What the Repsold Transit Instrument Building is
The National Astronomical Observatory Repsold Transit Instrument Building, put up in 1925 in Osawa, Mitaka City, Tokyo, is a single-storey reinforced concrete structure of 36 square metres with a sheet-metal roof, registered as a Registered Tangible Cultural Property in 2014.
A transit instrument is a telescope that observes stars as they cross the meridian, and from those crossings it fixes the longitude of the site and the time of day. It swings accurately in the north-south plane only, and the building that holds it is set on the same axis.
The Repsold instrument inside was made in Germany in 1880, with an effective aperture of 135 millimetres and a focal length of 2,120 millimetres. In the observatory's Azabu years it determined time and measured longitude, and the spot where it stood is counted as one of the origins of astronomical longitude in Japan. The building is now open as a small museum of transit instruments.
Why it is registered
Listed under the criterion of being difficult to reproduce, register number 13-0330.
The roof is what makes it so. The steel roof frame sits on rails, and at observation time the gable splits at the ridge and slides apart to clear the sky above the instrument. The roof keeps out the weather and then gets out of the way.
The observations made here produced foundational data for Japanese astronomy. Time determined at Azabu set the noon gun fired from the site of the Edo Castle keep; after the move to Mitaka the instrument was used from 1935 to fix the right ascensions of the moon, the planets and then the stars, work that produced the Mitaka zodiacal star catalogue of 1949 and the Mitaka equatorial catalogue of 1962.
What to look at
Outside, look at the tops of the pilasters and the canopy over the door, where straight-line patterns appear. This is Secession detailing of the early twentieth century, and it is the only ornament the building allows itself.
Find the line along the ridge where the two roof slopes meet. That is the joint that opens.
Inside, transit instruments of several kinds stand together. The National Astronomical Observatory Repsold Transit Instrument Building is now less the house of one telescope than a room showing how the type developed.
The Repsold itself was bought in 1881 for 15,200 marks. Few visitors get this close to a working scientific instrument more than 130 years old.
Visiting
The National Astronomical Observatory Repsold Transit Instrument Building is open inside as a transit instrument museum and can be entered freely during campus hours, with an audio guide available.
It is a small room, so at busy times visitors go through in turns. Reading the labels as you circle the instruments takes about fifteen minutes.
Photography is allowed for private use. Opening hours, reception 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
- Can I go inside the National Astronomical Observatory Repsold Transit Instrument Building?
- Yes. It is open as a museum of transit instruments during campus hours, with an audio guide.
- Where was the instrument in the National Astronomical Observatory Repsold Transit Instrument Building made?
- In Germany, by the Repsold firm, in 1880. It has an effective aperture of 135 millimetres and a focal length of 2,120 millimetres.
- Does the roof of the National Astronomical Observatory Repsold Transit Instrument Building really open?
- Yes. The steel roof frame runs on rails and the gable parts at the ridge to clear the sky above the instrument.
- When was the National Astronomical Observatory Repsold Transit Instrument Building built?
- In 1925. It was registered as a cultural property in 2014.
- What was observed at the National Astronomical Observatory Repsold Transit Instrument Building?
- Time and longitude in the Azabu years, then from 1935 the right ascensions of the moon, planets and stars, work behind the Mitaka star catalogues of 1949 and 1962.
Basic Information
| Name | National Astronomical Observatory Repsold Transit Instrument Building |
|---|---|
| Location | 2-1007-1 Osawa, Mitaka City, Tokyo |
| Designation | Registered Tangible Cultural Property (building) |
| Year Designated | Registered in 2014 (Heisei 26) |
| Quantity | 1 building |
| Dimensions | Reinforced concrete, single storey, sheet-metal roof, floor area 36 square metres |
| Owner | Inter-University Research Institute Corporation National Institutes of Natural Sciences |
| Public Access | Interior open during campus hours as a transit instrument museum |
References
- Agency for Cultural Affairs, National Cultural Properties Database: National Astronomical Observatory Repsold Transit Instrument Building
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00009915
- National Astronomical Observatory of Japan: Transit Instrument Museum
- https://www.nao.ac.jp/about-naoj/organization/facilities/mitaka/visit-facilities/repsold-transit-instrument.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
