PLANETARIUM is the name given to an arrangement, made by Zeiss of Jena, for producing an artificial sky. By optical methods images of the sun, moon, planets and stars are projected on a large hemispherical dome and by mechanical and electrical means the apparatus can be revolved so as to show the principal motions. The name planetarium is apparently given because of the success with which the apparent motions of the planets amongst the stars can be shown ; but the apparatus makes possible the representation of many other astronomical phenomena.
For several centuries orreries have been used to demonstrate the motion of the planets round the sun according to the Coperni can system. Perhaps the best arrangement of this kind is in the German Museum at Munich where the observer can be carried in a cage which is moved in a path representing the earth's orbit. By means of a periscope he can look out on the motions of the other planets and at the stars of the zodiacal constellations shown on the wall by means of small electric bulbs. But it is impossible in a model of this nature to make the relative distances correct and thus to get the proper alignment of the various bodies. When this instrument was ordered from Zeiss in 1913 it was suggested that another astronomical model might be made depicting the heavens as actually seen from the earth and it was suggested that the heavenly bodies might be represented by electric bulbs fixed or moving on a rotating dome. This suggestion, however, was soon discarded in favour of another, due to Dr. Bauersfeld, in which the images of the bodies were projected optically on a fixed dome. The resulting apparatus has been a great success. It is found that when a dome soft. or more in diameter is used, the appearance is extremely like that of the actual sky. For the stars it is merely necessary to have the projector mounted so that it can be rotated about an axis parallel to the earth's axis. For the sun, moon and planets small models, true to nature, are constructed. The angles between the orbit planes are allowed for and by the use of eccentric circular motion the elliptic motion is quite closely represented. For showing say the planet Venus, the model has pins representing Venus and the earth moving round the sun. A little projector is attached to the pin representing Venus and a linkage between the planets always directs the light away from the earth. Consequently, the image of the planet on the dome is in the correct position amongst the stars. The gearing is so arranged that the mean motions are accurately represented and only comparatively slight periodic errors affect the positions of the sun, moon and planets. The phases of the moon are shown by an occulting device.
The planetarium can be operated at varying speeds corre sponding to the phenomenon to be demonstrated so that even such a slow motion as that of the procession of the equinoxes is within its range. To show the rising and setting of the heavenly
bodies a day is compressed into an interval of from 1 to 4 minutes; to show that of the planets the diurnal motion can be put out of gear and a year run off in intervals varying from 7 seconds to 4 minutes. The original model was made for one latitude only, but in later ones, by introducing an additional axis of rotation, the latitude can be altered at will. The stars are shown brighter than they appear in the sky—the eyes of the spectators coming from the daylight would otherwise take too long to be sensitive enough to see them. It is, of course, im possible to make the sun as bright as in nature but this has the advantage of enabling the stars to be seen in the sky along with the sun and thus demonstrate the motion of the sun amongst the stars.
With the planetarium the spectator can see in an hour or so motions which can only be seen in the heavens by years of watching. When a year is compressed into a minute or two the planets can be seen chasing one another about the sky, describing loops, advancing and retrograding, just as they do so slowly in nature. Great attention has been paid to details so as to make the apparatus as perfect as possible; e.g., the brightness of the objects is dimmed as they approach the horizon and the light is practically cut off as the star actually reaches the horizon. The date cor responding to the configuration can be projected on the dome as well as the names of the constellations, and by means of a lumi nous pointer a lecturer can point out objects of interest. Planetaria have for some time engaged popular interest in a number of Ger man towns. In the United States the first to be erected was the Adler Planetarium at Chicago, but others are now to be found in Los Angeles, Philadelphia, and New York, the last named of which was opened in 1935.
JN.) PLANING-MACHINES rank in importance with the lathes and drilling-machines, and cut various kinds of plane sur faces in metal. The principle is that of attaching the metal to a long table which slides truly upon a massive bed, so that by holding a cutting tool in a slide overhead, the material becomes shaved off as the table moves. The width of surface is treated by giving the tool a cross-traverse. The slide moves on a hori zontal rail for this purpose, and the rail is adjustable up and down two uprights flanking the bed. Tool-slides are often put on the faces of the uprights as well, so that the sides of the work may be planed. Planing-machines are of value both for big and small articles; they may take one of the former and deal with it singly, or a whole string of small objects can be clamped in line on the table and surfaced off. (See also WOOD-WORKING MACHINERY ; MACHINE-TOOLS.)