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Harvard College Observatory History in Images

This is a personal project collecting and documenting early images of Harvard College Observatory, focusing on the site at Observatory Hill (previously Summer House Hill). Buildings, instruments, people, and observations.

Disclaimer: all content here is solely my own views, and in no way represents the views of my employer, or anyone else. Also, I'm documenting things as I learn about them, so expect frequent errors. Corrections will occur without notice and without a changelog at this point.


This page shows a list of all images tagged with "transit".


Earliest source: William Cranch Bond. "Description of the Observatory at Cambridge." Memoirs of the American Academy of Arts and Sciences vol. 4. Metcalf and Company, 1849.academybond

This is a drawing of the Observatory's best transit circle. It's a telescope, but it's mounted such that it can only rotate on a single axis. Most of these instruments were oriented along the meridian, and in that case were also called meridian circles. Meridian circles are used to precisely measure the timing of stars as they transit the meridian.

The Observatory possessed at least two meridian circles, as well as two "prime vertical" telescopes, which are transit circles oriented perpendicular to the meridian, rater than parallel to it.

This transit circle was mounted on one of the permanent piers built for that purpose, shown in other diagrams. It consists of a 4.5 inch refractor, made by Merz (same as the Great Rerfractor). This is mounted on a wheel four feet in diameter, consisting of a cast metal circle on each side, preciesly marked off around the perimeter in five minute increments. Eight "microscopes" mounted on the pier are used to read these markings, and a micrometer (in the eyepiece? or in the microscopes?) is used to accurately determine the direction of the telescope down to one arc-second, and estimate down to one fifth arc-second.

As early as 1844, the Observatory experimented with comparing remote transit times using telegraph connections to other observatories to determine longitudinal difference between locations. Telegraphs (which we now call the Victorian Internet) were a critical part of astronomical studies of that era. The Observatory had at least four permanent telegraph lines by 1887, and six by 1898.

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