What the Solar Spectrograph Building is
The National Astronomical Observatory Solar Spectrograph Building, completed in 1930 in Osawa, Mitaka City, Tokyo, is a tower-shaped observing structure that became a Registered Tangible Cultural Property in 1998. It is widely known as the Einstein Tower.
The tower is the telescope. Sunlight enters the dome at the top, is turned downward by a coelostat of two flat mirrors 60 centimetres across, falls the height of the shaft and is split into a spectrum in a half-basement darkroom extending north. The optical path runs 22 metres.
The building section of Tokyo Imperial University designed it and the Nakamura firm built it. The national database records reinforced concrete, a single storey and a floor area of 331 square metres; the observatory's own material describes five floors above ground and one below, with the tower about 20 metres high, so the two accounts differ. The half-basement spectrograph room was finished in 1926 and the tower in 1930.
Why it is registered
Registered on 23 July 1998 under the criterion of being difficult to reproduce, register number 13-0010. It was the first of the ten Mitaka entries to be listed.
The purpose of the building was to test the shift in solar spectral lines predicted by general relativity. Because it was planned with the same aim as the tower telescope at the Potsdam astrophysical observatory in Germany, the National Astronomical Observatory Solar Spectrograph Building carries the nickname Einstein Tower.
Structure and optics cannot be separated here, which is exactly what makes it irreproducible. Inside the shaft, the instrument room runs the full height while remaining structurally isolated from the building around it, with stairs and preparation rooms wrapped about it. Copying the outline would leave out the point.
The optics were rebuilt into their present form in 1957 around a 48-centimetre Cassegrain reflector, and research observation ended in 1968.
What to look at
The walls are clad in brown Scratch tile. Firing produced uneven colour, and the tiles were deliberately mixed so that no two neighbours quite match. It is worth standing close enough to see it.
The massing is all straight lines and squared blocks, except at the entrance, the canopies and the rooftop balcony, where the designer allowed curves. Three soft moments in an otherwise hard object.
The dome on top is about 5 metres across. It looks modest from the ground, until you remember that the light it gathers falls 20 metres inside the shaft below.
Follow the low wing running north from the National Astronomical Observatory Solar Spectrograph Building. Seeing the tower and that half-buried spectrograph room as one continuous object is what makes the description of the whole building as a single telescope click into place.
Visiting
The National Astronomical Observatory Solar Spectrograph Building is normally seen from the outside only; the interior is not part of the standard route. Walking round all four sides is the way to take in the tilework and the placing of the curves. An audio guide is available.
The exterior opened to visitors in 2000, and since 2010 guided viewing of the interior has been offered on occasion. These openings are not continuous, so anyone hoping to get inside should check in advance.
Photography along the route 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
- Why is the National Astronomical Observatory Solar Spectrograph Building called the Einstein Tower?
- It was built mainly to test a prediction of relativity, following the tower telescope built for the same purpose at Potsdam in Germany.
- Can I go inside the National Astronomical Observatory Solar Spectrograph Building?
- The standard visit is exterior only. Guided interior viewings have been offered since 2010, but not continuously.
- How does the National Astronomical Observatory Solar Spectrograph Building work as a telescope?
- A 60-centimetre coelostat at the top sends sunlight straight down the shaft to a half-basement darkroom on the north side, where it is dispersed. The tower acts as the tube.
- When was the National Astronomical Observatory Solar Spectrograph Building built?
- The half-basement spectrograph room dates from 1926 and the tower from 1930. It was registered in 1998.
- Is the National Astronomical Observatory Solar Spectrograph Building still used for research?
- No. Research observation ended in 1968 and the building is preserved as a registered cultural property.
Basic Information
| Name | National Astronomical Observatory Solar Spectrograph Building |
|---|---|
| Location | 2-21-1 Osawa, Mitaka City, Tokyo |
| Designation | Registered Tangible Cultural Property (building) |
| Year Designated | Registered in 1998 (Heisei 10) |
| Quantity | 1 building |
| Dimensions | Reinforced concrete, single storey, floor area 331 square metres, tower about 20 metres high |
| Owner | Inter-University Research Institute Corporation National Institutes of Natural Sciences |
| Public Access | Exterior only; the interior is not normally open |
References
- Agency for Cultural Affairs, National Cultural Properties Database: National Astronomical Observatory Solar Spectrograph Building
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00000565
- National Astronomical Observatory of Japan: Solar Tower Telescope
- https://www.nao.ac.jp/about-naoj/organization/facilities/mitaka/visit-facilities/solar-tower-telescope.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
