The conditions for observing vary, depending on the time of year and your location (latitude) on Earth. In summer, twilight may last throughout the whole night, making observation of the faintest stars impossible. In the Arctic and the Antarctic Circles, the sun does not set for 24 hours at least once during summer, which limits the amount of viewing that can take place.
There are 3 stages of twilight:
1. Civil twilight - where the sun is <6° below the horizon
2. Nautical twilight - where the sun is between 6 & 12° below the horizon. Here, only the very brightest stars are visible to the naked eye, which used to be used for navigation.
3. Astronomical twilight - where the sun is between 12 & 18° below the horizon, where the faintest stars can be seen overhead, but not near the horizon.
Full darkness only occurs when the sun becomes more than 18° below the horizon. This is the best for stargazing and visual astronomy/astrophotography.
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