Calculating dawn and sunset times using PyEphem

Question

Is it possible to calculate Dawn, Dusk, and sunset times using PyEphem? I've used PyEphem to produce day and night time, but I didn't find anything on sunset/dusk/dawn

Answer 1

The following script will calculate sunrise, sunset, and twilight times using PyEphem. The comments should be adequate to explain what each part is doing.

import ephem

#Make an observer
fred      = ephem.Observer()

#PyEphem takes and returns only UTC times. 15:00 is noon in Fredericton
fred.date = "2013-09-04 15:00:00"

#Location of Fredericton, Canada
fred.lon  = str(-66.666667) #Note that lon should be in string format
fred.lat  = str(45.95)      #Note that lat should be in string format

#Elevation of Fredericton, Canada, in metres
fred.elev = 20

#To get U.S. Naval Astronomical Almanac values, use these settings
fred.pressure= 0
fred.horizon = '-0:34'

sunrise=fred.previous_rising(ephem.Sun()) #Sunrise
noon   =fred.next_transit   (ephem.Sun(), start=sunrise) #Solar noon
sunset =fred.next_setting   (ephem.Sun()) #Sunset

#We relocate the horizon to get twilight times
fred.horizon = '-6' #-6=civil twilight, -12=nautical, -18=astronomical
beg_twilight=fred.previous_rising(ephem.Sun(), use_center=True) #Begin civil twilight
end_twilight=fred.next_setting   (ephem.Sun(), use_center=True) #End civil twilight

Relocating the horizon accounts for light refracting around the curvature of the Earth. PyEphem has the capability to calculate this more exactly given a temperature and pressure, but the U.S. Naval Astronomical Almanac prefers to ignore the atmosphere and simply relocate the horizon. I refer to the USNAA here because it's an authoritative source against which these sorts of calculations can be checked. You can also check answers on NOAA's website.

Note that PyEphem takes and returns values in UTC time. This means that you have to convert your local time to UTC and then convert UTC back to local time to find the answers you're probably looking for. On the date I wrote this answer Fredericton, Canada, in the ADT timezone was 3 hours behind UTC.

For kicks, I've also thrown in a calculation of solar noon. Note that this is an additional hour off from what you'd expect - that's a side-effect of our using daylight savings time.

All this returns:

begin civil twilight: 2013/9/4 09:20:46
sunrise:              2013/9/4 09:51:25
noon:                 2013/9/4 16:25:33
sunset:               2013/9/4 22:58:49
end civil twilight:   2013/9/4 23:29:22

Note that we'd have to subtract 3 hours to convert them to the ADT timezone.

This PyEphem documentation page contains much of the above, though I've tried to both clarify points I found confusing and include the important parts of that link here on StackOverflow.

Answer 2

For dawn amd dusk, see pyephem documentation regarding twilight
In a nutshell, dawn and dusk express the time when the center of the Sun is at a particular angle below the horizon; the angle used for this calculation varies for the definition of the "civilian", navigation (nautical) and astronomical twilights which use 6, 12 and 18 degrees respectively.

In opposition the sunrise corresponds to the time when the edge of the Sun appears (or disappears, for sunset) just above/below the horizon (0 degree). Hence everybody, civilians, sailors and astronomy enthusiasts alike get the same rise/set times. (see Naval Observatory risings and settings in pyephem documentation).

To summarize, once one has properly parametrized an pyephem.Observer (setting its lat, long, date, time, pressure (? for altitude?), etc.), the various twilight times (dawn, dusk) and the sunrise and sunset times are obtained from the
Observer.previous_rising() and Observer.next_setting() methods,
  whereby   the first argument is ephem.Sun() and
  the use_center= argument needs to be set to True for twilight calculations, and
  the horizon is either 0 (or 0:34 if you account for refraction of the atmosphere) or -6, -12 or -18.