Embedded Files
This movie shows the three levels of Twilight: civil, nautical and astronomical. Here's a file with the technical definitions of the three levels of twilight.The length of twilight is the same for Dawn and Dusk, and is essentially independent of the day of the year. Twilight is a function of latitude, increasing with latitude. This 180 degree movie shows how it never really gets dark on a Summer day at 55N latitude. The movie starts at sunset and ends at sunrise, with the horizon semi-transparent, so you can see the celestial path of the Sun.Keep in mind that twilight refers to both morning and evening. The twilight portion of the morning is called Dawn, and the dark part of evening twilight is called Dusk.
Since length of Twilight is dependent only on latitude, at what latitude is twilight the smallest amount of time? Given that the Sun moves on its celestial path at about 15 degrees per hour, how long does it take for Dusk to finish at this latitude?
If you're wondering why twilight increases with latitude, look at the picture on the left. For Dusk to finish, the center of the Sun must have descended vertically a full 18 degrees below the westerly horizon. Since the Sunset Angle is the observer's latitude, the Sun must moves through a greater celestial path to be positioned 18 degrees vertically below the westerly horizon.
Page updated
Google Sites
Report abuse