A node indicates that astronomical sunrise twilight time is detected/calculated. 

Astronomical twilight is the period when the center of the Sun is between 12 and 18 degrees below the horizon. It starts at astronomical dawn, early in the morning when the Sun is higher than 18 degrees below the horizon. From this point, it will be difficult to observe certain faint stars, galaxies, and other objects because the Sun starts to illuminate the sky. Astronomical twilight ends at astronomical dusk in the late evening, when those faint objects again can be visible because the Sun is lower than 18 degrees below the horizon. In locations north of 48°24' N or south of 48°24', it never gets darker than this near the middle of the summer solstice (June or December).

Technically, the start and end times are when the true geocentric position of the Sun is 108 degrees from the zenith position, or directly above the observer.

Astronomical twilight is the period when the center of the Sun is between 12 and 18 degrees below the horizon.

 | Data byte | Description                                                        | 
 | :---------: | -----------                                                        | 
 | 0         | User specified.                                                    | 
 | 1         | Zone for which event applies to (0-255). 255 is all zones.         | 
 | 2         | Sub-zone for which event applies to (0-255). 255 is all sub-zones. | 
