[R] julian day form POSIXt object
Prof Brian Ripley
ripley at stats.ox.ac.uk
Thu Sep 29 18:59:58 CEST 2011
On Thu, 29 Sep 2011, Jeff Newmiller wrote:
> On Thu, 29 Sep 2011, maxbre wrote:
[With the context excised, something the posting guide expressly asks
not be done.]
>> yes, you are perfectly right, sorry for that but for me was not so
>> clear the error message!
A phrase involving pots and kettles springs to mind.
The answer to the question asked was 'see ?julian'
>> ....and the third code example I posted (not working either) should be of
>> length one (if I'm not wrong again)
>> but most of all what is not included in my reproducible example and
>> get a bit confused my question is the case when the variable "date"
>> contains dates of many years and the objective is to count for each
>> given year the julian date starting from the first day of the
>> corresponding year
> So you aren't really interested in julian day, you are interested in day of
> year. Julian day maps all calendar time to number of days from a SINGLE
> epoch (origin) which may differ from one problem to the next, but not from
> one value to the next within the problem the way "beginning of year" changes
> for each value.
>> how would you deal with this problem?
> test$doy <- as.POSIXlt(test$date)$yday+1
Perhaps, but as described without the +1. The Julian day of the
origin is taken to be day 0.
I think the OP wanted what ISO 8601:2004 calls 'ordinal dates' and of
which Wikipedia says
'This system is sometimes incorrectly referred to as "Julian Date",
whereas the astronomical Julian Date is a sequential count of the
number of days since day 0 beginning 1 January 4713 BC Greenwich noon,
Julian proleptic calendar (or noon on ISO date -4713-11-24 which uses
the Gregorian proleptic calendar with a year ).'
>> thanks a lot for your help
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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