What the Great Equatorial Telescope Dome is
The National Astronomical Observatory Great Equatorial Telescope Dome, built in 1926 in Osawa, Mitaka City, Tokyo, became a Registered Tangible Cultural Property in 2002 and now opens to the public as the Observatory History Museum.
It is a reinforced concrete building of two storeys and 238 square metres, its cylinder carrying a copper-clad dome that brings the total height to 19.5 metres. The national database records the dome as 15 metres in diameter; the observatory's own material gives 14.5 metres.
The building section of Tokyo Imperial University designed it and Nakamura Yoshihei built it. Inside is a 65-centimetre Zeiss refractor, the largest refracting telescope in Japan, with a focal length of 10.21 metres.
Why it is registered
Listed on 14 February 2002 under the criterion of being difficult to reproduce, register number 13-0125.
A tube more than ten metres long had to be housed whole, and ordinary building practice could not do it. According to the observatory, the wooden dome was built with help from shipyard engineers. The Agency for Cultural Affairs database instead describes a copper-clad steel dome, so the two accounts of the framing do not agree.
The 65-centimetre refractor was completed in 1929 and remained the largest aperture in the country until the 188-centimetre reflector at the Okayama Astrophysical Observatory arrived in 1960. Its long focus suited precise measurement of stellar positions.
What to look at
From outside, the eye goes to the cornice running round the top of the cylinder, a band of small repeated arches in the Lombard manner, the only modelling on an otherwise plain concrete wall.
Inside, the floor is the remarkable part. The reddish-brown observing floor on the upper level once rose and fell like a lift: as the tube tilted towards the zenith the floor came up, and as it dropped towards the horizon the floor went down, so the astronomer never had to climb. It was fixed in place in 2000.
The telescope in the National Astronomical Observatory Great Equatorial Telescope Dome retired from research in March 1998 and is now preserved in a static state. Standing still, it lets you take in the sheer length of the tube.
The ground floor carries panels on the history of the observatory and a rotating display, changed twice a year, of facsimiles of rare items from its collection.
Visiting
The National Astronomical Observatory Great Equatorial Telescope Dome is open inside as the Observatory History Museum and can be visited freely during campus hours, with separate audio guides for the ground and upper floors.
The interest is split between two levels, so it is worth seeing the ground-floor display first and then climbing to the observing floor, where the full length of the tube reads clearly. Photography is allowed for private use.
Opening hours, the reception procedure and bus access are common to the site and are described on the page for the National Astronomical Observatory of Japan Mitaka Campus.
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Q&A
- What is the National Astronomical Observatory Great Equatorial Telescope Dome used for now?
- Since spring 2001 it has been open as the Observatory History Museum, with displays on the history of the observatory.
- Is the telescope in the National Astronomical Observatory Great Equatorial Telescope Dome the largest in Japan?
- It is the largest refractor in Japan at 65 centimetres, and until 1960 it was the largest telescope of any kind in the country.
- Does the observing floor in the National Astronomical Observatory Great Equatorial Telescope Dome still move?
- No. It once rose and fell like a lift, but it was fixed in position in 2000.
- How tall is the National Astronomical Observatory Great Equatorial Telescope Dome?
- 19.5 metres overall. The dome is recorded as 15 metres across in the national database and 14.5 metres in the observatory's own material.
- Can I take photographs in the National Astronomical Observatory Great Equatorial Telescope Dome?
- Yes, for private use, with the usual request that other visitors not be identifiable in anything you publish.
Basic Information
| Name | National Astronomical Observatory Great Equatorial Telescope Dome |
|---|---|
| Location | 2-21-1 Osawa, Mitaka City, Tokyo |
| Designation | Registered Tangible Cultural Property (building) |
| Year Designated | Registered in 2002 (Heisei 14) |
| Quantity | 1 building |
| Dimensions | Reinforced concrete, 2 storeys, floor area 238 square metres, total height 19.5 metres, dome 15 metres in diameter |
| Owner | Inter-University Research Institute Corporation National Institutes of Natural Sciences |
| Public Access | Interior open during campus hours as the Observatory History Museum |
References
- Agency for Cultural Affairs, National Cultural Properties Database: National Astronomical Observatory Great Equatorial Telescope Dome
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00002620
- National Astronomical Observatory of Japan: Observatory History Museum
- https://www.nao.ac.jp/about-naoj/organization/facilities/mitaka/visit-facilities/observatory-history-museum.html
- National Astronomical Observatory of Japan, Mitaka Campus: Visitor Information
- https://www.nao.ac.jp/about-naoj/organization/facilities/mitaka/visit.html
Last verified: 2026.09.13
