Single steady, bright emerald green light viewed briefly through 10" telescope and with naked eye.I was preparing to photograph the moon, using a 10" SCT telescope, looking through the eyepiece to ensure the moon was centered before attaching the camera. A round, bright, emerald green light flashed thru the eyepiece, left to right. I asked my husband "What was THAT?"; he looked up and saw a single green light slightly above us and to our southwest. I also observed the light without the scope. We immediately dismissed it as an aircraft as we are on the flight path for DC airports, as well as frequent overflights of private aircraft, military aircraft, helicopters, and high-altitude commercial aircraft. In retrospect, however, we realized virtually every passing aircraft has strobing lights, whereas this light was steady. Later, we deliberately observed aircraft on different nights and indeed all exhibited clearly strobing lights, normally red and/or white. At no time did we observe any aircraft exhibiting a single bright green light.We have also frequently observed satellites passing overhead, and this bore no resemblance to them, nor was its behavior or speed anything like meteors we have often observed. We have been intermittently involved in amateur astronomy for close to 30 years and admit, we are skeptical of most UFO reports (not that there is life in the universe, just knowing the distance to even the closest stars would require far more advanced civilizations to overcome the distance and time). Conditions at the time: partly cloudy, rural location with some horizon sky glow.