HALLEY ’S COMET—CAMPBELL. 955 
without telescopic assistance and how bright it will be at maximum 
are too uncertain for prediction. Certainly for a few months in the 
first half of 1910 it should be a conspicuous object. Comets brighten 
and develop their tails as they approach the sun, reaching their 
greatest development when in or near perihelion. For this 
reason it is their unfortunate practice to disappear from view in the 
sun’s glare just when they are largest, and Halley’s comet will be 
out of sight for a few days while it is passing on the other side of 
the sun, probably in March, 1910. We should see it at its best just 
after perihelion passage. 
The history of this most famous of comets prior to Halley’s first 
date, 1531, has been traced by three able English astronomers, Hind, 
Cowell, and Crommelin, as far back as B. C. 240. In all, twenty- 
nine appearances recorded in history have been identified. These 
have occurred at average intervals of seventy-six and three-quarters 
years. The individual values of the intervals have varied between 
seventy-four and a half and seventy-nine years, according as the 
disturbing actions of the planets combined to shorten or to lengthen 
the period. 
There are extant several quaint pictorial representations at many 
of its early returns. An especially interesting one, though of mini- 
mum scientific value is for the return in 1066—the year of William 
the Conqueror’s invasion—as preserved in the famous Bayeux tapes- 
try. Sir John Herschel’s drawing is probably our best record of 
its appearance at the 1835 return. Fortunately we now have pho- 
tography to make permanent records of both its general and its de- 
tailed structure. The dry plate puts down details which the eye 
can not see, and it does the work with great accuracy. Since Bar- 
nard’s pioneer success in the photography of comets at the Lick 
Observatory, about 1890, no one seriously attempts to “draw” a 
comet. An inspection of the two photographs of comet Morehouse, 
just visible to the naked eye in the fall of 1908, will show the rich- 
ness of structural detail, none of which could be seen in any existing 
telescope. 
The long elliptical orbit of Halley’s comet and the nearly circular 
orbits of the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are 
represented in the figure—approximately to the correct scale; but it 
should be said that the plane of the comet’s orbit makes an angle of 
18° with the earth’s orbit plane. The comet’s orbit therefore passes 
“ through ” the planetary orbits like the two adjacent links of a chain. 
The comet will approach within 56,000,000 miles of the sun, and 
then recede during thirty-eight years until it is far beyond Neptune’s 
path. In perihelion it must travel 34 miles per second, but at the 
outer turning its speed will be less than 1 mile a second. (See fig. 1.) 
