Page:Popular Astronomy - Airy - 1881.djvu/56

42 that that is what we mean by a sidereal day. I cannot tell you now what sidereal time is, and for this plain reason: I have not yet got the starting-point which marks the beginning of the sidereal day. All that I can at present say is, that the interval from the time of the passage of one star one day, to the time of the passage of the same star next day, is understood to be twenty-four hours of sidereal time.

Having proceeded so far in relation to the times of the passing of the stars, and the quantity of rotation which the globe must perform from the meridianal passage of a fixed star which we know, to that of a planet or similar object whose position we want to determine, I mentioned the use of the Mural Circle, by which we determine the altitude of the object when it is passing the meridian. And here I must observe, that one of the most important adjustments of the Mural Circle depends on reflection from the surface of quicksilver. It is not my province now to allude to optics as a science; I merely allude to it to indicate a thing important to our present purpose: the law of reflection of light from a surface of quicksilver. The surface of the quicksilver takes a position parallel to the horizon, with a degree of suddenness and certainty to which we know nothing similar. Light is reflected from the surface of the quicksilver, just as it would be from a looking glass. Now, the thing which I wished to point out as the great practical fact is this: that supposing SG and S′O, in Figure 13, to represent the direction of the light coming from the star, and OG′F′ the direction of the light reflected from the quicksilver; then the inclination of S′O or SG to the horizon is the same as the inclination of OG′ to the horizon; and if S′O or SG approach nearly to a flat with the horizon, OG′ will