256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
For purposes of description, it has been found convenient to divide 
the structure of a comet into three parts, as follows: 
1. The densest and brightest part near the center of the head, called 
“the nucleus.” Nearly all the mass of a typical comet resides in the 
nucleus. 
2. The coma, or envelope, of low density surrounding the nucleus. 
In occasional comets the head consists entirely of coma without an 
apparent nucleus. 
3. The tail, which always points approximately away from the 
sun. When the comet is traveling toward the sun the tail follows the 
head; when the comet is going away from the sun the tail precedes 
the head. This is illustrated in the drawing of the comet’s orbit. 
Fic. 1.—Orbits of the planets and the comet. (The smallest circle is the earth’s orbit.) 
The fact that a comet’s tail always points away from the sun was 
early recognized. There could be no doubt that some force origi- 
nating in the sun was repellent to the materials composing the tail; 
but to determine the nature of this force defied us for generations. 
Since the coming of photography and the accurate recording of 
details of comet structure utterly invisible to the eye, it has been 
possible to measure these motions. Comparisons of photographs of 
the same comet made two or three hours apart have shown that con- 
densations and other structural forms have moved rapidly outward 
during the interval; only a few miles per second at first, but faster 
and faster as the distance out in the tail increased. Some observed 
speeds have been nearly 50 miles per second. Fifty miles per second 
