Module: Ephem::Core::CalendarCalculations

Included in:
Segments::BaseSegment
Defined in:
lib/ephem/core/calendar_calculations.rb

Class Method Summary collapse

Class Method Details

.format_date(year, month, day) ⇒ String

Formats a calendar date as YYYY-MM-DD

Examples:

Formatting a modern date

format_date(2023, 2, 18)  # => "2023-02-18"

Formatting a BCE date

format_date(-44, 3, 15)   # => "-44-03-15"

Formatting with single-digit values

format_date(9, 3, 4)      # => "9-03-04"

Parameters:

  • year (Integer) —

    The year (negative for BCE)

  • month (Integer) —

    The month (1-12)

  • day (Integer) —

    The day of the month

Returns:

  • (String) —

    The formatted date string



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# File 'lib/ephem/core/calendar_calculations.rb', line 87

def format_date(year, month, day)
  format("%d-%02d-%02d", year, month, day)
end

.julian_to_gregorian(julian_date) ⇒ Array<Integer>

Note:

The algorithm handles the transition between Julian and Gregorian calendars at 1582-10-15 (Julian Date 2299161). Dates on or after this are interpreted as Gregorian calendar dates, while earlier dates are interpreted as Julian calendar dates.

Converts a Julian Date to a Gregorian calendar date

Examples:

Converting a date after Gregorian reform

julian_to_gregorian(2460000.0)  # => [2023, 2, 24]

Converting a date before Gregorian reform

julian_to_gregorian(2299160.0)  # => [1582, 10, 4]

Converting a BCE date

julian_to_gregorian(1721056.5)  # => [-1, 12, 31]

Parameters:

  • julian_date (Float) —

    The Julian Date to convert. The Julian Date is the number of days since noon on January 1, 4713 BCE (on the Julian calendar).

Returns:

  • (Array<Integer>) —

    An array containing [year, month, day]. For dates BCE, the year will be negative (-1 = 2 BCE, -2 = 3 BCE, etc.)



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# File 'lib/ephem/core/calendar_calculations.rb', line 31

def julian_to_gregorian(julian_date)
  # Convert Julian Date to Gregorian calendar date using the algorithm
  # from Fundamentals of Astrodynamics (Bate, Mueller, White) pg. 42

  # Adjust Julian Date and calculate integer and fractional parts
  jd = julian_date + 0.5  # Add 0.5 to Julian Date for correct rounding
  z = jd.floor            # Integer part of adjusted Julian Date
  f = jd - z              # Fractional part of adjusted Julian Date

  # Handle dates before and after the Gregorian calendar reform of 1582
  a = if z < 2299161 # 2299161 is the Julian Date of 1582-10-15T00:00:00Z
    z                # Use integer part directly for Julian calendar dates
  else
    # 1867216.25 is the Julian Date of 1500-03-01T00:00:00Z
    alpha = ((z - 1867216.25) / 36524.25).floor
    # Adjust for Gregorian calendar
    z + 1 + alpha - (alpha / 4).floor
  end

  # Calculate intermediate values
  # 1524 is the number of days from 1 BC to 1 AD
  b = a + 1524
  # 365.25 is the number of days in a Julian year
  c = ((b - 122.1) / 365.25).floor
  # Number of days since 1 BC
  d = (365.25 * c).floor
  # 30.6001 is the average number of days in a month
  e = ((b - d) / 30.6001).floor

  # Extract day, month, and year values from intermediate values
  # Fractional part gives the day of month
  day = b - d - (30.6001 * e).floor + f
  # Months are numbered from 1 (January) to 12 (December)
  month = (e < 14) ? e - 1 : e - 13
  # 4716 BC is the year of Julian Date 0
  year = (month > 2) ? c - 4716 : c - 4715

  [year, month, day.to_i]  # Return [year, month, day] as an array
end