. The Adolfo Stahl lectures in astronomy, delivered in San Francisco, California, in 1916-17 and 1917-18, under the auspices of the Astronomical Society of the Pacific. s lie in differentdirections with reference to the horizontal line. The anglebetween a fixed reference line and the axis is another earmarkof the orbit. It tells us how the orbit Hes in its own fourth earmark is given by the date of perihelion earmarks are expressed by symbols in astronomy asfollows: the semi-major axis by the letter a; the eccentricityby the letter e; the angle which the semi-major axis
. The Adolfo Stahl lectures in astronomy, delivered in San Francisco, California, in 1916-17 and 1917-18, under the auspices of the Astronomical Society of the Pacific. s lie in differentdirections with reference to the horizontal line. The anglebetween a fixed reference line and the axis is another earmarkof the orbit. It tells us how the orbit Hes in its own fourth earmark is given by the date of perihelion earmarks are expressed by symbols in astronomy asfollows: the semi-major axis by the letter a; the eccentricityby the letter e; the angle which the semi-major axis makes 184 The Adolfo Stahl Lectures with the reference Hne, usually the line of nodes, to be definedJDresently, by the Greek letter co: the time of perihelion passageby T. With these four earmarks and with the aid of Keplersimproved laws it is possible to determine the position of thebody in its orbit at any given time. In place of a we maychoose the period P or the mean motion u as the first a parabola the perihelion distance q is chosen as elementin place of the infinite semi-major axis. It remains to deter-mine the iX)sition of the body in Fig. 15. The Orbit of the Earth and of a Comet. In Figure 15 we see the Earths orbit. Its plane may betaken as a reference plane. Then we see the plane of the orbitof a comet which in this case is a parabola. The orbit planesintersect in a straight line called the line of nodes and at agiven angle. The point where the comet crosses the eclipticfrom south to north is called the ascending node. The angu-lar distance of the node from some fixed point in theEarths orbit, generally the Vernal Equinox, V, is designatedby the Greek letter Q, while the angle of inclination isdesignated by the letter /. These six elements or earmarksare the numerical characteristics or constants which distinguishthe orbits of different bodies, according to Keplers andNewtons laws. The accuracy with which the numerical values of theelements can
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectastronomy, bookyear19
