Dawn app for iPhone and iPad

Dawn is the time that marks the beginning of the twilight before sunrise. It is recognized by the presence of weak sunlight, while the sun itself is still below the horizon. Dawn should not be confused with sunrise, which is the moment when the leading edge of the sun itself appears above the horizon.
The duration of the twilight period between dawn and sunrise varies greatly depending on the observers latitude, from a few minutes in equatorial regions to many hours in polar regions. Dawn may easily be determined by observing a thread. When the color of a thread can be determined, changing from black to the threads distinctive color, the crack of dawn has occurred. This same measure of ambient light can be used to determine, conversely, the instant of dusk.