[ntp:questions] Google and leap seconds

unruh unruh at wormhole.physics.ubc.ca
Thu Sep 22 22:13:09 UTC 2011

On 2011-09-22, Terje Mathisen <"terje.mathisen at tmsw.no"> wrote:
> unruh wrote:
>> On 2011-09-22, David Woolley<david at ex.djwhome.demon.invalid>  wrote:
>>> 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.
> David is correct, the GPS sats are in fact using orbits that have a 
> period of almost exactly half sidereal day, or about 11 hours 58 minutes.
> This is not a requirement though, afaik the upcoming Galileo sats will 
> have significantly higher orbits/longer periods, but still a rational 
> factor of the sidereal day, so that they will return to the same point 
> after a number of days.

You and he are right, I am wrong. Although there is no particular reason
why that needs to be true. The GPS system would also work if the
sattelites were not in a siderial day orbit. 
> Terje

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