PHOTOGRAPH OF PLANET 2,800,000,000 MILES OFF.

Star (Christchurch), Issue 19268, 3 January 1931, Page 9

PHOTOGRAPH OF PLANET 2,800,000,000 MILES OFF.

ROTATION OF NEPTUNE SAME AS THE EARTH. SAN FRANCISCO, November 8. It takes a steady hand to “ shoot ” a photograph of'an object 2.800,000,000 miles away. Not steady for just a second or a minute, as in ordinary photography —but steady for hours. That kind of a hand used to steady astronomical clockwork has to solve at Lick Observatory of the University of California one of the longstanding mysteries of astronomy—the direction of rotation of the planet Neptune. Dr J. H. Moore and Dr D. H. Menzel have found it turns in the same direction as the earth. Neptune. 2,800,000,000 miles distant, appears even in the great telescope here only as a disc of light the size of the end of a pencil. There are no markings on this little face. To study rotation it was necessary to photograph only a thin slit, the equator, of this disc. For hours the telescope, bulkier than a 16-in gun, had to be held so that the slit lay unwavering across the round spot of light, a precision feat beyond the accuracy of the usual astronomical clockwork. Even wind tremors had to be guarded against. Exposures of the photographic plates lasted two to four hours each. The great scientific obstacle overcome was the uncertainty about location of Neptune's equator. It is not horizontal, but diagonal on the tiny image, because the planet is inclined away* from the plane of the earth s equator. This position was computed by aid of figures on a nodding motion long observed in the orbit plane of Neptune's only satellite. The nod is like the wobble of a spinning top that is beginning to slow down. . The slit photo showed the equator s spectrum, that is, light reflected from that part of the planet split into the lines which reveal its various colours, or wave lengths. Study of these lines revealed that one side of the equator was approaching the camera and the other receding from it, due to rotation, and thus fixed the direction of rotation.