_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. _The Meridian_ is that hour circle which passes through the zenith. _A Vertical Circle_ is any great circle that passes through the zenith. Is the meridian a vertical circle? _The Declination_ of a star is its angular distance north or south of the celestial equator. _The Right Ascension_ of a star is the angle included between its hour circle and the hour circle of a certain point on the equator which is called the _Vernal Equinox_. From spherical geometry we learn that this angle is to be measured either at the pole where the two hour circles intersect, as is done in the star map opposite page 124, or along the equator, as is done in the map opposite page 190. Right ascension is always measured from the vernal equinox in the direction opposite to that in which the stars appear to travel in their diurnal motion--i. e., from west toward east. _The Altitude_ of a star is its angular distance above the horizon. _The Azimuth_ of a star is the angle between the meridian and the vertical circle passing through the star. A star due south has an azimuth of 0°. Due west, 90°. Due north, 180°. Due east, 270°.