The transit circle took the place both of the old transit instrument and of the mural circle.
If he could not get such an instrument, he measured the positions of the stars with a transit circle.
But observations made with the transit circle must also be corrected for errors in the instrument itself.
The transit instrument was mounted as the transit circle is, between two solid stone piers, and moved in precisely the same way.
The observations with the altazimuth are rather more complicated than those with the transit circle.
The transit circle is turned directly downwards, and the mercury trough brought below it.
It will be remembered that with the transit circle any particular object can only be observed as it crosses the meridian.
In 1681 he took up his duties as astronomer at Copenhagen, and built the first transit circle on a window-sill of his house.
For many years the instruments in regular use were two only, the transit circle and the altazimuth.