![]() ![]() SearchRiseSet corrects for atmospheric refraction, but I believe most legal definitions of civil twilight ignore refraction and calculate the altitude angle as if there is no atmospheric distortion.I will follow your definition regarding the center of the body rather than the topmost part of the body. Sunrise, sunset, moonrise, and moonset are all defined in terms of when the topmost part of the object appears into (or disappears from) view, not when the center of the body crosses the horizon.Over the next few days I will add a new function for calculating these values. I think this functionality should be available in Astronomy Engine. This is very similar to what the existing function SearchRiseSet does. You would have to iteratively home in on the time when the Sun's altitude is -6. Currently, there is no simple way to calculate civil dawn/dusk using the existing Astronomy Engine functions. So I'm thinking about how to solve your problem. They vary quite a bit depending on the observer's latitude and the season (winter days are shorter, summer days are longer). In general, sunrise does not happen 6 sidereal hours before local noon, nor does sunset happen 6 hours afterward. So you can see that the hour angle doesn't tell you anything about when (or if) an object rises/sets. However, Polaris' altitude angle (how far it is above a given observer's horizon) is almost constant, and is approximately equal to the observer's latitude above the equator. About 12 hours later, Polaris will be directly below the north celestial pole by the same tiny angle. Sometimes Polaris is directly above the observer's north celestial pole by a tiny angle, meaning Polaris culminates with a 0 hour angle. However, its hour angle goes through the full range 0.24 hours every sidereal day. It just moves around in a tiny circle every 24 hours. For such an observer, Polaris never rises or sets. ![]() It is very close to the north celestial pole, so it is always in the sky for most observers in the northern hemisphere. However, the hour angle is not the same thing as altitude angle.Īs an extreme example, imagine you are looking at the "north star" Polaris. For dawn and dusk, what you are looking for is when the center of the Sun has an altitude angle of -6 degrees for the given observer. Hi Your code for finding noon is exactly right, because that is when the Sun has hour angle = 0.
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