Page:Popular Astronomy - Airy - 1881.djvu/135

Rh observed. I said that it might be observed by any person, if he watched, at a given hour of the night, the appearance of the stars on successive days and months: when he would find, on going from one month to the next in the order of succession, taking always the same hour of the night, that the stars appear to go round towards the right, or towards the west, (our faces being turned towards the south); which, as I explained, proves that the sun appears to go through the stars towards the left or the east. From this it is plain that the stars set a little earlier every day in reference to sun-time: or, that they pass the meridian a little earlier every day in reference to sun-time; and therefore, if we define the sidereal day to be the time that elapses from the passage of a star over the meridian one day to the passage of the same star another day, that interval or the sidereal day will be less than a solar day. It is, in fact, about 23 hours, 56 minutes, and 4 seconds of ordinary clock-time: the mean solar day being 24 hours. To sum it up in a few words, the stars appear to be going every day in their diurnal motion from east to west, and they appear to be passing the meridian quicker than the sun does. The sun appears to be travelling from west to east among the stars; and therefore, though the apparent diurnal motion of the sun through the heavens from east to west is quick, yet, in consequence of this apparent motion through the stars in an opposite direction, it passes the meridian slower than the stars do.

In speaking of surveys, I explained that by the use of the transit instrument and the theodolite, we might ascertain the angle made by any one side of a triangle with the meridian. This can of course be done at as many stations as we please; it is commonly