[time-nuts] Greenwich time ball

[Bob Marinelli]

Hi Bill,

The astronomers were busy doing their noon transit sighting at noon.   
They ball on Flamsteed House drops at 1:00.  The navigators on ships  
in the harbor were also likely doing a sight at noon, they set their  
clock at 1:00 by looking up at the observatory on the hill.  BTW, the  
view from Greenwich observatory over London is quite spectacular.  The  
above my recollection from a tour guide at the observatory.  Here is  
the official story (including an animation of the ball dropping):

http://www.nmm.ac.uk/places/royal-observatory/time-ball/

I guess this is the time to propose that, if possible, every Time Nut  
needs to visit Greenwich Observatory at least once :)

-Bob

On Nov 4, 2010, at 5:11 PM, WB6BNQ wrote:

> 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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