Page:A short history of astronomy(1898).djvu/267

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§ 161]

which the distance in terms of the radius of the earth, and hence in miles, can readily be deduced when desired.

The parallax of a heavenly body such as the moon, the sun, or a planet, being in the first instance defined generally (chapter ii., § 43) as the angle (o m p) between the lines joining the heavenly body to the observer and to the centre of the earth, varies in general with the position of the observer. It is evidently greatest when the observer is in such a position, as at q, that the line m q touches the earth; in this position m is on the observer's horizon. Moreover the angle o q m being a right angle, the shape of the triangle and the ratio of its sides are completely known when the angle o m q is known. Since this angle is the parallax of m, when on the observer's horizon, it is called the horizontal parallax of m, but the word horizontal is frequently omitted. It is easily seen by a figure that the more distant a body is the smaller is its horizontal parallax; and with the small parallaxes with which we are concerned in astronomy, the distance and the horizontal parallax can be treated as inversely proportional to one another; so that if, for example, one body is twice as distant as another, its parallax is half as great, and so on.

It may be convenient to point out here that the word "parallax" is used in a different though analogous sense when a fixed star is in question. The apparent displacement of a fixed star due to the earth's motion (chapter iv., § 92), which was not actually detected till long afterwards (chapter xiii., § 278), is called annual or stellar parallax (the adjective being frequently omitted); and the name is applied in particular to the greatest angle between the direction of the star as seen from the sun and as seen from the earth in the course of the year. If in fig. 69 we regard m as representing a star, o the sun, and the circle as being the earth's path round the sun, then the angle o m q is the annual parallax of m.

In this particular case Cassini deduced from Richer's observations, by some rather doubtful processes, that the sun's parallax was about 9"⋅5, corresponding to a distance from the earth of about 87,000,000 miles, or about 360 times the distance of the moon, the most probable value, according to modern estimates (chapter xiii., § 284), being