Re: [LEAPSECS] Be thankful for John Flamstead

From: Rob Seaman <>
Date: Thu, 10 Nov 2005 07:03:02 -0700

On Nov 10, 2005, at 3:48 AM, Ed Davies wrote:

>> The decision stemmed from the work 200 years previously of the first
>> English Astronomer Royal, John Flamsteed, who calculated that the
>> Earth rotated on its axis once every 24 hours.
> It must have been very confusing for people before it was
> realised that there were 24 hours in a day. You'd have
> thought somebody would have noticed the pattern before,
> though.
> And yes, my inner pedant has to note that it's once and a
> bit every 24 hours.

Not sure what work of Flamsteed's they are (poorly) referencing, but
you've piqued my interest. The notion of leap seconds seems to
function as the punchline for an unuttered joke. Not sure why that
is, but then there is humor in any academic pursuit of overtly
trivial details with large philosophical consequences. Monkey's
uncle? Ha, ha, ha! Nuclear fusion? Ho, ho! Atomic clock? He, he,
he! (

The length of the day is, of course, ~86164s. I've attached a slide
from my presentation at the UTC Birds-of-a-Feather session at the
recent Astronomical Data Analysis Software and Systems meeting in San
Lorenzo de El Escorial near Madrid. (I'm the one on the left in the
back of the conference photo wearing the eclipse viewing shades.) I
haven't previously tried to send an attachment through this list. If
that fails, the slide is also available from

I think a lot of the disrespect being directed at solar time during
this long and often interrupted conversation has been the result of
naive notions of large daily excursions in length of day due to the
equation of time. The EOT, of course, only represents the
accumulation of small daily differences that amount to no more than
half a minute out of a mean solar day of - yes - 24 hours. Leap
seconds represent a mechanism for introducing corrections to the very
slight non-zero slope of the mean.

I won't belabor my previous argument that civil time must mimic solar
time to better than a second per day. If you don't agree with that
statement, it is because you didn't have the patience (or stomach) to
read all the way to the end of my previous messages. But, I would
argue again that these two clocks that are - IN THE MEAN - required
to remain synchronized to better than +/- 1 second should best be
recognized as being the same clock. The existence of a long term
secular trend doesn't upset the much larger and more obvious physical
and philosophical effect.

If the underlying driver for the proposal to eradicate leap seconds
is really that the designers of GPS, GLONASS and now GALILEO each
chose different underlying time scales... And if the further
argument is that TAI is itself NOT an appropriate civil time
scale... Well, then - the only thing that ultimately ties these
three systems together is the underlying physical reality of mean
solar time.

Rob Seaman
National Optical Astronomy Observatory

Received on Thu Nov 10 2005 - 06:05:06 PST

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