Calculate sunrise and sunset times for Rothera Research Station

rsun.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Calculate local sunrise and sunset times for Rothera.
"""

import ephem
from datetime import date, datetime
import argparse

def sunrise(location, day, horizon, dawn='True'):
    """
    Calculate sunrise/set time and return it as a Julian Date.

    Args:
      location (object): an ephem.Observer object
      day (int/float): the Julian Day or Date
      horizon (int/float): the angle of the desired horizon e.g. astronomical dawn is -18
      dawn (bool): Dawn (True) or dusk (False)

    Returns:
      suntime (float): the Julian Date of the sunrise/sunset

    Raises:
      CircumpolarError: If sun does not rise or set during the Julian Day.
    """
    location.date = day
    location.horizon = horizon
    sun = ephem.Sun()
    sun.compute(location)
    if dawn is True:
        try:
            suntime = ephem.localtime(location.next_rising(sun))
        except ephem.CircumpolarError:
            suntime = ephem.Date(day).datetime()
    else:
        try:
            suntime = ephem.localtime(location.next_setting(sun))
        except ephem.CircumpolarError:
            suntime = ephem.Date(day).datetime()
    return suntime

def is_date(input_date):
    """Checks whether the input date is a valid date."""
    if input_date in ['tod', 'today']:
        day = ephem.Date(date.today())
    elif input_date in ['tom', 'tomorrow']:
        day = ephem.Date(date.today()) + 1
    elif input_date in ['yest', 'yesterday']:
        day = ephem.Date(date.today() - 1)
    else:
        try:
            day = ephem.Date(datetime.strptime(input_date, '%Y-%m-%d'))
        except ValueError:
            raise argparse.ArgumentTypeError('Invalid date – must be in format YYYY-MM-DD')
    return day

def main():
    parser = argparse.ArgumentParser(description="Calculates current sunset and sunrise for Rothera.")
    parser.add_argument("day", type=is_date, nargs='?', default=ephem.Date(date.today()), help="Date in YYYY-MM-DD. Defaults to today.")
    args = parser.parse_args()
    rothera = ephem.Observer()
    rothera.lat = '-67.57'
    rothera.lon = '-68.13'
    rothera.pressure = 0
    # -6º is civil twilight; –0:34º with 0 pressure is the NOAA spec for
    # sunrise/set to take into account atmospheric refraction
    horizons = ['-6','-0:34']
    day = args.day
    srises = [sunrise(rothera, day, horizon, True) for horizon in horizons]
    ssets = [sunrise(rothera, day, horizon, False) for horizon in reversed(horizons)]
    now = datetime.now()
    if ssets[1] < now:
        daylight_left = 'None'
    else:
        if now > srises[0]:
            daylight_left = str(ssets[1] - now)
        else:
            daylight_left = str(ssets[1] - srises[0])
    sunrises = [srise.strftime('%H:%M') for srise in srises]
    sunsets = [sset.strftime('%H:%M') for sset in ssets]
    output = sunrises + sunsets
    print("Dawn: {0}   Sunrise: {1}   Sunset: {2}   Dusk: {3}".format(*output))
    print("Daylight left: {0}".format(daylight_left))

if __name__ == "__main__":
    main()