The Great Equatorial Telescope

The Great Equatorial Telescope at the Royal Observatory kept Britain at the forefront of astrophysics and greatly expanded our knowledge of stars.

"How many ecstatic hours does the astronomer spend at the eye-end, high up on his scaffold-like observing chair, communing with other worlds during the darkest hours of the night? No wonder, then, that the making of colossal 'equatorials' should be replete with wondrous incident, and the details of their history almost beyond belief."

Fitzgerald, Strand Magazine, 1896

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 designed to keep the Royal Observatory at the forefront of contemporary astronomy and more active in the growing disciplines of astrophysics and photography. The job of constructing the telescope was given to Howard Grubb, an Irish optical manufacturer who was then the world leader in the field.

Although the telescope was removed from Greenwich to Herstmonceaux in 1957, it was used for research into double star systems throughout its working life until its retirement in the late 1960s. It was returned to Greenwich in 1971, and has become a central part of educational programmes at the Royal Observatory. 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.

A revolutionary design

The 28-inch lens was to be of revolutionary design, allowing the telescope to perform as an observational and photographic instrument but that proved to be a flawed idea in practice. After detailed testing, the proposed role of the Great Equatorial Telescope was changed to double star measurement and over the next 70 years it completed many successful observation programmes.

The mount

The mount on which the Great Equatorial Telescope now sits was built for an earlier instrument and predates it by over thirty years. The mount is an 'English equatorial' type, with the telescope mounted on an axis tilted parallel to the Earth's axis of rotation. The telescope can then follow a star from east to west across the sky merely by rotating the mount.

The telescope

The telescope tube is over 28 feet long and passes between the central spokes of the mount. The mount was not originally intended for such a large telescope and it was necessary to taper the middle section of the tube in order to attach it. This is why the telescope has the curious characteristic of being round at each end but rectangular in the middle.

The Onion Dome

The original dome construction was a riveted iron frame covered with papier mache. It was damaged by an air raid in October 1940 and was stripped of its covering when a V1 flying bomb fell in Greenwich Park on 15 July 1944. The telescope was removed from the dome in 1947, and the dome was dismantled in 1953. The present dome is a fibreglass replica of the 1893 dome, installed after the 28-inch returned to Greenwich in 1971.

Double star observing

Double stars are stars that share a common centre of gravity and are in orbit around that centre. Little was known about the nature of stars in the 19th century and the idea that they could be associated physically as well as visually was new. Today it is known that at least 50% of stars are binary stars in orbit around one another.

Measurement of the orbits of binary stars allows their masses to be determined by applying Newton's Law of Gravitation. This is the only method by which astronomers can directly measure the masses of stars.

The 28-inch was moved to Herstmonceaux in 1957, but continued to be involved in double star programmes on and off until its retirement from research in the late 1960s.

The Royal Observatory is open daily from 10am

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