THE SOLAR SYSTEM — ^LOVELL 285 



meters the corona of the sun, or the radio sun, has been traced out to 

 something like 20 or 30 solar radii. All this is compatible with the 

 picture we have already formed of the influence of the solar atmos- 

 phere extending throughout great distances of mterplanetary space. 



METEORS OR SHOOTING STARS 



In addition to the complex of radiation and ionized particles in 

 space, there is a vast debris of small particles mainly composed of 

 stone or iron. The most common manifestation of these is the ap- 

 pearance in the sky of a meteor or shooting star. Occasionally the 

 particles are so big that they penetrate the atmosphere and fall to the 

 earth as meteorites. The common shooting stars occur when the earth 

 encounters this debris in its journey through space; the particles are 

 heated by friction as they enter the outer layers of the atmosphere, and 

 have generally evaporated completely at about 100 kilometei*s above 

 the surface of the earth, leaving behind a transient trail of light. 

 The occurrence of these meteors has been known for centuries and they 

 may be seen in any clear dark sky with the naked eye at a rate of 

 about 10 per hour. These are the sporadic meteors wliich appear to 

 be distributed with a fair degree of uniformity in interplanetary 

 space. On the other hand at particular times of the year, say in 

 August or in December, the rate rises to 50 to 100 per hour for a few 

 nights. These are the shower meteors which appear to radiate from 

 a particular point in the sky and generally occur with considerable 

 regularity from year to year. 



The question of the origin of these meteors in the solar system 

 is another problem of great contemporary interest. The earth moves 

 in its orbit around the sun at a speed of about 29 km./sec. ; as well as 

 this motion of the earth around the sun, the sun itself and the entire 

 solar system are moving through space with a velocity of about 20 

 km./sec., in the direction of the star Vega. To an observer outside 

 the solar system the motion of the earth would appear to be like that 

 of a giant corkscrew. In this journey the earth occasionally runs 

 mto the streams of debris which are concentrated in orbits around the 

 sun. These enter the atmosphere of the earth and give rise to the 

 showers of meteors. 



These concentrations of debris are, in many cases, closely associated 

 with comets. Although the origin of the comets is uncertain, we 

 know that they are contained within the solar system moving under 

 the gravitational control of the sun and are not visitors from inter- 

 stellar space. The nucleus of the comet is an icy conglomerate of 

 various carbon compounds, and most of the comets have a long tail 

 wliich may stream behind the head for millions of kilometers. In 

 this tail or in the orbit of the comet we have these very large numbers 

 of small specks of dust which may have been evaporated from the 



