154 The Sky 
invisible to the naked eye. Seen through a telescope, it has 
a greenish appearance. The discovery of the planet in 1846, 
as the result of computations by Leverrier, is considered one 
of the great triumphs of mathematical astronomy. Neptune, 
which is known to have one satellite, has a period of revolu- 
tion around the sun equal to 164 earth years. 
Pluto which is the most remote known planet of our solar 
system was an object of search for many years. Dr. Percival 
Lowell, founder and director of the Lowell Observatory in 
Flagstaff, Arizona, predicted its orbit and the research which 
he originated in 1905 led to the location of the planet in 1930 — 
by C. W. Tombaugh of the Lowell Observatory. Pluto is in- — 
visible to the naked eye and as seen in the telescope is of — 
yellowish color. It completes its revolution about the sun in ~ 
248 years and is about half the size of the earth. 
FIERY TAILS 
Comets occasionally appear in the sky as brilliant starlike | 
bodies from which long tails of luminous material extend. 
The tails are composed of very thin and tenuous gases which 
are driven away from the main body of the comet (head of 
comet) by the pressure of the sun’s radiation and are hence 
always pointed away from the sun. Moreover, the tails, and 
therefore the visibility of comets, are present only when the 
comet is near to the sun. At all other points of the comets — 
orbits around the sun they are invisible. Once their orbits — 
have been determined, the return of comets may be con- 
fidently predicted, although some comets have broken up and 
been destroyed (Biela’s Comet) and others presumably 
approach the sun only once and then are forever lost again 
in the remoteness of interstellar space. 
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