# [ntp:questions] Re: Extracted fom comp.risks - What Time Is It?

H. Peter Anvin hpa at zytor.com
Tue Aug 19 21:15:05 UTC 2003

```Followup to:  <87ptj2wy8a.fsf at netnews.comcast.net>
By author:    Dale Worley <worley at dragon.ariadne.com>
In newsgroup: comp.protocols.time.ntp
>
> "David Schwartz" <davids at webmaster.com> writes:
> > I don't believe anyone corrects for gravitational effects. Time dilation
> > definitely requires compensation when you have to fly an atomic clock from a
>
> The clocks in the GPS satellites are (I've read) corrected for the
> gravitational red shift.  But I would expect that for clocks on the
> surface of the Earth, the gravitational red shift effects would be
> negligible.
>
> http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/gratim.html says
> that the dilation factor is 1 + g h / c^2.  I get that the difference
> is about 3.3E-13 for 10,000 feet vertical distance, or 3.3E-15 for 100
> feet.  That would build up to 100 ns within some days, if I've done
> the calculation right.
>

According to the same website you indicated, the gravitational
redshift at the surface of the Earth is about 1e-9 compared to a

Scout Rocket Experiment

In 1976 the Smithsonian Astrophysical Observatory sent aloft a Scout
rocket to a height of 10,000 km. At this height, a clock should run
4.5 parts in 10^10 faster than one on the Earth. During two hours of
free fall from its maximum height, the rocket transmitted timing
pulses from a maser oscillator which acted as a clock and which was
compared with a similar clock on the ground. This result confirmed the
gravitational time dilation relationship to withing 0.01%.

-hpa
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```