Class: Suntrack::RAstro

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

Overview

Astronomical methods, designed to be used in tandem with Suntrack::Point3D, from “Astronomy on the Personal Computer”, by Montenbruck and Pfleger (1991).

Constant Summary collapse

B1950_EPOCH =

Epoch constants for 1950 and 2000. Discussion on p. 15 of M&P.

-0.500002108
J2000_EPOCH = 
0
SIRIUS_DECLINATION =

Declination and right ascension of Sirius, for testing purposes, measured in degrees.

-16.7306
SIRIUS_RA = 
6.75242

Class Method Summary collapse

Class Method Details

.frac(x) ⇒ Object

Utility fraction method



98
99
100
101
102
 
# File 'lib/suntrack/RAstro.rb', line 98

def self.frac(x)
  x = x - (x.to_i)
  x = x + 1 if x < 0
  x
end

.get_dms(ddd) ⇒ Object

Utility method to get degrees/minutes/seconds from decimal degrees. M&P, page 11



56
57
58
59
60
61
62
63
64
 
# File 'lib/suntrack/RAstro.rb', line 56

def self.get_dms(ddd)
  pt = Suntrack::Point3D.new(0,0,0)
  pt.x = ddd.abs.to_i
  d1 = (ddd.abs - pt.x) * 60
  pt.y = d1.to_i
  pt.z = (d1 - pt.y) * 60
  pt.x = -pt.x if ddd < 0
  pt
end

.get_time(jd) ⇒ Object

Utility method to convert Julian date to time needed by Sun position method



93
94
95
 
# File 'lib/suntrack/RAstro.rb', line 93

def self.get_time jd
  (jd - 2451545)/36525
end

.jd(mjd) ⇒ Object

Utility method to convert modified Julian date to Julian dare M&P, page 12



86
87
88
 
# File 'lib/suntrack/RAstro.rb', line 86

def self.jd mjd
  mjd + 2400000.5
end

.lmst(mjd, lambda) ⇒ Object

Local Mean Sidereal Time M&P, page 38.



45
46
47
48
49
50
51
52
 
# File 'lib/suntrack/RAstro.rb', line 45

def self.lmst(mjd,lambda)
  mjd0 = mjd.to_i
  ut = (mjd - mjd0) * 24
  t = (mjd0 - 51544.5) / 36525.0
  gmst = 6.697374558 + 1.0027379093*ut + (8640184.812866+(0.093104-6.2e-6*t)*t)*t/3600.0
  lmst = 24 * frac((gmst-(Float(lambda)/15)) / 24)
  lmst
end

.sun_position(t) ⇒ Suntrack::Point3D

Sun position method. Returns Sun location, in declination and right ascension, given time in Julian centuries

Parameters:

  • t (Float) —

    time in Julian centuries

Returns:



21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
 
# File 'lib/suntrack/RAstro.rb', line 21

def self.sun_position(t)
  # Montebruck & Pfleger, page 36
  # Low-precision Sun position as a function of t
  # t is the number of T is the number of Julian centuries between the
  # epoch and J2000
  p2  = 6.283185307
  coseps = 0.91748
  sineps = 0.39778
  m  = p2*frac(0.993133+99.997361*t) 
  dl = 6893.0*sin(m)+72.0*sin(2*m)
  l  = p2*frac(0.7859453 + m/p2 + (6191.2*t+dl)/1296000);
  sl = sin(l)
  x = cos(l)
  y = coseps*sl
  z = sineps*sl
  rho = sqrt(1.0-z*z)
  declination = (360.0/p2)*atan(z/rho)
  right_ascension  = ( 48.0/p2)*atan(y/(x+rho))
  right_ascension = right_ascension + 24.0 if right_ascension < 0
  Suntrack::Point3D.new(0,declination,right_ascension)
end

.to_mjd(date_time) ⇒ Object

Utility method to convert Ruby DateTime to modified Julian date M&P, page 12



68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
 
# File 'lib/suntrack/RAstro.rb', line 68

def self.to_mjd(date_time)
  a = (10000 * date_time.year) + (100 * date_time.month) + date_time.mday
  m = date_time.month
  y = date_time.year
  if date_time.month <= 2
    m = date_time.month + 12
    y = date_time.year - 1
  end
  leapDays = (y/400).to_i - (y/100).to_i + (y/4).to_i
  if a <= 15821004.1
    leapDays = -2 + ((y+4716)/4).to_i - 1179
  end
  a = (365 * y) - 679004
  a + leapDays + (30.6001 * (m + 1)).to_i + date_time.mday + (Float(date_time.hour)/24) + (Float(date_time.min)/1440) + (Float(date_time.sec)/86400)
end