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Chronograph at the Detroit Observatory in Ann Arbor September 27, 2013

Posted by aquillam in Astronomy.

The chronograph is a very precise instrument for recording time. It can be used to ensure that your clocks are running at the right rate. They were commonly used to trigger the drop of a time ball. This one has a telegraph paddle on the bottom, and may have been used to send the time signal to the Michigan Central railroad terminal down by the river (now the Gandy Dancer restaurant.) However, a lot of things in the building have telegraph paddles, but few lines, so it is difficult to say if this was actually used to send the signal.


It is located right next to the Meridian Circle room at the Detroit Observatory. In the 19th century, an observer using the transit telescope could send a signal to the chronograph. This would make a little tick mark on paper. The sidereal clock in the transit room would also send a signal, which would make a smaller very regular tick mark. The paper on the chronograph now is a replica of one made by Director Franz Brunnow on April 25, 1861. Just above where the pen is sitting, the paper is labeled Tau Virginis.




Astronomers could do one of two things with this paper: check the accuracy of the chronograph (and the clock in the transit room), or measure the position of a star. Since those two things are somewhat complicated, I’ll leave them to another post.

Both the telegraph and the button in the transit room required electricity, so there was an open cell battery under the cabinet.

In the top image on the right is an odd looking brass thing with two round weights attached to crossed arms. That is the governor, and it controls how fast the chronograph runs. The weights are not symmetric, so the speed of the governor is changed by rotating the weights, which are marked so you can tell how much you’ve moved them. Like any good observatory, the Detroit Observatory was not heated. Brass is affected by temperature, so the temperature would have to be measured regularly during the night, and the temperature changes would have to go into the calculations for checking the time and the stars’ positions.

I haven’t been able to get a look at the gearing system to see how the roll and pen work.



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