Module: Astronoby::Util::Time

Defined in:
lib/astronoby/util/time.rb

Class Method Summary collapse

Class Method Details

.local_sidereal_time(time:, longitude:) ⇒ Object 



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# File 'lib/astronoby/util/time.rb', line 76

def local_sidereal_time(time:, longitude:)
  ut_to_gmst(time.utc) + longitude.hours
end

.lst_to_gmst(lst:, longitude:) ⇒ Object 



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# File 'lib/astronoby/util/time.rb', line 80

def lst_to_gmst(lst:, longitude:)
  gmst = lst - longitude.hours

  # If gmst negative, add 24 hours to the date
  # If gmst is greater than 24, subtract 24 hours from the date
  gmst += 24 if gmst.negative?
  gmst -= 24 if gmst > 24

  gmst
end

.lst_to_ut(date:, longitude:, lst:) ⇒ Object 



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# File 'lib/astronoby/util/time.rb', line 35

def lst_to_ut(date:, longitude:, lst:)
  gmst = lst_to_gmst(lst: lst, longitude: longitude)
  julian_day = date.ajd
  jd0 = ::DateTime.new(date.year, 1, 1, 0, 0, 0).ajd - 1
  days_into_the_year = julian_day - jd0

  t = (jd0 - Epoch::J1900) / Epoch::DAYS_PER_JULIAN_CENTURY
  r = BigDecimal("6.6460656") +
    BigDecimal("2400.051262") * t +
    BigDecimal("0.00002581") * t * t
  b = 24 - r + 24 * (date.year - 1900)

  # If t0 negative, add 24 hours to the date
  # If t0 is greater than 24, subtract 24 hours from the date
  t0 = BigDecimal("0.0657098") * days_into_the_year - b
  t0 += 24 if t0.negative?

  a = gmst - t0
  a += 24 if a.negative?

  ut = BigDecimal("0.99727") * a

  absolute_hour = ut.abs
  hour = absolute_hour.floor
  decimal_minute = 60 * (absolute_hour - hour)
  absolute_decimal_minute = (60 * (absolute_hour - hour)).abs
  minute = decimal_minute.floor
  second = (
    60 * (absolute_decimal_minute - absolute_decimal_minute.floor)
  ).round

  ::Time.utc(
    date.year,
    date.month,
    date.day,
    hour,
    minute,
    second
  )
end

.ut_to_gmst(universal_time) ⇒ Object

Source:

Title: Practical Astronomy with your Calculator or Spreadsheet
Authors: Peter Duffett-Smith and Jonathan Zwart
Edition: Cambridge University Press
Chapter: 12 - Conversion of UT to Greenwich sidereal time (GST)



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# File 'lib/astronoby/util/time.rb', line 12

def ut_to_gmst(universal_time)
  julian_day = universal_time.to_date.ajd
  t = (julian_day - Epoch::J2000) / Epoch::DAYS_PER_JULIAN_CENTURY
  t0 = (
    (BigDecimal("6.697374558") +
    (BigDecimal("2400.051336") * t) +
    (BigDecimal("0.000025862") * t * t)) % 24
  ).abs

  ut_in_hours = universal_time.hour +
    universal_time.min / 60.0 +
    (universal_time.sec + universal_time.subsec) / 3600.0

  gmst = BigDecimal("1.002737909") * ut_in_hours + t0

  # If gmst negative, add 24 hours to the date
  # If gmst is greater than 24, subtract 24 hours from the date
  gmst += 24 if gmst.negative?
  gmst -= 24 if gmst > 24

  gmst
end