. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. arate the effects of parallax and aberration,the following method was adopted by the astronomer of attempting to determine by direct observation thechange of position of the star whose parallax was sought, heselected another star of much less magnitude, and therefore sup-posed to be at a much greater distance, which lay very nearly inthe same direction as the first star, and observed the changes inthe distance between these two stars and in the direction of theline joining them, during th
. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. arate the effects of parallax and aberration,the following method was adopted by the astronomer of attempting to determine by direct observation thechange of position of the star whose parallax was sought, heselected another star of much less magnitude, and therefore sup-posed to be at a much greater distance, which lay very nearly inthe same direction as the first star, and observed the changes inthe distance between these two stars and in the direction of theline joining them, during the year. Fig. 76 will serve to ex-plain the general principle of this amethod. Let S be the position of thestar whose parallax is sought, and *the position of the smaller star, bothbeing projected on the surface of thecelestial sphere. By the motion of theearth in its orbit the star S will describethe parallactic ellipse ABB C, and thestar s, the ellipse adbc. When S ap-pears to be at A, s will appear to be ato; when S is at D, s will be at d, & is evident that Aa and Bb will lie. 222 MAGNITUDE OF THE STARS. in different directions, and that Dd will be greater than (7c; and,therefore, by observing the different directions of the line joiningthe two stars, and also its different values, during the year, wcmay obtain the difference of parallax of the two stars, and, ap-proximately, the parallax of S. 277, Results. —This method was applied by Bessel to the star01 Cygni. For the sake of greater accuracy he made uee oftwo very small stars, situated very near to that star, whose ab-solute parallaxes he assumed to be equal. The parallax which heobtained for this star was Later observations make : and we see by Art. 274 that this corresponds to adistance of about forty-two trillions of miles, a distance whichlight would require seven years to traverse. The parallaxesof a few other stars have been obtained with more or less ac-curacy. The star about whose parallax ther
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