8 
THE GUIDE TO NATURE 
comet has been known to be a periodic 
comet and its return has been expected 
at regular intervals of between five and 
six years. The comet was seen in 1863, 
1869, 1875, 1886, 1892, 1898, 1909 and 
1915. It was not seen in 1880 and 1904 
because of its unfavorable positions at 
those times. 
The comet is a periodic comet of 
short period which has been seen many 
times. For this reason alone it is in- 
teresting. It has never been a conspicu- 
ous comet. None of these short period 
comets are. Their many returns to 
positions near the sun seem to exhaust 
their power to produce tails and gradu- 
ally wear the comet itself away. 
The influence of Jupiter on the 
comet’s path is great. In 1819, for in- 
stance, the inclination of the plane of 
the comet’s path to that of the earth 
was io° 43', and the comet’s perihelion 
distance was 72,000,000. These quan- 
tities are now 19 0 and 94,000,000 
miles, respectively. The comet was at 
A, the point farthest from the sun. 
about August, 1918. At this time Jupi- 
ter was at C. This close approach to 
fupiter has changed the comet’s path 
to a considerable path in the last revo- 
lution. 
Figure 2 shows that a part of the 
comet’s path lies very close to that of 
the earth. The comet is now in this part 
of its path. This part is shown in more 
detail in Figure 3. In considering this 
figure the fact must be borne in mind 
that the comet does not move in the 
same plane as the earth. Until June 16 
the comet is above or north of the 
earth’s plane ; after that date it is south. 
The angle between the two planes is 
nineteen degrees. This angle is shown 
on the figure. 
As we knew about where the comet 
would be when close enough to be seen, 
search was made for it. It was seen 
first by photography, having been dis- 
covered by Professor Barnard of the 
Yerkes Observatory on a plate taken 
for the purpose on April 10. The posi- 
tions of the comet and the earth at ten 
day intervals are shown in the figure. 
The earth moves with a nearly uniform 
velocity of i8j^ miles per second. The 
comet moves with a velocity which 
varies with its position. When nearest 
to the sun June 13 it moves at the rate 
of twenty-four miles per second, the 
velocity elsewhere being slightly less. 
Thus the comet gained on the earth and 
passed it in May. Because of the in- 
clination, however, the comet was not 
closest to us at that time. This occurs 
early in June. 
This comet passes through the plane 
of the earth’s orbit on June 16. Its dis- 
tance from the sun at that place is al- 
most exactly equal to the distance of 
the earth from the sun at that place, so 
that there is very nearly a real inter- 
section of the paths themselves. If the 
comet and the earth reached this point 
at the same time the bodies would be 
very close together indeed. As will be 
seen, the comet passes this point June 
16 and the earth June 25, so that the 
bodies are far apart at the time, as the 
comet is nine days’ motion ahead of 
the earth, about 18,000,000 miles. 
Even if the intersection were exact 
the chances of collision are so small as 
to be negligible, as the earth is so small 
and space so large. If the head of the 
comet were 1,000,000 miles in diameter, 
and that is about the largest known, 
the earth might collide with the head 
about nine hours before or after passing 
the intersection. In this case the only 
observed efifect would be a fine meteoric 
shower. If the nucleus of the comet, 
which is the solid part, were as large 
as the earth, which is the size of the 
largest, the collision could only occur in 
the eight minutes before or after the 
earth passed the intersection. In this 
case there might be some damage where 
the collision occurred. The fulfillment 
of such conditions is so unlikely that a 
collision with a comet is an exceedingly 
remote possibility. No comet has been 
known to have approached the earth 
closer than that of 1770, which was 1,- 
400,000 miles away. 
When the earth passes through the 
