The Declination of a star is analogous to geographical latitude and it is the angle between the celestial equator and the star, measured in the direction of the north or south pole. It is given in degrees, arcminutes and arcseconds
Because the apparent positions of stars change with time due to precession, aberration and nutation it is usual when making a catalogue to specify a particular date, called the Epoch, to which all entries have been reduced.
Kepler’s laws are:
I. The planets revolve in ellipses with the Sun at one focus.
II. The line between a planet and the Sun sweeps out equal areas in equal times.
III. The square of the period of revolution of a planet is proportional to the cube of its mean orbital radius.
The Meridian is an imaginary line in the sky joining the celestial poles and passing through the zenith of a particular place.
The Parallax of a star is the angle, as viewed from it, subtended by the radius of the earth’s orbit. It is measured, for example, by photographing a nearby star against the background of much more distant stars on two occasions six months apart. Knowing the size of the earth’s orbit, the distance of the star can then be be worked out.
The Parallax of a solar system object is the angle, as viewed from it, subtended by the earth’s radius. It is measured, for example, by photographing the object against the starry background from two well-separated points on the earth’s surface simultaneously. Knowing the earth’s radius, the distance can be worked out. (p.169)
The axis of the earth is not fixed in space but moves in a circle of angular radius about 23½° over a period of about 25,800 years. This movement is called Precession and is a much larger effect than the axial nutation discovered by Bradley and described in Chapter 1, Programme of research.
The Right Ascension of a star is analogous to its geographic longitude. It is simply the exact time, measured by a sidereal clock, when it is seen to cross the meridian, i.e. when it goes from the eastern side of the sky to the west due to the rotation of the earth. It is measured in hours, minutes and seconds.
Just as ordinary time is given by the rotation of the earth beneath the Sun, Sidereal time is given by the rotation of the earth beneath the stars. A sidereal day is about four minutes shorter than a solar day because the earth goes around the Sun in 365 days.