Stand beneath the magnificent onion dome and see the Greenwich Great Equatorial Telescope, one of the biggest telescopes in the world.
The 28-inch Greenwich refracting telescope is the largest of its kind in the UK and the seventh largest in the world. Completed in 1893, it was commissioned in 1885 by William Christie, Astronomer Royal between 1881 and 1911.
It was built to research double star systems and remained in use until the late 1960s. With the recent addition of a computer-aided guidance system and CCD camera, it continues to work as an excellent visual aid to observing the night sky.
See for yourself
You can visit the telecope for free at the Royal Observatory Greenwich, or book for one of our night sky observation evenings during the winter months. Booking in advance is advised as these events are very popular.
The distinctive onion dome was designed specifically to house the Great Equatorial Telescope. At its widest, the dome bulges out about 5 feet (1.5 metres) beyond the tower walls on which it sits, and the iron frame used to be covered with papier mâché. It was damaged by an air raid in October 1940 and stripped of its covering by another bomb in July 1944. The present dome is a fibreglass replica of the 1893 dome, installed in 1971.
The telescope’s lens has a diameter of 28 inches (71 cm) and weighs nearly 200 lbs (91 kg). It took eight years to make and was a great success in Royal Observatory’s programme of double star observations. It rests on a mount built 30 years earlier for a smaller telescope. The telescope tube is over 28 feet long and is round at each end but rectangular in the middle. This is because its mount was built for a much smaller telescope, so the middle section of the tube had to be tapered in order to fit. Attached to the outside of the tube are smaller telescopes, used as guides to target an object or to magnify the telescope’s scales. The mount allows the telescope to be rotated on the same axis as the Earth, so it can follow a star from east to west across the sky.
War disprupts observations
Twice in its history, observations with the twenty-eight-inch refractor have been disturbed. The first time was during the First World War. Then, in 1939, the valuable object glass was sent to a place of safety during the Second World War. This was just as well since the Observatory was damaged by bombing and in 1944 the covering of the dome itself was stripped off by a V1 flying bomb.
In 1947, the telescope was dismantled and sent to Herstmonceux, leading the departure of the astronomers from Greenwich. It was fully operational there from 1957 to 1970, but was then 'retired' and sent back to Greenwich to mark the Observatory's 300th anniversary in 1975.