F. A. P. Barnard on the Zodiacal Light. 231 
If we take the season when, at the close of twilight, the eclip- 
tic makes the largest angle with the horizon—that is to — 
when the vernal equinox is in the western horizon at that m 
ment—the apparent altitude of the extremity of the light eae 
be less than eight degrees*, and this extremity should be 90° dis- 
tantin azimuth from the sun, while the base should be distant 
about 42°, making the length of the luminosity, about 48°. _ Its 
5. nee inclination to the horizon should be 193 degrees. The 
ecliptic, in the mean time, will have an a mpeg inclination of 
79° to the horizon, and its setting point will be distant in azi- 
muth from the sun less than four degrees. Upon the supposition 
We are now considering, the zodiacal light should never be seen 
in the evening sky in the winter, until the close of January, and 
it would reach its maximum conspicuousness during the first 
week in March. It would be too faint however, to be noticeable 
until probably the middle of pcre or later, so that its sensi- 
ble duration would be but a few wee 
* That the oe inclin: ecliptic to the 6. 
ostien we stm the eclipt 
resent the ecliptic cidtecctiae ‘ha me eridian t E, the 
equator at A, and the horizon at EP; Aw. ill be the vernal 
uinox, Also, the triangle, EPR, be bent & a 
minati : en, 
. Cos inclination (EPR) = cos ER X sin PER= sin EZ X sin PER, But ZE= 
pe — EQ = latitude minus the deciin ioe of the culminatin int of the ecliptic. 
tI= inclination, L= latit — D = declination EQ, and E =angle PER;; then, 
ded it setae = sin (L— —D) sin i 
nd if declination south ey: ded as negativ e formula will express 
atace y the inclination for the ee P’E’ of the ‘cline gt A is the autumnal 
But, sin (L—D) =sin L cos D — cos Iisin D; whence 
cos I= sin L cos $ sin E — cos L sin D sin 
a in the ine, ge AES. cos D sin E = cos EAQ (= spies 2 of the cclipti} 
ich put = = 0 and sin D sin E= sin ZAQ sin AQ (right as cension of the 
of the exp) which put Then, 
‘os I =sin maphitperne 
in which the only vaviahle j is A, the right ascension of the culminating point of the 
“ti van Manifest that cos T i A is at its 
is least, and I gre test, when sin A is at its positiv 
cone —that is to say, when the R a e ts gee of the ge ap on the treed is 
bel In like manner, cos I is grea I Jeast, w oe 0° eget na 
> or in e 
ear when the R. A. of the sa i af Fe the Mpeg a tei : ibe — 
limits of time a ph ; 
will be sbecreatl! : ooh eb Such aml 
i thea whose visibility di nds on this inclination. Such an 
ne * of it a little further “ue sat it is on this account, and not = 
sake of the Proposition directly demonstrated, that it is introduced h 
