Convert a POSIXt time to a Julian day, using the method provided in
Chapter 3 of reference 1. It should be noted that
Meeus and other astronomical treatments use fractional days, whereas the
present code follows the R convention of specifying days in whole numbers,
with hours, minutes, and seconds also provided as necessary. Conversion is
simple, as illustrated in the example for 1977 April 26.4, for which Meeus
calculates julian day 2443259.9. Note that the R documentation for
julian()
suggests another formula, but the point of the present
function is to match the other Meeus formulae, so that suggestion is ignored
here.
julianDay( t, year = NA, month = NA, day = NA, hour = NA, min = NA, sec = NA, tz = "UTC" )
t  a time, in POSIXt format, e.g. as created by


year  year, to be provided along with 
month  month, numbered with January being 1. 
day  day in month, starting at 1. 
hour  hour of day. 
min  minute of hour 
sec  second of hour 
tz  timezone 
A JulianDay number, in astronomical convention as explained in Meeus.
Meeus, Jean, 1982. Astronomical formulae for Calculators. WillmannBell. Richmond VA, USA. 201 pages
Other things related to astronomy:
angle2hms()
,
eclipticalToEquatorial()
,
equatorialToLocalHorizontal()
,
julianCenturyAnomaly()
,
moonAngle()
,
siderealTime()
,
sunAngle()
,
sunDeclinationRightAscension()
Other things related to time:
ctimeToSeconds()
,
julianCenturyAnomaly()
,
numberAsHMS()
,
numberAsPOSIXct()
,
secondsToCtime()
,
unabbreviateYear()
t < ISOdatetime(1977, 4, 26, hour=0, min=0, sec=0, tz="UTC")+0.4*86400 expect_equal(julianDay(t), 2443259.9) # example from Meeus