The Altazimuth Pavilion was constructed between 1894 and 1896 to the design of William Crisp who also designed the nearby New Physical Building (now known as the South Building). It was grade II listed on 21 August 2002.The Pavilion was built to house the new Altazimuth Instrument proposed by Christie in 1892. Of solid construction, and of two floors, it was erected in the only space that Christie had available, which was on the South Ground just to the north of the Magnetic House. To avoid excessive interference with the magnetic observations, the site selected was on the magnetic meridian of the declination magnet and 90 feet to the north of it. The centre of the building is 141 feet east and 166 feet south of the Airy Transit Circle, with the telescope being mounted 20 feet above the ground.
Like the South Building, it is constructed of red brick and terracotta with a ring of porthole like windows to light the observing floor inside. Christie sometimes referred to the telescope as a universal transit circle, and to this end, as well as the main telescope, there were collimating telescopes to the north and south side of the building which could be swung into position from beneath the movable roofs that covered them.
The building has tunnel like entrances to the north and south that sit beneath the supporting masonry for the collimators. Most of the internal space is taken up by the massive supporting column for the main telescope. This sits on a six foot deep concrete foundation on which the whole of the building rests.
The terracotta decoration was limited by Crisp to the porticos surrounding the two entrances. The dome was built by Sir Howard Grubb and opened in a quite different way to any of the others at Greenwich. It had no shutters. Instead, the two halves of the dome could be slid outwards to create a 4½ foot wide opening. The weathervane represents Halley's Comet and was added in June 1901.
The Altazimuth Instrument was never a success. It was dismantled by Spencer Jones at the start of WW2 and replaced with a new instrument to the observatory – a small transit instrument by Cooke Troughton and Simms that had been made for time determinations in Singapore, but never used.
The building was struck during the blitz on the night of 21–22 October 1940. This destroyed the southern portico as well as stripping the dome of its covering. The small transit instrument inside was thrown from its mounting and broken in half. Its object glass however was undamaged. There was also shrapnel damage to the brickwork. The large gap over the south portico was covered in 1941/2 with a wooden framework covered with felting. The dome, (some of the ribs of which still show signs of war damage), was recovered with papier mâché in 1945/6, and the building repaired in 1947 (which included replacement of the shrapnel damaged bricks). Only a small amount of the terracotta was replaced, the damaged terracotta being disposed of by tipping it down the hillside immediately to the west of the building. Pieces remained visible until 2009 when the gardens there were refurbished.
After the announcement had been made in 1946 that the Observatory would be moving to Herstmonceux, the Ministry of Works, to whom the site was to revert on behalf or the Crown, drew up plans to return the southern half of the Observatory site to the Park. Under this plan, only two buildings were to escape demolition, the South Building Store which was destined to become a public lavatory and the Altazimuth Pavilion which was scheduled to be adapted as a shelter by the opening up of the windows on both the east and the west side. Its future looked in doubt when during a ministerial visit that took place on 22 April 1958; Hugh Molson (the minister) indicated that he wanted it demolished.
In the end, the building was retained and passed into the care of the National Maritime Museum where it was adapted for use both as a shelter and as a new home for the Sheepshanks Equatorial. To this end, the staircase was retained and only the western half adapted as a shelter. The dome was re-clad in fibreglass somewhere around 1965. The Sheepshanks Equatorial remained in place from 1963 until 1982, when it was replaced by the 4-inch Dallmeyer photoheliograph, and the Newbegin 6¼-inch refractor following their discontinued use at Herstmonceux.
The dome was (and still is) opened and turned by hand. There does not appear to have been any mechanism to prevent the wind from blowing it shut, or causing it to rotate. Whilst undoubtedly irritating, this would not have damaged the original Altazimuth Instrument, but the present instruments are too large for the dome and are vulnerable to being knocked by the internal structure, not only if the wind catches the dome, but also when they are being manipulated into position.
During the construction of the Peter Harrison Planetarium, the ground floor of the Altazimuth Pavilion was remodelled and the windows on the west side reinstated.