Part 20

Occultations, 161 explained, 165

Occultations, use of, 166, 167

Orion nebula, 30

Pacific islands, their longitude and time, 125

Parallax, solar, 67, 106 stellar, 94, 106 measured with heliometer, 158

Paris, congress of astronomers, 1887, 102 exposition of 1900, 176

Periodic motion of earth's pole, 133

Perseus, constellation, temporary star in, 46

Philippine Islands, their time, 127

Photography, asteroid, invented by Wolf, 104 congress of astronomical, 102 cumulative effect of light, 84 distance of light-source, 83 double telescopes for, 86 general star-catalogue, 102 IN ASTRONOMY, 81 in discovery of asteroids, 64, 104 in solar physics, 109 in spectroscopy, 108 length of exposure, 84 measuring-machine, Rutherfurd, 93 motion of telescope for, 87 "mouse" control of telescope, 88 of eclipses, 109 of inter-stellar motion, 99 Paris congress, 1877, 102 polar, 191 Rutherfurd pioneer in, 90 star-clusters, 98 star-distances measured by, 94 summarized, 110 wholesale methods in, 103

Piazzi, discovers first asteroid, 59, 106

Pitkin, report to House of Representatives, 168

Planetary nebulæ, 31

PLANET OF 1898, 58

Planetoids, see Asteroids.

Planets known to ancients, 58

PLEIADES, 10 gravitation among, 212 motion among, 14, 16, 98 nebular structure, 17 number visible, 11

Polar axis, of telescope, 173

Polar photography, 191 at Helsingfors, 195

Pole, celestial, 184 of the earth, motions of, 131 THE ASTRONOMER'S, 184

POLE-STAR, 18 as a binary, 25 as a triple, 18, 26 change of, 187 its four-day cycle, 24 motion toward us, 24

Positive, and negative, in photography, 82

Potsdam, observatory, photographic star-catalogue, 103

Practical uses of astronomy, 112

Precession, explained, 186

Prize, for invention of chronometer, 8

Ptolemaic theory of universe, 56

Ptolemy, writes concerning Hipparchus, 39