Page:Popular Astronomy - Airy - 1881.djvu/48

34 Now, allow me to point out what we have obtained with regard to these celestial objects.

By the use of the transit instrument, when properly adjusted, and the clock, we observe the time of transit of a principal star, and we observe the time of transit of any other objects, smaller stars, planets, or whatever else they may be. By means of these observations, we have a difference of times of transit. We can place no reliance upon the clock, except this, that it gives us the difference of time between the passage of principal stars, and that of other objects. Suppose that our clock is so adjusted, that if we observe the time of that principal star passing the instrument to-day, and again observe the time at which it passes the instrument to-morrow, the clock describes accurately twenty-four hours. If it does not describe accurately twenty-four hours, we know how great its error is in twenty-four hours, and we can apply a proportionate part of the error to every interval of time; so that it is in every respect as serviceable as if it were accurately adjusted. Supposing, then, that our clock was adjusted in such a manner that it indicated twenty-four hours, from the time of a principal star passing to the time at which the same object passed again—this amounts to saying, that it indicates twenty-four hours in the time in which the whole heavens turn round. Assuming, then, that the planet which we have observed, passes the telescope one hour after the principal star passes, then we must conclude that the heavens have turned for one hour; or have performed one twenty-fourth part of their whole revolution, before that part of the heavens in which the planet is seen, passes our meridian. And this is precisely one of the co-ordinates which, as I said, serves to determine the position of