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302 THE ADVANCE OF PHOTOGRAPHY
pyramids, four thousand years ago, beheld the constellations much as we do. It is only the finest astronomical measurements that show any change within a limited number of years. However, the study of the proper movement of the fixed stars has now begun, and requires very accurate measurements carried on for generations.
Another interesting point comes into consideration in connection with this subject. On the one hand, the fixed stars are not without movement ; on the other, their distances from the earth are very various, the nearest being enormously great. The photographer who wishes to take a graphic view of an object, will always try to take it from different points. Two views of a moderately remote object taken from two points only two inches apart, appear different to the eye, and produce, when viewed in a special manner, a stereoscopic effect. No distance on earth is great enough to give different pictures of the same constellation ; nevertheless, within the space of one year we describe a circle round the sun having a diameter of 184 millions of miles, so that in half a year we are 184 millions of miles from our present position. This enormous distance is in certain cases just sufficient to show a change in the relative position of certain stars, though not to the naked eye. By this means the distance of the nearest fixed stars has been determined, amounting to billions of miles.
By careful comparative measurements of positions of neighbouring stars, continued for years and centuries, a change can be proved to exist, and the proper movement of the stars can be calculated. The distance of the stars can be deduced by carefully collating the yearly recurring changes in the positions of the stars. It is evident that photography, which affords the means of fixing these positions, must be of the greatest value for both these astronomical problems.
Photography of the stars was first introduced nearly