[time-nuts] Setting clocks 100 years ago

Alan Melia alan.melia at btinternet.com
Fri Nov 5 01:02:41 UTC 2010


Good question, probably to allow the Astronomer Royal's staff to take a
transit and check their clocks for that day.
Alan

----- Original Message ----- 
From: "WB6BNQ" <wb6bnq at cox.net>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Friday, November 05, 2010 12:11 AM
Subject: Re: [time-nuts] Setting clocks 100 years ago


> OK Bob,
>
> I'LL bite !  Why is it 1:00 pm for the ball ?
>
> Bill....WB6BNQ
>
>
> Bob Marinelli wrote:
>
> > Hi Murray,
> >
> > Actually, the ball at Greenwich drops at 1:00 pm every day.  For
> > everyone who can get to London, the observatory is well worth at least
> > a half day visit, they have several working Harrison clocks and yes
> > you can set your wristwatch at 1:00 when the ball drops :) there is
> > also a wonderful transit.
> >
> > Cheers,
> >
> > Bob
> >
> > On Nov 4, 2010, at 4:30 PM, Murray Greenman wrote:
> >
> > > Navigators used chronometers to determine their longitude. If they
> > > were
> > > stopped in one place long enough, they could work out longitude by a
> > > complicated process of star and lunar observations; however, when they
> > > left an established port, they usually took with them a time standard
> > > based on local measurements of the sun and the known location of the
> > > port.
> > >
> > > These measurements used a device called a 'Transit', which was a
> > > simple
> > > telescope mounted so that it pivoted in elevation, but was fixed N-S
> > > in
> > > azimuth. Midday was marked by the time at which the sun transited the
> > > telescope. It thus had higher resolution than a sundial. Getting N-S
> > > axis correct involved determining by iteration and surveying the axis
> > > that gave maximum elevation at time of transit.
> > >
> > > Once the transit was observed, a large ball on top of the building was
> > > dropped, indicating midday, and in some locations a cannon was also
> > > fired. Ships in port could observe the ball drop and hear the
> > > cannon. To
> > > this day the ball drops at midday at Greenwich.
> > >
> > >
> > > 73,
> > > Murray ZL1BPU
> > >
> > >
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