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Re: Some more inconsistencies

From: Garmt de Vries <G.deVries~at~phys.uu.nl>
Date: Fri, 16 Jun 2006 16:40:27 +0200 (CEST)
To: Jules Verne Forum <jvf~at~Gilead.org.il>

On Fri, 16 Jun 2006, jcrovisier~at~free.fr wrote:

> I wish to correct my preceding message. There is no error for the date(s)
> of the 1896 solar eclipse. This eclipse is referred to in the litterature
> either as the 8 August or the 9 August 1896 eclipse. The reason is the
> convention used by astronomers to count time and dates at that epoch. The
> "mean time" is counted from, and the change of date occurs at NOON, not at
> MIDNIGHT.

As I understand it, astronomers use the Julian Date (JD), which is the
number of days elapsed since 12 noon GMT on 1 January, 4713 BC. Counting
from noon to noon was useful because the sky was observed at night, and it
was easier if all observations done in one night were assigned the same
date.

Of course, nowadays, with radio astronomy, satellite telescopes and
intensive international collaboration, it doesn't make much sense anymore
to count from noon to noon. In 1957, with the launch of the Sputnik, the
Modified Julian Date (MJD) was introduced: the number of days elapsed
since midnight at the beginning of 17 November, 1858. Hence, there's a
difference of 2,400,000.5 between MJD and JD.

As a physicist, I sometimes have trouble adapting to the units and
conventions used by astronomers, especially when going into the details of
ephemerides...

Cheers,
Garmt.
Received on Fri 16 Jun 2006 - 17:40:43 IDT

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