T stands for the time by the clock at which the star crossed the meridian. A is the right ascension of the star, and U is the correction of the clock. Use the + sign in the equation whenever the clock is too slow, and the - sign when it is too fast. U may be found from this equation when A and T are given, or A may be found when T and U are given. It is in this way that astronomers measure the right ascensions of the stars and planets.
Determine U from each star you have observed, and note how the several results agree one with another.
21. Definitions.—To define a thing or an idea is to give a description sufficient to identify it and distinguish it from every other possible thing or idea. If a definition does not come up to this standard it is insufficient. Anything beyond this requirement is certainly useless and probably mischievous.
Let the student define the following geographical terms, and let him also criticise the definitions offered by his fellow-students: Equator, poles, meridian, latitude, longitude, north, south, east, west.
Compare the following astronomical definitions with your geographical definitions, and criticise them in the same way. If you are not able to improve upon them, commit them to memory:
The Poles of the heavens are those points in the sky toward which the earth's axis points. How many are there? The one near Polaris is called the north pole.
The Celestial Equator is a great circle of the sky distant 90° from the poles.
The Zenith is that point of the sky, overhead, toward which a plumb line points. Why is the word overhead placed in the definition? Is there more than one zenith?
The Horizon is a great circle of the sky 90° distant from the zenith.
An Hour Circle is any great circle of the sky which passes through the poles. Every star has its own hour circle.