Page:The story of the comets.djvu/220

166 its stay near the region of planets lying in wait, as it were, for comets is limited in time. It runs down to the ecliptic: is quickly across, and off again on the other side, and is soon out of reach of a planet prowling along the ecliptic to see what it can catch.

The motion of a comet is said to be "Direct" (represented by +) when it moves in the order of the signs of the Zodiac, and "Retrograde" (represented by −) when it moves contrary to the signs of the Zodiac.

In the case of an elliptic orbit, given q and e we can ascertain the length of the major axis (a).

Given the daily motion we can obtain the period in days by dividing 1,296,000 (the number of seconds in a circle of 360 degrees) by the value of.

Astronomers are in the habit of performing all these calculations by logarithms because of the ease and convenience of doing so.

Be it remembered that the "eccentricity" is not the linear distance of the centre of the ellipse from either focus, but the ratio of that quantity to the semi-axis major.

Fig. 94 represents the usual way of drawing an ellipse on paper. A line joining AE would be the "axis" of the ellipse, a line at right angles to this at A would be the "directrix" of the ellipse. B and D are the foci; and taking B to be the principal focus, B would be the place of the Sun, supposing the ellipse to represent the path of a comet revolving in an elliptical orbit. AC would be the "semi-axis major" or "mean distance"; and the eccentricity would be the ratio of BC to AC, which in this particular diagram would be about 7 to 10, or decimally 0⋅7. The diagram therefore represents nearly the shape of the orbit of Winnecke's comet.

Up to the present time the orbits of nearly 400 different comets have been calculated. This does not include orbits of comets which have returned one or more times after their first discovery and recognition as being periodical comets.