Module: SunTimes

Defined in:
lib/sun_times.rb

Defined Under Namespace

Modules: VERSION

Constant Summary collapse

DEFAULT_ZENITH = 
90.83333
KNOWN_EVENTS = 
[:rise, :set]
DEGREES_PER_HOUR = 
360.0 / 24.0

Class Method Summary collapse

Class Method Details

.calculate(event, date, latitude, longitude, options = {}) ⇒ Object

Calculates the sunrise or sunset time for a specific date and location

Parameters

  • event - One of :rise, :set.

  • date - An object that responds to yday.

  • latitude - The latitude of the location in degrees.

  • longitude - The longitude of the location in degrees.

  • options - Additional option is :zenith.

Example

SunTimes.calculate(:rise, Date.new(2010, 3, 8), 43.779, 11.432)
> Mon Mar 08 05:39:53 UTC 2010



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# File 'lib/sun_times.rb', line 64

def SunTimes.calculate(event, date, latitude, longitude, options = {})
  raise "Unknown event '#{ event }'" if KNOWN_EVENTS.find_index(event).nil?
  zenith = options.delete(:zenith) || DEFAULT_ZENITH

  # lngHour
  longitude_hour = longitude / DEGREES_PER_HOUR

  # t
  base_time = event == :rise ? 6.0 : 18.0
  approximate_time = date.yday + (base_time - longitude_hour) / 24.0

  # M
  mean_sun_anomaly = (0.9856 * approximate_time) - 3.289

  # L
  sun_true_longitude = mean_sun_anomaly +
                      (1.916 * Math.sin(degrees_to_radians(mean_sun_anomaly))) +
                      (0.020 * Math.sin(2 * degrees_to_radians(mean_sun_anomaly))) +
                      282.634
  sun_true_longitude = coerce_degrees(sun_true_longitude)

  # RA
  tan_right_ascension = 0.91764 * Math.tan(degrees_to_radians(sun_true_longitude))
  sun_right_ascension = radians_to_degrees(Math.atan(tan_right_ascension))
  sun_right_ascension = coerce_degrees(sun_right_ascension)

  # right ascension value needs to be in the same quadrant as L
  sun_true_longitude_quadrant  = (sun_true_longitude  / 90.0).floor * 90.0
  sun_right_ascension_quadrant = (sun_right_ascension / 90.0).floor * 90.0
  sun_right_ascension += (sun_true_longitude_quadrant - sun_right_ascension_quadrant)

  # RA = RA / 15
  sun_right_ascension_hours = sun_right_ascension / DEGREES_PER_HOUR

  sin_declination = 0.39782 * Math.sin(degrees_to_radians(sun_true_longitude))
  cos_declination = Math.cos(Math.asin(sin_declination))

  cos_local_hour_angle =
    (Math.cos(degrees_to_radians(zenith)) - (sin_declination * Math.sin(degrees_to_radians(latitude)))) /
    (cos_declination * Math.cos(degrees_to_radians(latitude)))

  # the sun never rises on this location (on the specified date)
  return nil if cos_local_hour_angle > 1
  # the sun never sets on this location (on the specified date)
  return nil if cos_local_hour_angle < -1

  # H
  suns_local_hour =
    if event == :rise
      360 - radians_to_degrees(Math.acos(cos_local_hour_angle))
    else
      radians_to_degrees(Math.acos(cos_local_hour_angle))
    end

  # H = H / 15
  suns_local_hour_hours = suns_local_hour / DEGREES_PER_HOUR

  # T = H + RA - (0.06571 * t) - 6.622
  local_mean_time = suns_local_hour_hours + sun_right_ascension_hours - (0.06571 * approximate_time) - 6.622

  # UT = T - lngHour
  gmt_hours = local_mean_time - longitude_hour
  gmt_hours -= 24.0 if gmt_hours > 24
  gmt_hours += 24.0 if gmt_hours <  0

  case
  when date.respond_to?( :offset )
    offset_hours = date.offset * 24
  when date.respond_to?( :gmt_offset )
    offset_hours = date.gmt_offset / 3600
  else
    offset_hours = nil
  end

  if ! offset_hours.nil?
    if gmt_hours + offset_hours < 0
      next_day = Date.new(date.year, date.month, date.day + 1)
      return calculate(event, next_day, latitude, longitude, options = {})
    end
    if gmt_hours + offset_hours > 24
      previous_day = Date.new(date.year, date.month, date.day + 1)
      return calculate(event, previous_day, latitude, longitude, options = {})
    end
  end

  hour = gmt_hours.floor
  hour_remainder = (gmt_hours.to_f - hour.to_f) * 60.0
  minute = hour_remainder.floor
  seconds = (hour_remainder - minute) * 60.0

  Time.gm(date.year, date.month, date.day, hour, minute, seconds)
end

.rise(date, latitude, longitude, options = {}) ⇒ Object

Helper method: calculates sunrise, with the same parameters as calculate



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# File 'lib/sun_times.rb', line 43

def SunTimes.rise(date, latitude, longitude, options = {})
  calculate(:rise, date, latitude, longitude, options)
end

.set(date, latitude, longitude, options = {}) ⇒ Object

Helper method: calculates sunset, with the same parameters as calculate



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# File 'lib/sun_times.rb', line 48

def SunTimes.set(date, latitude, longitude, options = {})
  calculate(:set, date, latitude, longitude, options)
end