[time-nuts] Danjon Astrolabe meridian transit timing errors

Bill Hawkins bill at iaxs.net
Sat Sep 30 14:47:51 EDT 2006

OK, there are serious sources of error in making a one-time
solar transit measurement.

What I propose is a differential method, a favorite of
instrument makers to reduce errors. This is possible
because the equation of time makes a correction of only
one percent or so.

A steady platform with a single axis of motion is
turned by a precision synchronous motor driven by a
frequency derived from a cesium standard. This provides
a fixed reference rotation speed.

Use a mirror mounted so that it turns on the horizontal axis
and is in turn turned on the vertical axis by the accurate
frequency. Use a simple sun-tracking servo to keep the image
of the sun on a mirror attached to a galvanometer assembly.

Use an analog servo to generate a current that will keep the
solar image from the galvo centered horizontally on a target.
Use high frequency dithering to improve accuracy. Filter the
galvanometer current to remove the dither and measure it with
a computer. Use math tricks to subtract the equation of time,
looking for a drift rate at a frequency much less than one
cycle per day, but larger than the drift rate of the standard.

Systematic errors in the instrument should be revealed. If they
are temperature dependent, they can be compensated. The stability
of the mounting for the apparatus becomes a problem over long
periods of time. Perhaps ways can be found to compensate can be
found, but I can't think of something that doesn't require a
stable reference platform.

The fact is, no other physical property can be measured to the
same accuracy as frequency, because atomic motion provides a
stable reference.

The question is, then, can long-term averaging remove the small
errors in measuring the position of the sun relative to a
rotating reference platform?

If this is feasible, where can I find a Maxwell clamp? Google
can't find one.

Bill Hawkins

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