What the Gautier Meridian Circle Building is
The National Astronomical Observatory Gautier Meridian Circle Building, completed in 1924 in Osawa, Mitaka City, Tokyo, is a single-storey reinforced concrete structure of 129 square metres with a sheet-metal roof, registered as a Registered Tangible Cultural Property in 2014.
A meridian circle measures the right ascension and declination of a star at the moment it crosses the meridian, and it turns accurately in the north-south plane alone. The building was therefore set with its axis exactly on north and south.
The Gautier instrument it houses was made in France in 1903 and bought in 1904 for about 20,000 yen, a considerable sum at the time. It has an aperture of 20 centimetres and a focal length of 310 centimetres. After trial use at Azabu it became the observatory's principal instrument once the move to Mitaka was made in 1924.
Why it is registered
Listed under the criterion of being difficult to reproduce, register number 13-0331.
The essential trick is that the instrument never touches the building. The meridian circle stands on its own central foundation, isolated from the walls, so that movement in the structure cannot reach the optics. Foundations and side walls are reinforced concrete, the roof is steel, and the centre of the roof opens for observation.
The instrument has a history of its own. When the Great Kanto Earthquake struck in 1923 the Gautier circle was already crated for the move and came through undamaged. It stepped back from front-line work in 1982 when an automatic photoelectric meridian circle was built, but from 1992 it was fitted with a CCD micrometer and spent about a decade measuring the positions of faint objects such as quasars, finishing around 2000.
What to look at
Seen head on, the elevation is a half circle, and a trapezoidal roof over the entrance is planted against it. The design interest of the National Astronomical Observatory Gautier Meridian Circle Building lies in that deliberate collision of a curve and a wedge.
There are two doors, east and west. The north-south axis was spent entirely on observation, so people were sent in from the sides. The circulation reveals what the building was arranged around.
The roof is steel and opens down the middle. From outside, look for the seam running across the top.
A curved wall registers light as a gradient rather than as flat planes. On a clear afternoon the shadow across it visibly moves.
Visiting
The National Astronomical Observatory Gautier Meridian Circle Building is one of the facilities on the visitor route, with an audio guide available.
Standing in front of the entrance puts both the half-round elevation and the trapezoidal porch roof in view at once. Step back a little and the way the two shapes meet becomes clearer.
Photography along the route is allowed for private use. Opening hours, reception and bus access are common to the site and are set out on the page for the National Astronomical Observatory of Japan Mitaka Campus.
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Q&A
- Where was the instrument in the National Astronomical Observatory Gautier Meridian Circle Building made?
- In France, in 1903, and bought the next year for about 20,000 yen. It has a 20-centimetre aperture and a focal length of 310 centimetres.
- Why is the National Astronomical Observatory Gautier Meridian Circle Building half round?
- Because observation was confined to the north-south plane, the elevation and the opening were shaped to that movement. The centre of the roof opens for observing.
- How long was the instrument in the National Astronomical Observatory Gautier Meridian Circle Building used?
- It left front-line work in 1982, returned with a CCD micrometer in 1992 for about ten years of faint-object astrometry, and finished around 2000.
- When was the National Astronomical Observatory Gautier Meridian Circle Building registered?
- On 25 April 2014, with the register number 13-0331.
- Is the National Astronomical Observatory Gautier Meridian Circle Building on the visitor route?
- Yes. It is one of the facilities open to visitors and has an audio guide.
Basic Information
| Name | National Astronomical Observatory Gautier Meridian Circle 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 129 square metres |
| Owner | Inter-University Research Institute Corporation National Institutes of Natural Sciences |
| Public Access | On the visitor route |
References
- Agency for Cultural Affairs, National Cultural Properties Database: National Astronomical Observatory Gautier Meridian Circle Building
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00009916
- National Astronomical Observatory of Japan: Gautier Meridian Circle
- https://www.nao.ac.jp/about-naoj/organization/facilities/mitaka/visit-facilities/gautier-meridian-circle.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
