Class: DateTime

Inherits:
Object
  • Object
show all
Defined in:
lib/miriad.rb

Overview

The MIRIAD package adds astronomy-related and other utilty methods to the DateTime class.

Constant Summary collapse

J2000 =

The J2000 epoch

civil(2000,1,1,12)
OMEGA =

The sidereal rotation rate of Earth in radians per second

7.29211538e-5

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.ajd(d) ⇒ Object

Create a DateTime object from a numeric Astronomical Julian Date.



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# File 'lib/miriad.rb', line 138

def self.ajd(d)
  J2000 + d - J2000.ajd
end

.ut1 ⇒ Object

UT1 (expressed in hours) at the current time.



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# File 'lib/miriad.rb', line 190

def self.ut1
  now.ut1
end

.utc ⇒ Object

Create a DateTime object from a numeric Astronomical Julian Date.



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# File 'lib/miriad.rb', line 180

def self.utc
  now.to_utc
end

Instance Method Details

#by ⇒ Object

Besselian year at the time represented by self.

Adapted from hpiers.obspm.fr/eop-pc/models/constants.html



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# File 'lib/miriad.rb', line 145

def by
  2000+(amjd-51544.5)/365.242189813
end

#eqeq ⇒ Object

Equation of the Equinoxes at the time represented by self.

Adapted from aa.usno.navy.mil/faq/docs/GAST.php



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# File 'lib/miriad.rb', line 228

def eqeq
  # The local variables here have been slightly renamed from the reference to
  # be more ruby-esque.  Here is the mapping...
  # 
  # Local  Reference
  # jdut1  JD
  # d      D
  # omega  Omega (Greek letter)
  # dpsi   Delta Psi (Greek letters)
  # eps    epsilon (Greek letter)

  # The UT Julian date of the time of interest
  jdut1 = ajd + ut1_utc/(24*60*60)
  # Days and fraction of day since J2000 epoch
  d = jdut1 - J2000.ajd
  # Longitude of the ascending node of the Moon
  omega = 125.04 - 0.052954 * d
  # Mean Longitude of the Sun
  l = 280.47 + 0.98565 * d
  # Delta psi
  dpsi = -0.000319 * Math.sin(omega.d2r) \
         -0.000024 * Math.sin(2*l.d2r)
  # Obliquity
  eps = 23.4393 - 0.0000004 * d
  # eqeq
  dpsi * Math.cos(eps.d2r)
end

#gast ⇒ Object

Greenwich Apparent Sidereal Time at the time represented by self.



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# File 'lib/miriad.rb', line 257

def gast
  (gmst + eqeq) % 24.0
end

#gmst ⇒ Object

Greenwich Mean Sidereal Time at the time represented by self.

Adapted from aa.usno.navy.mil/faq/docs/GAST.php



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# File 'lib/miriad.rb', line 197

def gmst
  # The local variables here have been slightly renamed from the reference to
  # be more ruby-esque.  Here is the mapping...
  # 
  # Local  Reference
  # jdut1  JD
  # jd0    JD0
  # h      H
  # d      D
  # d0     D0
  # t      T

  # The UT Julian date of the time of interest
  jdut1 = ajd + ut1_utc/(24*60*60)
  # Julian date of the previous midnight (0h) UT
  jd0 = (jdut1-0.5).floor + 0.5
  # Hours of UT elapsed since jd0
  h = (jdut1 - jd0) * 24.0
  # Days and fraction of day since J2000 epoch
  d = jdut1 - J2000.ajd
  # Integer days (and a half) since epoch
  d0 = jd0 - J2000.ajd
  # Fractional centuries since epoch
  t = d/36525.0
  # gmst
  (6.697374558 + 0.06570982441908 * d0 + 1.00273790935 * h + 0.000026 * t**2) % 24.0
end

#last(longitude = 0.0) ⇒ Object

Local Apparent Sidereal Time in hours at the time represented by self. longitude is in degrees.



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# File 'lib/miriad.rb', line 269

def last(longitude=0.0)
  (gast + longitude.d2h) % 24.0
end

#lmst(longitude = 0.0) ⇒ Object

Local Mean Sidereal Time in hours at the time represented by self. longitude is in degrees.



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# File 'lib/miriad.rb', line 263

def lmst(longitude=0.0)
  (gmst + longitude.d2h) % 24.0
end

#to_utc ⇒ Object

A DateTime object corresponding to the same time as self, but with an offset of 0. Returns self if self.offset == 0.



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# File 'lib/miriad.rb', line 175

def to_utc
  (offset == 0) ? self : new_offset(0)
end

#ut1 ⇒ Object

UT1 (expressed in hours) at the time represented by self



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# File 'lib/miriad.rb', line 185

def ut1
  ((amjd % 1) * 24 + ut1_utc/(60*60)) % 24
end

#ut1_utc ⇒ Object

UT1-UTC (unit is seconds) at the time represented by self

This is an increasingly out-of-date approximation only. Taken from www.iers.org/products/6/11769/orig/bulletina-xxi-052.txt



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# File 'lib/miriad.rb', line 165

def ut1_utc
  # TODO Get more accurate measurement of this value from usno
  d = ENV.has_key?('UT1_UTC') ?
    ENV['UT1_UTC'].to_f : 0.3987 - 0.00094 * (amjd - 54832) - ut2_ut1
  d -= 1 if amjd < 54832 # 2009/01/01 (hack for leap second)
  d
end

#ut2_ut1 ⇒ Object

UT2-UT1 (unit is seconds) at the time represented by self

Taken from www.iers.org/products/6/11136/orig/bulletina-xx-042.txt



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# File 'lib/miriad.rb', line 153

def ut2_ut1
  pi2t=Math::PI*2*(by%1)
  ENV['UT2_UT1'] || 0.022 * Math.sin(pi2t) \
  - 0.012 * Math.cos(pi2t) \
  - 0.006 * Math.sin(2*pi2t) \
  + 0.007 * Math.cos(2*pi2t)
end