# [time-nuts] Sidereal Time and Lady Heather.

Bill Beam wbeam at gci.net
Thu Dec 20 04:52:01 UTC 2012

```On Thu, 20 Dec 2012 14:39:21 +1100, Neville Michie wrote:

>I have just found out how to get sidereal time from Lady Heather.
>(You put t=GMST in the command line, or alias target line, and
>the Lady comes up in Sidereal Time, ticking away in Sidereal seconds.)
>(Also you can get LMST, LAST or GAST.)
>Now, as a time nut, I have questions. How is a leap second handled?

Leap seconds are handled by the GPS receiver, not by Lady Heather.
Some receivers add a 60th second for the leap second, others insert an
extra 00th second.

>Sidereal time is time observed by watching stars rather than the sun.
>In fact it is the source of all knowledge about how fast our planet rotates,
>and the basis for all our time scales.
>A sidereal second is shorter than a physical second by about one second in six minutes,
>so as to fit 366.25 sidereal days into the year of 365.25 solar days.
>At the Vernal Equinox there is a situation where Solar seconds exactly match
>the Sidereal second count, as zero, but how can a time nut know if this
>is in error? How accurate is it? In microseconds, nanoseconds?
>My interest in Sidereal time is because I have two pendulum clocks mounted
>on the same brick wall and they interfere with each other.
>By running one one sidereal time they are independent.
>The problem is to get a source of sidereal time to measure the performance of the
>sidereal clock. It is no problem to divide 10 MHz down to sidereal seconds, but how do you synch.
>the seconds? This is where the Lady helps. But I do not know how I can get really accurate
>seconds markers as convenient as the PPS from my Thunderbolt.

Even though Lady Heather can display  LMST, LAST or GAST it runs at solar rate, not siderial rate.
Each second still rolls over at the top of the solar second.  Thus there is a one second sawtooth error
in siderial time with a 365+ second repitation period.

I have two Thunderbolts running copies of Lady Heather and two other GPS receivers running
Tac32.  Tac32 also runs at solar rate, but can display siderial time to nearest 0.01 sec.  So I was
able to compare Lady Heather with Tac32.

>cheers,
>Neville Michie

Bill Beam
NL7F

```

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