I'm a french Guide of french military heritage. I have to prepare visit about "L'observatoire de la Marine, moulins du faouëdic" (Former windmill which served as an astronomical and meteorological observatory for the French Navy) in Lorient (famous french port).

The refracting telescope was oriented exclusively to the North. So, I've a question :

Is there a reason for the telescope to point north? Why this choice ? How it works ?

Thanks for yours answers ! I'm sorry for my level of English... Have a nice day ! Former windmill

  • $\begingroup$ Do you mean it is fixed to face North ? $\endgroup$ Commented Jan 22, 2022 at 12:50
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    $\begingroup$ It seems that this wasn't a general scientific observatory, but it had a particular job: adjusting chronometers for ships to true astronomical time - that might explain partly why the telescope was fixed North. $\endgroup$
    – James K
    Commented Jan 22, 2022 at 13:05
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    $\begingroup$ Potentially useful link: The port mill marine observatory $\endgroup$ Commented Jan 22, 2022 at 15:23
  • $\begingroup$ What James said. That telescope was probably used to observe the meridian passage of stars and the Sun. See en.wikipedia.org/wiki/Meridian_circle Do you have a photo of the telescope? $\endgroup$
    – PM 2Ring
    Commented Jan 23, 2022 at 2:13

1 Answer 1


(Partial answer)

The telescope in Lorient was not a general scientific instrument for studying the night sky. It had a very practical use: calibrating and synchronizing ships chronometers to terrestrial time.

To accurately measure time, you determine the interval between successive culmination of a star. That is the time when it crosses the Meridian, the line in the sky through the North and South poles. As the time between successive culminations of a star depends only on the rotation of the Earth (and not on its elliptical orbit) it is very regular and can be the basis for very accurate timekeeping.

Ships need accurate chronographs to determine longitude. But the chronographs need to be calibrated, and astronomical observations were used to do that.

But you only need to view the meridian for this application. It probably doesn't matter much if you observe the northerly or southerly meridian. There may have been non-astronomical reasons for having the telescope point North: eg "No window in the south facing part of the building" or "The window in the South holds a gun battery facing the ocean."

So the telescope points north so it can observe the meridian, which is all it needed to do for the specific task that it had.


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