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The great instrument makers of C18 and C19, in striving to produce
perfectly divided circles, were very much conscious of the effects of
temperature on their machines and instruments. In dividing or
measuring to a precision of a second or two, even the heat of the
observer's body was noted to have an effect. I cannot say what was the
magnitude of the effect but can give an example of a little experiment
I carried out to give an idea of what can happen. At an ambient
temperature of 23 degrees I left my fingers(skin temp ca. 32 degrees)
in contact with one side of an autocollimator tube for  fifteen
seconds. This produced an easily measurable deflection of the a/c axis
of about 1.5 seconds.

An excellent book on the history of dividing, with many references to
relatively easy-to-obtain primary sources is Chapman, Allan. Dividing
the circle:the development of critical angular measurement in
astronomy 1500 - 1850. Chichester 1990. ISBN 0-13-217555-X

Bill

On Jan 29, 8:20�am, Jim  wrote:
> Great post.
>
> You mentioned:
>
> "The instruments need to be given time to reach
> the ambient temperature, overnight if possible,"
>
> Does anyone have a sense of how much error temperature can induce?
>
> Thanks,
>
> Jim H.
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