The limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished;
at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions
Each twilight phase is defined by the solar elevation angle, which is the position of the Sun in relation to the horizon. During civil twilight, the geometric center of the Sun's disk is at most 6 degrees below the horizon. In the morning, this twilight phase ends at sunrise; in the evening it begins at sunset. Sunrise and sunset are the moments when the Sun's upper edge touches the horizon.
Lawmakers have enshrined the concept of civil twilight. Such statutes typically use a fixed period after sunset or before sunrise (most commonly 20-30 minutes), rather than how many degrees the sun is below the horizon. (since this would be very hard to "prove"). Adding a "range" makes it easier to argue from both sides since not only does there exist an acceptable range, but also acknowledges that there is some flexibility and deviation in the very definition of that range.
It seems to me that if it takes the earth 24 hours, or 1440 minutes to rotate 360 degrees, then it's reasonable to assume it takes about 4 minutes to rotate 1 degree (1440/360=4), or, by that math, 24 minutes to rotate 6 degrees (6x4=24), so a general "legal" reference of 20 - 30 minutes (as made above) seems also correct, in a less-definitive sort of way, and while 24 minutes is perhaps more precise, it would rarely, if ever, be exact.
Daylight-only operations, or civil twilight (30 minutes before official sunrise to 30 minutes after official sunset, local time) with appropriate anti-collision lighting.
- Crop Science
- Drone Guns
- Ground Control Points
- Public Concern
- State PArks
- Terrain Mapping
- Volume Measurement
- Wine Country
- Mar 2018