Duration of his civil dusk in march 21st?

[atomqwerty]

Homework Statement

Evening civil twilight (civil dusk) begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon. Calculate for an observer at latitude = 45º the duration of his civil dusk in march 21st (when \delta_{sun}=0º

Homework Equations

The equations of the transformation between horizontal coordinates and ecuatorial coordinates

"http://en.wikipedia.org/wiki/Celestial_coordinate_system#Equatorial_to_horizontal_coordinates"

The Attempt at a Solution

Let us do a = 0 (sun over the horizon). This implies cos h = -tan d tan f (h: hour angle, d: declination, f: latitude). If d = 0 (delination of the sun is zero on march 21st), then we have h = 90º. Thus A = 90º too (azimut). Is this correct?
doing the same for a =-6º I obtain that the difference between hour angles for the two positions (sun in a=0 and sun in a=-6) is 8.51º. How can I transform this into minutes?

Thanks

 

[gneill]

360deg/24hr = 15deg/hr

24hr/360deg = 4min/deg = 240sec/deg

 

[atomqwerty]

360deg/24hr = 15deg/hr

24hr/360deg = 4min/deg = 240sec/deg

I think that relation is useless here... am I wrong?

 

[gneill]

I think that relation is useless here... am I wrong?

Not completely useless, just inconvenient.

On March 21st (equinox) the Sun follows a trajectory along the celestial equator, since the Sun is over the equator and the Earth's rotational inclination lies in a plane that is perpendicular to the Earth-Sun line.

So the Sun follows the projected arc of the equator on the sky. After sunset, when the Sun is so many vertical degrees below the local horizon, it actually followed the longer, slanting path of the equatorial arc, which it traversed at the rate of 15 degrees per hour.

If you have the angle between the equatorial plane and the local vertical, and you have the angular distance representing the twilight duration, then you can work out the angular length of the path of the Sun on the equatorial arc (approximated by a right angle triangle).

Unless, of course, I've mucked up in my thinking somewhere along the line...