Explain four different sunrises and four different sunset times, please?
In heaven-above.com, after choosing your observing location in the upper left and clicking on SUN under the Astronomy section; you’ll see the sunrises first, followed by the four sunsets. All three twilights means sunlight is destroying some of the visibility of the stars.
Sunrises: Remember, -0.8, 6, 12, 18 degrees as those are the key sun positions off the horizon.
Astronomical Twilight: (18-12 degrees) When the sun is 18 degrees or more below the horizon, it is still as dark as it can get at your location but at 18 degrees the sun starts lighting up your location some as Astronomical Twilight starts so only the brightest 100 or so objects are visible instead of thousands.
Nautical Twilight: (12-6 degrees) When the sun rises to 12 degrees below the horizon, Nautical Twilight begins meaning you can still navigate by the North Star (Polaris, the 50th brightest star) but it may disappear near the end of Nautical Twilight.
Civil Twilight: (6 to -0.8 degrees) When the sun rises to 6 degrees below the horizon only a few stars might still be visible, planets and the moon. (You know this as dawn). I noticed that ten minutes into civil twilight was light enough to start my walk into our hill country with my neighbor as the start of civil twilight was too dark yet.
Finally, you actually see the sun starting to appear and yet the top of the sun is still below the horizon 0.8 degrees but our atmosphere refracts (bends) the light to our eyes.
Sunsets: The four sunsets are in reverse. Since we usually have observation sessions in the evenings, these are the only four that I record.
Sunset: I try to be at the observation session at this time, deciding where to place the table and the telescopes. I note anything that can block our view of the sky (like trees or buildings or lights) and I note if I can set up our of the wind.
Civil Twilight: (-0.8 to 6 degrees) Goes from sunset to until the sun is 6 degrees below the horizon. During civil sunset the brightest objects become visible (planets, the summer triangle or the winter triangle stars) Navigators would want to see these before too many stars become visible as lots of starlight actually makes it harder to locate the brightest stars for navigating.)
Nautical Twilight: (6 to 12 degrees) means the navigational North Star will become visible for navigating and so around 50 stars are visible, instead of 3.
Astronomical Twilight: (12 to 18 degrees) I’m most interested when this is over as it’s as dark as it’s going to get so it will not get any better for viewing.
Sunrises: Remember, -0.8, 6, 12, 18 degrees as those are the key sun positions off the horizon.
Astronomical Twilight: (18-12 degrees) When the sun is 18 degrees or more below the horizon, it is still as dark as it can get at your location but at 18 degrees the sun starts lighting up your location some as Astronomical Twilight starts so only the brightest 100 or so objects are visible instead of thousands.
Nautical Twilight: (12-6 degrees) When the sun rises to 12 degrees below the horizon, Nautical Twilight begins meaning you can still navigate by the North Star (Polaris, the 50th brightest star) but it may disappear near the end of Nautical Twilight.
Civil Twilight: (6 to -0.8 degrees) When the sun rises to 6 degrees below the horizon only a few stars might still be visible, planets and the moon. (You know this as dawn). I noticed that ten minutes into civil twilight was light enough to start my walk into our hill country with my neighbor as the start of civil twilight was too dark yet.
Finally, you actually see the sun starting to appear and yet the top of the sun is still below the horizon 0.8 degrees but our atmosphere refracts (bends) the light to our eyes.
Sunsets: The four sunsets are in reverse. Since we usually have observation sessions in the evenings, these are the only four that I record.
Sunset: I try to be at the observation session at this time, deciding where to place the table and the telescopes. I note anything that can block our view of the sky (like trees or buildings or lights) and I note if I can set up our of the wind.
Civil Twilight: (-0.8 to 6 degrees) Goes from sunset to until the sun is 6 degrees below the horizon. During civil sunset the brightest objects become visible (planets, the summer triangle or the winter triangle stars) Navigators would want to see these before too many stars become visible as lots of starlight actually makes it harder to locate the brightest stars for navigating.)
Nautical Twilight: (6 to 12 degrees) means the navigational North Star will become visible for navigating and so around 50 stars are visible, instead of 3.
Astronomical Twilight: (12 to 18 degrees) I’m most interested when this is over as it’s as dark as it’s going to get so it will not get any better for viewing.