[ntp:questions] Google and leap seconds

unruh unruh at wormhole.physics.ubc.ca
Thu Sep 22 16:09:59 UTC 2011


On 2011-09-22, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> unruh wrote:
>> On 2011-09-21, David Woolley <david at ex.djwhome.demon.invalid> wrote:
>>> unruh wrote:
>>>> On 2011-09-21, David Woolley <david at ex.djwhome.demon.invalid> wrote:
>>>>> Richard B. Gilbert wrote:
>>>>>
>>>>>> It's unfortunate that the earth DOES NOT rotate exactly 360 degrees in 
>>>>>> exactly 24.000000000000 hours. This bit of poor design causes all sorts 
>>>>> It's nothing like it.  It out by approximately 1 degree a day!
>>>> well, not if you define the rotation with respect to the "mean sun"
>>>> rather than the stars. Then it is only out by a few PPB.
>>>>
>>> It makes a big difference for GPS, though.  I believe their orbital 
>>> period is half a sidereal day, not 12 hours or even 43200 TAI seconds.
>> 
>> The orbit is far faster than that. You mean the period of the orbit (not
>> in the orbit) as seen from earth. 
>> 
>
> I don't understand.
>
> The orbit plane is fixed relative to the fixed stars, so, for the orbit 
> to cover the same ground each time, it has to have a period that exactly 
> divides the sidereal day.  The period is between successive maximum 
> North points.

It is the sattelite you see, not the orbital plane. The sattelite goes
around its orbit in a much shorter time period than one siderial day. 




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