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# Sidereal clock needed

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#### captainkirksdog

##### Member
Hi,
Anyone have a schematic (US) for building a sidereal time clock? Thanks.

OK, I'll bite. What is it?

Sidereal clock explanation

Hi Gene,
A sidereal clock measures "star time" rather than solar time. An average day is 24 hours long, but a sidereal day is 23 hours, 56 minutes, and about 14 seconds. As the earth rotates, it has to turn a little bit farther each day, as it moves along its orbit, in order for the same exact spot to be overhead the next time. This is why the stars change with the seasons. I was able to obtain several old style satellite dishes and I want to make an el cheapo radiotelescope. (Remember Charlie Sheen in The Arrival?) Because I can use it during daylight hours, I have to have a way to "point and shoot". Hence, the sidereal clock. If you can come up with any ideas, I'd be glad to hear them. Thanks.

Got it! Since no one responded, I figgured that I was not the only person who didn't know what you were talking about.

This is somewhat similar to the tide clock I am finishing up. In my case, the (lunar) day lasts 24 hours 50 minutes. That's a rough number because tide action appears to vary by day and by location. But the problem is similar in that my clock pulses must be divided such that (in the case of the hour indicator) one pulse appears each 62.0833 minutes (24 hr 50 min / 24). In your case, the hour pulse comes at 59.834 minutes - both slightly different time bases.

Rather than design the clock from the ground up like I did, you might look for a commercial digital clock that is fed from a 555 oscillator chip. Then, it's relatively easy to adjust the 555 R-C or crystal control such that the pulses from the 555 are in-line with your time measurement plan (faster for the shorter day). 555 timing is generally the circuit attached to pins 6 and 7. If you have an oscilliscope, you can measure the 555s pulses at pin #3 and calculate your timing adjustment needed.

An alternative would be to build it from a schematic for a standard digital clock with a 555 oscillator (that could be tweaked).

A sundial is a sideral clock.

Second thoughts, maybe its not.

Hi captainkirksdog,

I would suggest you line up the moveable axis north-south,
then use parts from an electric clock.
The motor in an electric clock has very little torque,
but after its been geared down to operate the hour hand
it has adequate torque for most things.

I guess you would only want your dish to follow the chosen
area of sky for an arc of movement, their wouldn't be much
point in aiming at something below the horizon.

You're looking at a difference of movement of 144 to 150
aproximately.

If you are capable of modifying the gears, i think that
would be the best way, then you can just use the mains
supply.

If you want to do this electronically, you're looking at
making an oscillator to give you 2 cycles different from
the 60 cycles per second supply that is common on the
western side of the Atlantic or about 1.8 cycles different
from the 50 cycles per second supply that is common here
in England on the eastern side of the Atlantic.

Fortunately the power required to run an electric clock
motor is very small, but you will need to use a transformer
to step the voltage up from a low voltage oscillator.

I would guess that a simple LC type oscillator would be
stable enough if run from a stabilised supply. I guess that
it would only be used over a few hours, so cumulative
error would not be a problem.

I would use the step-up transformer as the L part of an
LC oscillator, such circuits are simple and dont have many
components.
Simple emitter follower stabiliser circuits are easy to
build too, and dont have many parts either.
If you take this approach, then set the frequency with the
clock motor connected, as it may alter the tuning slightly.

I would be interested to know the aproximate angle of the
arc you intend to cover.
If its only small, there may be easier ways of doing this.

Regards, John

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