An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

AXilI"}tATr ION<. 109
1 95. AmxmxA'r roxl 1 ils -:t pc'rent cJll.2ngye of place,in, tthe
stars, oeea(tsio ned by thu.e joint jc t ls of the, mnotion of' te earth
1in, its orbit, (tnd? tiw proygressive n2otion, of liyit.
Suppose the carthl to -move froml 0 to L, whlile the lighlt firom
S describes the line )E. If theyl arrive togetler at thle point
F1, the ilmpr'cssion onl thoe eye will not be Fig. 43.
the same as if fhe observer ltad b)een at
rest, btt it will appear to come in the direction of ST., the star )being apparently.D
thlrown forward ftom S X to S'. For, make
i-:.;l:l)El, and complete the parallelogram
CO; and suppose, according to tile principle of equal action and reaction, thatd tthe
light has a motion EIQJ given to it, in place
of the eartlt's motion CE; then tell two
motions EA antd E:Qt wvill pri'oduce the, resultfant:EB, as thoiugh the light had come A?
fiom S' insteadl of S.
Since the earth moves 19 miles, anld light 192,000 per second,
if S is in a direction lperpendicular to the line of the earthi's
motion, the right-angled tnriangle 1 lECl gives altout 20".5 fob
the displacemenC t of the star. I'n fact, howeveir, it was tile obsefved displalcemellt of 20".5 ill all stars situated 900 fi'oml the
direction in which thle eart't is moving at any time, which led
to the knowledge of the velocity of light; thl1us,
taln 20.5: it: 19': 192,000.
If there were no change in the ablerration of a starl tihe fact
of such ablerration couldl lever lrve been disco-vered. AW1hen
we move directly towatrd or f/'omt a star, it plainly hlas no aberration; but thrlle month's )tefore thatt timle our ll 1otion crosses
the line of tle rays at ri ght angles, and (also tllree monttls after,
but in a contrarlty direction. Ifence anttly star in the l)latt ot
the ecliptic appnIarently moves back and fortlt over ar arte of
41" (2 x 20"I.5) i a year, Those out of the ecliptic seeCI to
describe e(lliptic orbits, whose major axis is 41.", and of vtarious
eccentricity5 the minor axis increasing withl their latitude.