1919.] 
Astronomical Notes. 
Ill 
relative quantities of energy transformed in equal times in the two classes 
of collision. 
The impact of two stars like our Sun is over in less than an hour, as 
may be calculated from the known dimensions and velocities. The sudden 
rise of most novae to their maximum brightness also supports this con¬ 
clusion. In this brief time the kinetic energy of the parts that actually 
come into collision is nearly all changed into heat. During an equal time 
the quantity of energy transformed by the impact of a star with a nebula 
would be only 10 -14 times as great. Even if we suppose the nebulous matter 
to be drawn in from surrounding space, so that the effective area of impact 
is increased a hundred times, still the heat energy liberated in the first 
hour would be only one billionth (10 12 th) part of that transformed in the 
same interval in the purely stellar clash. Thus the effect produced in an 
hour in one case is spread over 114,000,000 years in the other. 
Impacts of stars with nebulae are doubtless occurring continually, but 
there is nothing catastrophic about them. The increase of temperature is 
so slow as to be imperceptible. It is to the impact of denser bodies that we 
must look for the origin of novae, and it is the grazing character of such 
impacts that furnishes the key to the solution of the problem. 
A. C. G. 
The Date of Easter. 
The British Act of Parliament defining Easter Day states that “ Easter 
Day is always the first Sunday after the full moon which happens upon, 
or next after, the 21st March ; and if the full moon happens upon a Sunday, 
Easter Day is the Sunday after.” 
A number of arithmetical rules have been devised for the calculation 
of the date of Easter, but most of them are subject to exceptions. How¬ 
ever, A. M. W. Downing’,* a former Superintendent of the British Admiralty 
Nautical Almanac , quotes a rule which has the great advantage of being 
subject to no exceptions. The rule was sent to Nature (April 20, 1876) by 
a “ New York correspondent,” and is as follows 
Divide By 
Quotient. 
Remainder. 
The year 
19 
• « 
a 
The year 
100 
b 
c 
b 
4 
d 
e 
5 + 8 .. 
25 
f 
« « 
6 — / -f 1 
3 
9 
• e 
19a -f- b — d — g -f- 15 
30 
« . 
h 
c 
4 
i 
j 
32 -j- 2e -b 2 i — h — j 
7 
„ e 
k 
a -J- 11 h 22 k . . 
451 
l 
» » 
h + k — 7^ + 114 
31 
m 
n 
Then m is the number of the month of the 
year, 
and it + 1 is 
the number 
of the day of the month on which Easter falls. 
Example : Find Easter Day for the year 1919 
a = 0 ; 5 = 19; c = 19 ; d = 4 ; e = 3 ; 
A.D. 
/ = i; g = 
6 ; h = 24 ; 
i = 4 ; j — 3 ; k = 5 ; £ = 0 ; m 
Hence Easter Day is April 20. 
= 4; 
n = 19. 
C. E. A. 
* A. M. W. Downing, The Observatory, May, 1916, p. 217. 
