Eight of the Greenwich Instruments are known to have been used with meridian marks. In order of commissioning they are:
Halley’s 5-foot Transit Instrument was the first transit telescope to be used at the Observatory. Its key role was to delineate a meridian and maintain it as a standard of adjustment for the other instruments, most notably the 8-foot Iron Mural Quadrant by George Graham, installed in 1725. Like the later transit instruments it was also used for the determination of time.
The meridian of a transit telescope is established from observations of the circumpolar stars. These stars are always present in the sky and transit (pass over) the meridian twice each (sidereal) day as the Earth spins on its axis. When a transit telescope is correctly aligned, the measured time between successive transits of a given circumpolar star is constant. If a good catalogue was to hand, the right ascensions of a high and a low star could be used instead. Once a transit telescope had been adjusted to the meridian, it was possible to create a mark on the horizon for use as a quick alignment check.
The standard way of checking the collimation (alignment of the optics) of a transit telescope prior to the introduction of collimators in the 19th century was to adjust the telescope to a distant point or mark on the horizon and then reverse it in its mountings. Other things being equal, if the optics were correctly aligned, the mark would still be seen in the centre of the field of view. Reversing the telescope was a time consuming and a potentially hazardous operation. Once a transit telescope had been initially collimated; provided correctly placed meridian marks were available to both the north and the south, there was no further need to reverse the telescope unless, when aligned to one mark, it was out of alignment with the other. By then reversing the telescope, it was possible to determine if the problem was one of collimation or a shifting of one or other of the marks. A further way of checking the collimation independently of any mark (and one occasionally used by Maskelyne) was to reverse the telescope during the passage of Polaris.
There were a variety of factors relating to the setting up, use and maintenance of the marks at Greenwich which the Astronomers Royal had to consider and contend with. These included: