What the Second Meridian Mark Room is
The National Astronomical Observatory Second Meridian Mark Room, built in 1925 about 100 metres north of the Gautier Meridian Circle Building in Osawa, Mitaka City, Tokyo, is a two-storey reinforced concrete structure of 7.2 square metres, registered as a Registered Tangible Cultural Property in 2014.
It is the twin of the First Meridian Mark Room, placed symmetrically on the other side of the meridian circle. One to the south, one to the north, so that a reference target lies in view whichever way the instrument is turned.
The mark inside the National Astronomical Observatory Second Meridian Mark Room rests on a reinforced concrete foundation that is structurally separated from the building around it. The walls and roof may shift; the mark does not.
Why it is registered
Listed under the criterion of being difficult to reproduce, register number 13-0333, one digit away from its twin.
The Agency for Cultural Affairs describes it as a structure that protects a precision instrument from the natural conditions an observatory has to live with. Keeping an outdoor reference point rigidly in place through wind, rain and temperature swings is the problem this shape answers.
Structural isolation runs through the whole campus. The meridian circle stands on its own foundation, the solar tower keeps its instrument room separate from the shell around it, and the mark in the National Astronomical Observatory Second Meridian Mark Room is protected the same way. The idea is concentrated into 7.2 square metres here.
What to look at
From outside there is a concrete box half sunk into the ground and a stair attached to it. No ornament at all.
Compare it with its southern twin and the shapes turn out to be almost identical: 7.2 square metres against 7.3, a difference of 0.1 square metres. They were designed as a near-symmetrical pair.
The meaning is not in the object but in where it stands. Draw a line on the map through the three points and the line is the meridian.
Visiting
The National Astronomical Observatory Second Meridian Mark Room is not among the facilities open to visitors and lies off the marked route. The interior is closed.
Stepping off the route or into the undergrowth is not permitted, so there is no close approach. Locating it on the campus map, among the trees to the north, is as far as a visit goes.
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 is the National Astronomical Observatory Second Meridian Mark Room?
- About 100 metres north of the Gautier Meridian Circle Building on the Mitaka campus, with the First Meridian Mark Room the same distance to the south.
- Can I visit the National Astronomical Observatory Second Meridian Mark Room?
- No. It is off the marked route, the interior is closed, and visitors may not leave the route to approach it.
- How does the National Astronomical Observatory Second Meridian Mark Room differ from the first?
- Barely at all. The floor areas are 7.2 and 7.3 square metres, and the two sit symmetrically north and south of the meridian circle.
- How is the mark in the National Astronomical Observatory Second Meridian Mark Room mounted?
- On a reinforced concrete foundation that is structurally separated from the building, so movement in the structure does not reach it.
- When was the National Astronomical Observatory Second Meridian Mark Room registered?
- On 25 April 2014, with the register number 13-0333.
Basic Information
| Name | National Astronomical Observatory Second 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 7.2 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 Second Meridian Mark Room
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00009918
- 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
