An observing window is defined as a period of time (allowing for observing conditions) that provides at least 1 and 1/2 hours of observing after astronomical twilight, which occurs one and one half hours after sunset, and which takes place when the Moon provides less than 25% illumination.

Observing sessions or activities should take place only when clear, mostly clear or a few clouds from time to time are forecast, based on both the National Weather Service and the Clear Sky Clock. Because of these variables, scheduled observing sessions or activities should be planned with a rain date in mind, which should be announced no later than 6:00 pm on the day prior to an event.

1. Visit the Charlottesville Astronomical Society Website
2. Go to the Sky Conditions drop-down menu and choose the Clear Sky Clock for the Observatory on Fan Mountain – a tool used by amateur astronomers for a two-day look-ahead at upcoming observing conditions. It can be a bit on the conservative side but can be useful for re-scheduling an event or activity, if necessary. Then use the Weather Forecast tool in conjunction with the Clear Sky Clock.

Nautical Twilight (an hour after sunset): Observing the planets and brighter objects, plus satellites

Astronomical Twilight (an hour and a half after sunset): Nelson’s dark skies make their appearance, with the caveats mentioned above.