PLANETOID 



4696 



PLANING MACHINE 



would weigh 215,000 pounds on Jupiter; that 

 is, he would be heavier than a locomotive. 

 Another distinction between the planets in 

 these groups is that those of the terrestrial 

 group shine by reflected light, while there is 

 evidence that those of the major group emit 

 some light of themselves. Between the ter- 

 m-trial and major planets is a great space in 

 which are found a large number of small plan- 

 lied asteroids, or planetoids. 

 Movements of the Planets. Every planet has 

 two motions a revolution around the sun and 

 a rotation on its axis. The time required for 

 a complete revolution in its orbit constitutes 

 the year, or period, of the planet; the time re- 

 quired for its rotation constitutes its day. Be- 

 cause of their smaller orbits the terrestrial 

 planets have shorter years than those of the 



PLANE TREE , a species of tree which grows 

 to giant size, easily recognized by its smooth 

 bark, which flakes off, exposing the white layer 

 beneath and giving the tree a mottled, white- 

 washed appearance, and also by its seed balls, 

 strung like beads, three or four upon each long 

 stem. It received its name (from a Latin word 

 meaning broad) because of its wide leaves. 

 The European species is gaining popularity 

 as a shade tree in America, where others of 

 the family, the buttonwood, or buttonball, and 

 sycamore, grow extensively. In ancient times 

 the Greeks admired and worshiped the plane 

 tree, pouring wine upon its roots and festoon- 

 ing its branches with jewelry. The story is 

 told that Xerxes once stopped his vast army for 

 several days to feast his eyes upon a particu- 

 larly beautiful specimen, and afterwards had 



major group, but the major planets rotate on 

 their axes more rapidly than the smaller ones. 

 Jupiter's day, for instance is less than one-half 

 of our day. 



The table on this page gives the distance 01 

 each planet from the sun, the length of its 

 year and its diameter, in round numbers. In 

 measuring distance from the sun the earth's dis- 

 tance is taken as the unit. F.ST.A. 



Related Subjects. For further information 

 and more detailed description, the reader is re- 

 ferred to the following articles in these volumes : 

 Asteroids Neptune 



Astronomy Satellite 



Earth Saturn. 



Jupiter Solar System 



Mars Star 



Mercury Uranus 



Nebular Hypothesis Venus 



PLANETOID, plan' et old, one of many small 

 planets usually called asteroids, revolving about 

 the sun between the orbits of Jupiter and 

 Mars. About 500 planetoids have been dis- 

 covered and it is considered that their num- 

 ber is almost countless, though their small 

 size renders them invisible. See ASTEROIDS. 



PLANETESIMAL HYPOTHESIS, plan e tes 

 i mal hipoth'esis. See NEBULAR HYPOTHESIS. 



a gold medal engraved with a picture of it. 

 Giant plane trees with trunks over forty feet 

 in diameter and thought to be 4,000 years old 

 may be seen in Europe; these are venerable 

 relics of a past age which are slowly falling into 

 decay. The strong wood of this tree is used 

 in making boxes, crates and butcher's blocks. 



PLANING MACHINE, an adaptation of the 

 principle of the ordinary hand plane to ma- 

 chinery for planing or smoothing wood and 

 metal. It is usually operated by steam or 

 electricity, power being transmitted by belts 

 and shafting. As used for planing wood it con- 

 sists of a drum with cutters attached, rotating 

 on a horizontal axis. The wood to be planed 

 or smoothed passes underneath the drum. 



In metal planers the object to be smoothed is 

 fixed to a traversing table and is moved against 

 a stationary cutter. Planing machines for 

 metal, or planers, as they are generally called, 

 are of various kinds and are usually named 

 according to the articles they are intended to 

 plane, as nut planer, rod planer, plate-edge 

 planer, etc. In shipbuilding yards planers are 

 used with sufficient cutting power to remove 

 large strips from the thickest and strongest 

 armor plate made. 



