. Elementary physical geography . heavenly bodies whichdo not appear to be regular parts of the system. Thestrangest of these are comets. Some 500 of these havebeen recorded as visible to the naked eye; and in addition,over 200 have been detected by the aid of the telescope, someof these being millions of miles in length. When near thesun, they usually have a relatively dense head and a vaporoustail, through which stars are visible (Fig. 11). Some haveregular elliptical orbits, andtheir time of appearance canbe closely calculated; butthe orbits of others are ap-parently parabolas, so thatif th
. Elementary physical geography . heavenly bodies whichdo not appear to be regular parts of the system. Thestrangest of these are comets. Some 500 of these havebeen recorded as visible to the naked eye; and in addition,over 200 have been detected by the aid of the telescope, someof these being millions of miles in length. When near thesun, they usually have a relatively dense head and a vaporoustail, through which stars are visible (Fig. 11). Some haveregular elliptical orbits, andtheir time of appearance canbe closely calculated; butthe orbits of others are ap-parently parabolas, so thatif they ever return to thesolar system, it is only afterlong periods of time haveelapsed, and after havingmade a journey far beyond ^ ^^ ^ the outermost limits of the solar system. Perhaps these may be mere wanderers throughspace, which after one visit to the solar system, depart neverto return again. What they are, whence they came, whitherthey are going, or what relation they bear to the solar sys-tem, is still an unsolved 16 PHYSICAL GEOGRAPHY. Comets have an added interest to us, from the fact thatsome shooting stars and meteors seem to be remnants ofcomets, which at some former time have crossed the orbitof the earth. Thus the November meteorites are due to thefact that in its movement around the sun the earth en-counters particles that are left in the trail of a comet (Tem-pers) which has a period of revolution of about thirty-threeyears; and the August meteors (Fig. 12) appear to havea similar origin. Meteors and shooting stars (meteors are large shootingstars) enter the earths atmosphere at a high rate of speed, and are burned up?*•, 0/August Mehpre in the higher layers of the atmosphere,often at an eleva-tion as great as 100miles from the sur-face of the burning isthe result of fric-tion with the air, which produces a high heat, because inaddition to the movement of the meteor, there is oftenadded the motion of the earth itself, which is about 98,000f
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