What the First Equatorial Telescope Dome is
The National Astronomical Observatory First Equatorial Telescope Dome, completed in 1921 in Osawa, Mitaka City, Tokyo, is the oldest surviving observing building on the Mitaka campus and became a Registered Tangible Cultural Property in 2002.
It was designed by the building section of Tokyo Imperial University and built by Nishiura Chodayu: a cylindrical reinforced concrete structure of two storeys and 50 square metres, carrying a steel dome. The Agency for Cultural Affairs records the dome as 5.6 metres across, while the observatory's own material gives 6 metres in diameter and 7.8 metres in height.
Under the dome sits a Zeiss refractor of 20 centimetres aperture and 359 centimetres focal length. Its mounting is driven by a falling weight through a clock train, so the telescope tracks a star without a motor.
Why it is registered
The National Astronomical Observatory First Equatorial Telescope Dome was listed on 14 February 2002 under the criterion of being difficult to reproduce, register number 13-0126. The Great Equatorial Telescope Dome was listed on the same day.
It is hard to reproduce because building and instrument were designed as one thing. The roof is opened by hand and the whole dome is turned by hand. Function came first and the shape followed, so copying the outline would not recreate what is inside.
The instrument's working life counts as well. From 1938 the 20-centimetre refractor was used for more than sixty years to sketch sunspots, and research observation ended in 1998.
What to look at
The exterior is symmetrical, with covered walkways projecting on either side of the cylinder. There is almost no ornament. The plainness of the National Astronomical Observatory First Equatorial Telescope Dome comes from the directness of its purpose rather than from any austerity of style.
Inside, it becomes obvious that everything moves by muscle. A member of staff turns a handle and the slit overhead travels round. This is what observing looked like before electric drives.
Look at the base of the telescope for the weight drive. A descending weight is geared down into steady rotation, the same principle as a long-case clock, and it solved the problem of following a star without electricity.
On clear days staff point the refractor at the sun and project the image so that visitors can count sunspots on an instrument that recorded them for six decades.
Visiting
The National Astronomical Observatory First Equatorial Telescope Dome is open inside. It stands on the visitor route and can be entered freely during opening hours, with an audio guide and a sign-language guide available.
When the sky is clear, staff run a free solar viewing session with the 20-centimetre refractor. No booking is needed, but the session pauses from June to September because of the heat, and building work occasionally closes the room.
Photography is allowed for private use. Opening hours, reception and bus access apply to the whole site and are set out on the page for the National Astronomical Observatory of Japan Mitaka Campus.
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Q&A
- Can I go inside the National Astronomical Observatory First Equatorial Telescope Dome?
- Yes. It is on the visitor route and open during campus hours, with audio and sign-language guides.
- What telescope is in the National Astronomical Observatory First Equatorial Telescope Dome?
- A Zeiss refractor of 20 centimetres aperture and 359 centimetres focal length, on a weight-driven clock mounting.
- Can I see the sun at the National Astronomical Observatory First Equatorial Telescope Dome?
- On clear days staff run a free solar viewing session with the refractor, though it pauses from June to September.
- How old is the National Astronomical Observatory First Equatorial Telescope Dome?
- It was completed in 1921, making it the oldest observing building on the campus, and was registered in 2002.
- How big is the dome of the National Astronomical Observatory First Equatorial Telescope Dome?
- The national database gives a steel dome 5.6 metres in diameter; the observatory's own material states 6 metres across and 7.8 metres high.
Basic Information
| Name | National Astronomical Observatory First 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 50 square metres, dome 5.6 metres in diameter |
| Owner | Inter-University Research Institute Corporation National Institutes of Natural Sciences |
| Public Access | Interior open during campus hours |
References
- Agency for Cultural Affairs, National Cultural Properties Database: National Astronomical Observatory First Equatorial Telescope Dome
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00002621
- National Astronomical Observatory of Japan: 20-cm Telescope Dome
- https://www.nao.ac.jp/about-naoj/organization/facilities/mitaka/visit-facilities/20cm-telescope-dome.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
