The Monument - London, UK
Posted by: 
marcius
N 51° 30.608 W 000° 05.150
30U E 702215 N 5710582
The Monument was designed as a type of telescope known as a zenith telescope, used to observe stars that pass directly overhead.
Waymark Code: WM16G23
Location: London, United Kingdom
Date Posted: 07/25/2022
Views: 7
This large column was erected next to the Thames as a memorial to the Great Fire of London of 1666, and was completed in 1677. But what is not so well known is that its designers, Christopher Wren and Robert Hooke, intended it to be used as a telescope.
Wren and Hooke hoped that by looking at one star in particular they might be able to detect stellar parallax - a change in the position of an observed object caused by a change in the observer's position. This was something that should be observed if the Earth was moving around the Sun, but astronomers had not yet been able to detect it. In fact, it was only in the nineteenth century that stellar parallax was finally observed.
Here, the whole structure was the telescope. The observer sat in a room in the basement and looked up through the 'tube' created by the spiral staircase. The flaming urn on top had a hinged lid that opened for viewing. Sadly, it didn't prove to be up to the job because it expanded and contracted in different temperatures and swayed in the wind.
SOURCE - (Royal Museums Greenwich)
Zenith telescope
A zenith telescope is a type of telescope that is designed to point straight up at or near the zenith. They are used for precision measurement of star positions, to simplify telescope construction, or both.
A classic zenith telescope, also known as a zenith sector employs a strong altazimuth mount, fitted with levelling screws. Extremely sensitive levels are attached and the telescope has an eyepiece fitted with a micrometer. They are used for the measurement of small differences of zenith distance, and used in the determination of astronomic latitude.
Other types of zenith telescopes include the Monument to the Great Fire of London, which includes a central shaft meant for use as a zenith telescope. High-precision (and fixed building) zenith telescopes were also used until the early 1980s to track Earth's north pole position e.g. Earth's rotation axis position (polar motion). Since then radio astronomical quasar measurements (VLBI) have also measured Earth's rotation axis several orders of magnitude more accurately than optical tracking.
The NASA Orbital Debris Observatory, which used a 3 m diameter aperture liquid mirror, and the Large Zenith Telescope, which uses a 6 m diameter aperture liquid mirror, are both zenith telescopes, as the use of liquid mirror meant that these telescopes could only point straight up.
SOURCE - (Wikipedia)
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