What the First Meridian Mark Room is
The National Astronomical Observatory First Meridian Mark Room, built in 1925 about 100 metres south of the Gautier Meridian Circle Building in Osawa, Mitaka City, Tokyo, is a two-storey reinforced concrete structure covering just 7.3 square metres, registered as a Registered Tangible Cultural Property in 2014.
What it holds is a meridian mark. To measure star positions with a meridian circle, an observer needs a fixed target that proves the telescope is truly aligned north and south. This little building keeps that target at a known distance from the instrument.
So the National Astronomical Observatory First Meridian Mark Room is not a room for people. It is a case for a reference point, which is why its floor area is less than a thirtieth of the great dome up the hill.
Why it is registered
Listed under the criterion of being difficult to reproduce, register number 13-0332.
Most of the value lies in the position. The distance and the bearing are the point, and the same box built anywhere else would not be a meridian mark room. Its job is to hold the accuracy of the north-south axis on which the meridian circle depends.
The north face is a triangular retaining wall with earth banked against the back. The mark itself sits on the upper level, reached by a stair attached to the front of the retaining wall. That is a good deal of engineering for 7.3 square metres.
What to look at
The scale is the first thing. A building nobody was ever meant to live or work in was still framed in reinforced concrete over two levels.
The triangular retaining wall on the north takes the banked earth behind it, so the meeting of structure and ground is handled quite unlike an ordinary building.
It is identical to the Second Meridian Mark Room, and the two sit symmetrically north and south of the meridian circle. Knowing that its twin stands 100 metres away is what turns the National Astronomical Observatory First Meridian Mark Room from an odd little box into one component of an instrument.
Visiting
The National Astronomical Observatory First Meridian Mark Room is not among the facilities open to visitors and lies off the marked route. The interior is closed.
Visitors to the campus are asked to keep to the route and to stay out of the undergrowth, where mosquitoes, wasps and snakes are a real concern, so there is no approaching this one on foot.
From the western part of the route, look toward the Gautier Meridian Circle Building and picture the two mark rooms set north and south of it; the layout of the grounds makes much more sense that way. Opening hours and the reception procedure are covered on the page for the National Astronomical Observatory of Japan Mitaka Campus.
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Q&A
- What is the National Astronomical Observatory First Meridian Mark Room for?
- It houses the meridian mark that serves as the reference point for the meridian circle, holding the accuracy of its north-south axis.
- Can I visit the National Astronomical Observatory First Meridian Mark Room?
- No. It is not among the facilities open to visitors and lies off the marked route, with no interior access.
- Where is the National Astronomical Observatory First Meridian Mark Room?
- About 100 metres south of the Gautier Meridian Circle Building on the Mitaka campus.
- How big is the National Astronomical Observatory First Meridian Mark Room?
- It has a floor area of 7.3 square metres over two reinforced concrete levels, among the smallest of the ten registered entries.
- When was the National Astronomical Observatory First Meridian Mark Room registered?
- On 25 April 2014, with the register number 13-0332.
Basic Information
| Name | National Astronomical Observatory First Meridian Mark Room |
|---|---|
| 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, 2 levels, floor area 7.3 square metres |
| Owner | Inter-University Research Institute Corporation National Institutes of Natural Sciences |
| Public Access | Closed; outside the visitor route |
References
- Agency for Cultural Affairs, National Cultural Properties Database: National Astronomical Observatory First Meridian Mark Room
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00009917
- National Astronomical Observatory of Japan, Mitaka Campus: Facilities Open to Visitors
- https://www.nao.ac.jp/about-naoj/organization/facilities/mitaka/visit-facilities/
- 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
