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Re: JULDAY-CALDAT problem [message #38265 is a reply to message #38264]

Thu, 26 February 2004 06:52

btupper
Messages: 55
Registered: January 2002

Member

On Thu, 26 Feb 2004 07:19:10 -0700, David Fanning <david@dfanning.com>
wrote:

> Luciano writes:
>
>> Hi, maybe somebody has a clue about the following:
>>
>> IDL> aa=JulDay(11,18,1990,2,0,0)
>> IDL> Caldat,aa,m,d,y,h,mi,s
>> IDL> print,m,d,y,h,mi,s
>> 11 18 1990 2 0 5.3644181e-005
>>
>> Why does s=5.3644181e-005 and not s=0 as it should be?
>
> I think it is a question of using a computer to do the
> calculations rather than your fingers. :-)
>
> Floating point numbers have about 7 significant figures.
> Assuming 60 seconds in a minute, this number starts to
> vary in the seventh place. So, about as close to zero
> as you gonna get, I think.
>
> For a more complete explanation, see this article:
>
> http://www.dfanning.com/math_tips/sky_is_falling.html
>

Hello,

One additional thing to add to David's explanation is that JULDAY adds
a small offset to the seconds argument. I forget why, but the reason
is described in the documentation for JULDAY. You could test it out
by editing the JULDAY function so it does not add the offset. I'll
bet you still will not get the answer you expect because of the
condition David describes.

Ben

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[Message index]

		Re: JULDAY-CALDAT problem By: James Kuyper on Fri, 27 February 2004 08:17
		Re: JULDAY-CALDAT problem By: Paul Van Delst[1] on Fri, 27 February 2004 08:06
		Re: JULDAY-CALDAT problem By: David Fanning on Fri, 27 February 2004 05:28
		Re: JULDAY-CALDAT problem By: lucianor on Fri, 27 February 2004 02:30
		Re: JULDAY-CALDAT problem By: David Fanning on Thu, 26 February 2004 08:51
		Re: JULDAY-CALDAT problem By: David Fanning on Thu, 26 February 2004 08:41
		Re: JULDAY-CALDAT problem By: btupper on Thu, 26 February 2004 08:17
		Re: JULDAY-CALDAT problem By: David Fanning on Thu, 26 February 2004 07:22
		Re: JULDAY-CALDAT problem By: btupper on Thu, 26 February 2004 06:52
		Re: JULDAY-CALDAT problem By: David Fanning on Thu, 26 February 2004 06:19
		Re: JULDAY-CALDAT problem By: Mark Hadfield on Sun, 29 February 2004 14:50

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