. | PHYSICAL ASTRONOMY. 143 
orbits around the sun. The eccentricity of their orbits is very great. 
It is now also known that telescopic comets, or those only visible to the 
eye when assisted by a good telescope, occur in far greater numbers than 
others, and that multitudes of them are probably always present in the space 
belonging to our solar system. 
There are thus far, only three comets whose return has been calculated 
several times, and whose orbits are accurately known. The first of these 
is Halley’s comet. Halley found that the comets of 1456, 1531, 1607, and 
1682 were one and the same, which, he predicted, would return in the begin- 
ning of 1759. His prediction was nearly fulfilled. It again appeared in 1835, 
and is next expected in the end of 1911. The period of revolution of this comet, 
which can approach within 48 millions of miles of the sun, and recede from 
him 2920 millions of miles, embraces 76 years. The inclination of its orbit 
is 17° 44’, and its eccentricity is {75 of the semi-major axis. The motion of 
this comet is retrograde. 
The second comet, discovered by Pons in Nov. 26, 1818, is the direct 
comet of Encke, with the very short period of 34 years. Encke found 
that it had been already observed in 1786, 1795, and 1805, that its perihelion 
distance was 26 millions of miles, its aphelion 340 millions, the inclination 
of its orbit 13°, and the eccentricity ;83, of the semi-major axis. The pre- 
dictions of later reappearances of this comet, whose course is affected by 
the resistance of the ether (sec. 50), were fulfilled in 1825, 1828, 1832, &c. 
The third, also a direct comet, was discovered by Biela, 28th Feb. 1826, 
and named after him. This comet revolves in 6? years about the sun. Its 
aphelion distance amounts to 508 millions of geographical miles, while its 
perihelion comes very near the earth’s orbit. This, as is well known, is 
the comet which in 1832 passed near the earth, and on that account excited 
universal apprehension. The inclination of its orbit amounts to 13°, and 
the eccentricity to 4, of the semi-major axis. ; 
_ Besides these three comets, there are some others recently discovered, 
whose periods have been found with great accuracy. ‘Their return in the 
calculated time must, however, be determined by experience. Of these, the 
best known is the one discovered in the constellation of the Fly, by Olbers, 
March 6, 1815, and named after him. The motion of this comet is direct. 
It approaches to within 100 millions of geographical miles of the sun, and 
recedes to a distance of 2840 millions. The inclination of its orbit amounts 
to 44°, its eccentricity to 23, of the semi-major axis, and its period to a little 
more than 74 years. Its re-appearance may consequently be looked for 
about the year 1889. 
While in earlier times comets were looked upon as harbingers of misfor- 
tune, or as indicative of Divine wrath, more recently the fear has been 
excited, lest, on account of their great number, and the various positions of 
their orbits with respect to that of the earth, a comet may at some time or 
other come in contact with the earth. This fear, however, Olbers, more 
than 40 years ago, by his copious investigations of the numbers and orbits 
of comets, sought to remove as far as possible. 
In conclusion, it remains to say that very recently, two more great and 
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