Class: Astronoby::GreenwichMeanSiderealTime

Inherits:
GreenwichSiderealTime show all
Defined in:
lib/astronoby/time/greenwich_mean_sidereal_time.rb

Overview

Greenwich Mean Sidereal Time (GMST). Computed from UTC using the IERS Conventions 2010 ERA-based expression, with a polynomial fallback for dates outside the IERS EOP data range.

Constant Summary collapse

JULIAN_CENTURIES_EXPONENTS = 
[
  6.697374558,
  2400.051336,
  0.000025862
].freeze
SIDEREAL_MINUTE_IN_UT_MINUTE = 
0.9972695663
UT_TO_SIDEREAL_RATIO = 
1.002737909

Constants inherited from SiderealTime

SiderealTime::TYPES

Instance Attribute Summary

Attributes inherited from SiderealTime

#date, #time, #type

Class Method Summary collapse

Instance Method Summary collapse

Methods inherited from GreenwichSiderealTime

apparent_from_utc, mean_from_utc, #to_lst

Methods inherited from SiderealTime

#apparent?, #mean?, normalize_time, #normalize_time, validate_type!

Constructor Details

#initialize(date:, time:) ⇒ GreenwichMeanSiderealTime

Returns a new instance of GreenwichMeanSiderealTime.

Parameters:

  • date (Date) —

    the calendar date

  • time (Numeric) —

    GMST in hours



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# File 'lib/astronoby/time/greenwich_mean_sidereal_time.rb', line 70

def initialize(date:, time:)
  super(date: date, time: time, type: MEAN)
end

Class Method Details

.from_utc(utc) ⇒ Astronoby::GreenwichMeanSiderealTime

Creates GMST from UTC using the IERS Conventions 2010 ERA-based expression.

Source:

Title: IERS Conventions (2010)
Chapter: 5.5.7 - ERA based expressions for Greenwich Sidereal Time

Parameters:

  • utc (Time) —

    the UTC time

Returns:



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# File 'lib/astronoby/time/greenwich_mean_sidereal_time.rb', line 29

def self.from_utc(utc)
  date = utc.to_date
  gmst_hours = begin
    gmst_radians = IERS::GMST.at(utc)
    gmst_radians * 12.0 / Math::PI
  rescue IERS::OutOfRangeError
    from_utc_polynomial(utc)
  end
  gmst_hours = normalize_time(gmst_hours)

  new(date: date, time: gmst_hours)
end

.from_utc_polynomial(utc) ⇒ Numeric

Polynomial fallback for dates outside the IERS EOP data range.

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)

Parameters:

  • utc (Time) —

    the UTC time

Returns:

  • (Numeric) —

    GMST in hours



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# File 'lib/astronoby/time/greenwich_mean_sidereal_time.rb', line 52

def self.from_utc_polynomial(utc)
  julian_day = utc.to_date.ajd
  t = (julian_day - JulianDate::J2000) / Constants::DAYS_PER_JULIAN_CENTURY
  t0 = (
    (JULIAN_CENTURIES_EXPONENTS[0] +
      (JULIAN_CENTURIES_EXPONENTS[1] * t) +
      (JULIAN_CENTURIES_EXPONENTS[2] * t * t)) % Constants::HOURS_PER_DAY
  ).abs

  ut_in_hours = utc.hour +
    utc.min / Constants::MINUTES_PER_HOUR +
    (utc.sec + utc.subsec) / Constants::SECONDS_PER_HOUR

  UT_TO_SIDEREAL_RATIO * ut_in_hours + t0
end

Instance Method Details

#to_utc ⇒ Time

Converts GMST back to UTC.

Source:

Title: Practical Astronomy with your Calculator or Spreadsheet
Authors: Peter Duffett-Smith and Jonathan Zwart
Edition: Cambridge University Press
Chapter: 13 - Conversion of GST to UT

Returns:

  • (Time) —

    the UTC time



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# File 'lib/astronoby/time/greenwich_mean_sidereal_time.rb', line 83

def to_utc
  date = @date
  julian_day = @date.ajd
  t = (julian_day - JulianDate::J2000) / Constants::DAYS_PER_JULIAN_CENTURY

  t0 = (
    (JULIAN_CENTURIES_EXPONENTS[0] +
      (JULIAN_CENTURIES_EXPONENTS[1] * t) +
      (JULIAN_CENTURIES_EXPONENTS[2] * t * t)) % Constants::HOURS_PER_DAY
  ).abs

  a = normalize_time(@time - t0)

  utc = SIDEREAL_MINUTE_IN_UT_MINUTE * a

  Util::Time.decimal_hour_to_time(date, 0, utc)
end