Class: Astronoby::GreenwichSiderealTime

Inherits:
Object
  • Object
show all
Defined in:
lib/astronoby/time/greenwich_sidereal_time.rb

Constant Summary collapse

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

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(date:, time:) ⇒ GreenwichSiderealTime

Returns a new instance of GreenwichSiderealTime.



41
42
43
44
 
# File 'lib/astronoby/time/greenwich_sidereal_time.rb', line 41

def initialize(date:, time:)
  @date = date
  @time = time
end

Instance Attribute Details

#date ⇒ Object (readonly)

Returns the value of attribute date.



13
14
15
 
# File 'lib/astronoby/time/greenwich_sidereal_time.rb', line 13

def date
  @date
end

#time ⇒ Object (readonly)

Returns the value of attribute time.



13
14
15
 
# File 'lib/astronoby/time/greenwich_sidereal_time.rb', line 13

def time
  @time
end

Class Method Details

.from_utc(utc) ⇒ 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)



20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
 
# File 'lib/astronoby/time/greenwich_sidereal_time.rb', line 20

def self.from_utc(utc)
  date = utc.to_date
  julian_day = utc.to_date.ajd
  t = (julian_day - Epoch::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

  gmst = 1.002737909 * ut_in_hours + t0
  gmst += Constants::HOURS_PER_DAY if gmst.negative?
  gmst -= Constants::HOURS_PER_DAY if gmst > Constants::HOURS_PER_DAY

  new(date: date, time: gmst)
end

Instance Method Details

#to_lst(longitude:) ⇒ Object 



71
72
73
 
# File 'lib/astronoby/time/greenwich_sidereal_time.rb', line 71

def to_lst(longitude:)
  LocalSiderealTime.from_gst(gst: self, longitude: longitude)
end

#to_utc ⇒ Object

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



51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
 
# File 'lib/astronoby/time/greenwich_sidereal_time.rb', line 51

def to_utc
  date = @date
  julian_day = @date.ajd
  t = (julian_day - Epoch::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 = @time - t0
  a += Constants::HOURS_PER_DAY if a.negative?
  a -= Constants::HOURS_PER_DAY if a > Constants::HOURS_PER_DAY

  utc = SIDEREAL_MINUTE_IN_UT_MINUTE * a

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