowed spotting overtaking triangular craft.This occurred, most likely, a few days after Halloween of 1996. Verification easy to obtain. This was an identifiable flight, as the pilot's seat was replaced at maybe 1 a.m. in a delayed stopover` at Reno from San Francisco where I departed. Wile I am an old private pilot, I still can't resist covering my head so I can stare out the window and watch the earth pass below. The city we were passing appeared to be very large and moving off to our left, northern, side at an angle of maybe 30 degrees. The subject craft appeared in silhouette below and just far enough out from us for me to spot it from a passenger window. It was an EQUILATERAL triangle with NO protrusions of nose, canard, tailfin, fuselage or jet tube. There were no lights of any kind and it appeared totally black. It overtook us rapidly. My guess is that it was 500 feet below us and the equilateral edges were maybe 60 feet or more per edge. I reasoned that we were doing 580 or so and it was moving past at 100 kts faster. The time it was in my view, as the city lights did not extend but maybe half way, was maybe three or four seconds.((NUFORC Note: Witness indicates that date of the incident is approximate. PD))