Celestial globe

Just as a terrestrial globe shows the distribution of land and water on the Earth's surface, topography, and lines of latitude and longitude, this device treats the sky around us as a sphere and depicts stars, constellations, the celestial equator, ecliptic, time circles, and isoclinations (lines connecting points of equal declination on the celestial sphere. Declination isoclinations), allowing us to read various phenomena on the actual celestial sphere, such as the rising and setting of celestial bodies, as well as their altitude and direction. However, since it is not possible to make a sphere that can be viewed from inside, the celestial sphere is depicted on the sphere as seen from outside (the reverse of what is actually seen). Generally, celestial globes were not simply maps of the sky, but were used for practical purposes. In other words, a model celestial sphere was made to rotate around an axis that passes through the celestial north and south poles (S and N), and this axis was tilted at an angle equal to the latitude to the horizontal ring (the plane containing the horizontal ring was made to pass through the center of the model celestial sphere). If a scale is engraved on a horizontal ring with compass markings, and a scale is made that allows the angle from the horizontal ring to a celestial object to be measured, it is possible to read the altitude, direction, and rise/set direction of a celestial object at any given time. It is also possible to read the time if appropriate conditions are given, such as the state of its diurnal motion or the season. A set of devices made to enable such measurements is called a celestial globe, and in the past, they were actively produced in Europe and elsewhere, not only as practical astronomical instruments, but also as works of art. The first celestial globe was made in Japan during the Edo period.

[Naoaki Owaki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

地球儀が地球表面の水陸分布や地形、経緯度線などを表すように、われわれを取り巻く空を一つの球に見立て、星や星座、天の赤道や黄道、時圏、等赤緯線（天球上の赤緯の等しい点を結ぶ線。赤緯等圏）などを描き表し、実際の天球での諸現象、たとえば天体の出没や高度・方位などを読み取れるようにした装置。ただし、球の内側から見るようにはつくれないので、天球の外から見た天球の状況（現実に見るものの裏返しになる）を球面に描く。一般に天球儀は、単なる天の図ではなく、実用に供されていた。すなわち、模型天球を、天の南北極（SおよびN）を通る軸の周りを回転できるようにし、この軸が水平環（水平環を含む平面は模型天球の中心を通るようにする）と緯度に等しい角だけ傾けられるようにしておく。水平環に方位目盛りを刻み、さらに水平環から天体までの角を測れるような尺度をつくっておくと、天体の任意の時刻の高度・方位、出没方位が読み取れる。また日周運動の状況、あるいは季節など適当な条件を与えれば時刻を読み取ることもできる。このような測定ができるようにつくられた一式の装置が天球儀で、かつてはヨーロッパなどで天文学上の実用器機としてはもとより、芸術品としても盛んに製作された。日本でも江戸時代に最初の天球儀がつくられた。

［大脇直明］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例