Re: datetime

Hi Bob,

now we're getting into the insteresting stuff :-)

On Fri, 22 Apr 2005, Rob Seaman wrote:
> Patricio Ortiz says:
>
> > a "datetime" (however it is represented) corresponds to an instant in
> > time.
>
> Well, simultaneity is not just an issue for GR. A datetime, whatever
> time scale is used, only makes sense with respect to the location of
> the observation (or a derived location such as the barycenter). Some
> representation with a complexity similar to STC is required to
> unambiguously capture that instant in full.

Yes, I've seen several flavours of time scales. The picture I painted was the most simplistic of all!

> > Julian Date and Modified Julian Date (which do assume the usage of UTC)
> > seem to be one of the most appropriate way of representing such instant
> > in time. Using the ISO standard (string representation) is quite
> > common,
> > but any application wanting to compare instants in time needs to
> > convert
> > it to a floatinng number (double).
>
> If the precision required is larger than a second (a typical case for
> ground based observations), an integer (long) may be preferable. A
> simple epsilon test may be sufficient to test for equality.

In any case it is an 8-byte representation. People determining periods usfing fourier transform methods can obtain results with several significant digits of a second.

For practical applications, I doubt that equality will be of much interest, but I may be totally wrong in this one.

> A more
> general case may be constructing a histogram representing the passage
> of a wavefront (or non-EM event) - in that case we may want to consider
> more obscure questions like whether the histogram intervals are
> half-open on the top or the bottom. Deciding the simultaneity of two
> events that are extended in time rather than instantaneous is a more
> interesting question than simply whether two intervals overlap. (Even
> ignoring the differing physics of signal generation and propagation
> that may apply to two different wavelength regimes.)

Indeed that's one of the most interesting aspects, eg, observed solar events (flares or whatever) with atmospheric phenomena (aurorae or other), or GRBs with their subsequent X-ray/optical/IR/radio afterglow. Even orbit determination of solar system objects may fall into that category, one is not interested in simultaneous events in space-time, but events which are somehow related.

Few things are instantaneous anyways, observations (other than perhaps, in photon counting mode) extend over a period of time (fractions of a second to hours or days). Perhaps "instants" can only be defined at a post-processing stage (eg, Peak time of a SN light curve), but even in those cases, the presence of an error bar in the determination of such instants convert them into intervals for comparison purposes.

Cheers,

Patricio

---
Patricio F. Ortiz			pfo-at-star.le.ac.uk
Department of Physics & Astronomy	Phone: +44 (0)116 252 2015
University of Leicester			
Leicester, LE1 7RH, UK