julianDay {oce} | R Documentation |
Convert a POSIXt time to a Julian day
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 |
The method is taken from Chapter 3 of Meeus (1982). 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.
A Julian-Day number, in astronomical convention as explained in Meeus.
Dan Kelley
Meeus, Jean, 1982. Astronomical formuae for Calculators. Willmann-Bell. Richmond VA, USA. 201 pages
t <- ISOdatetime(1977, 4, 26, hour=0, min=0, sec=0, tz="ET")+0.4*86400 jd <- julianDay(t) cat(format(t), "is Julian Day", format(jd, digits=14), "\n")