234 F. A. P. Barnard on the Zodiacal Light. 
at the close of twilight, so long as the angle between the ecliptic 
and the vertical passing through the sun is less than 67° 26’; 
which, the sun being depressed 18°, will make the minimum in- 
elination of the ecliptic to the horizon, at which it can be seen, 
28° 34’ ow as the minimum inclination of the ecliptic to the 
horizon in any Jatitude (which occurs in the northern hemisphere 
when the autumnal equinox is on the western horizon) is evi- 
dently equal to the co-latitude diminished by the obliquity of 
the ecliptic to the equator, we may easily ascertain within what 
limits of latitude the light ought never to be absent from either 
the evening or the morning sky. For this purpose we have the 
equation, 
Co-lat.—obliq. =28° 34’. 
Whence  Co-lat.=52° 2’, and Lat.=37° 58’, or 38° nearly. 
At Tuscaloosa, however, which is considerably within this limit, 
the zodiacal light is very far from being constantly present. 
Similar considerations enable us to find a limit beyond which 
the supposed ring can never be visible. This is obtained from 
the equation, 
Co-lat.+-oblig.=28° 34’. 
Whence, Co-lat.=5° 6’ and Lat.=84° 56’, or 85° nearly. 
Thus, in favorable positions of the ecliptic, the light should be 
e pole 
would be elevated only about 13°, but the middle portion of the 
arch would approach 6° of altitude. The arch, however, could 
not be a circle of the sphere, but a spherical conic section. This 
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