248 
The N.Z. Journal of Science and Technology. 
[Nov. 
ON THE VARIATIONS IN SIZE OF A COMET’S HEAD. 
By A. C. Gifford. 
In The Story of the Comets (p. 8) G. F. Chambers makes the following 
statement:— 
“ Few things are more remarkable to witness, and more paradoxical to 
explain, than the changes of bulk which the head of a comet generally 
undergoes in approaching to, or receding from, the Sun. One might expect, 
reasoning from terrestrial analogy, that as a comet approaches the Sun the 
increased heat to which it is submitted would expand its head, whereas 
the effect observed is the contrary—it grows smaller as it grows hotter. 
And when receding from the Sun the observed changes are of a converse 
character : the comet’s head seems to expand as it gets farther away and 
grows cooler. No satisfactory explanation of this anomaly has been given 
unless it is permissible to accept Sir J. Herschel’s idea that the change of 
bulk is due to some part of the cometary matter remote from the nucleus 
being evaporated, as it were, under the influence of the Sun’s heat, just 
as a morning mist is evaporated and disappears as the Sun rises in the 
heavens and its radiant heat becomes more potent.” 
Similar statements are made by Professor F. R. Moulton* and Dr. 
W. W. Campbell,! the former pointing out that the jatio of the largest to 
the smallest volume is sometimes as great as 100,000 to 1, and the latter 
giving some details with regard to Encke’s periodic comet. All these 
writers agree that no satisfactory explanation of the contraction and 
expansion of the head has yet been found. This is rather surprising, as the 
explanation which naturally suggests itself is too evident to have been 
overlooked, and, though insufficient by itself, appears to lead inevitably to 
a more complete explanation. 
The exceedingly small mass and the enormous volume of a comet’s 
head suggest that it is of the nature of a widely scattered meteor-swarm. 
Whilst the comet is very far away from the Sun the separate solid particles 
are doubtless moving amongst one another like the stars in a globular cluster. 
As the comet approaches the Sun the velocities due to the mutual gravita¬ 
tion of the members of the swarm are compounded with those due to 
solar attraction, the latter becoming more important as the distance from 
perihelion diminishes. 
The path of any particular meteor in the swarm will be a complicated 
curve—very nearly an ellipse of high excentricity, but differing from a true 
ellipse somewhat as the path, of the Moon with respect to the Sun does. 
But at every moment, whilst some particles within the swarm are moving 
towards its centre, others are moving outwards to take their places. 
Instead, therefore, of following individual particles throughout their orbits, 
we may consider points related in a definite way to the centre of mass of 
* F. R. Moulton, An Introduction to Astronomy, p. 316, New York, 1918. 
t What we know about Comets, Adolfo Stahl Lectures in Astronomy, p. 37, Stanford 
University Press, 1919. 
