4. The celestial sphere

The celestial sphere

Figure 2

The distances of the stars distributed over the surface of the spherical vault of the sky above us, differ greatly, but for practical purposes we are only concerned with their direction. In order to record their directions it is convenient to imagine the stars as distributed over a spherical surface, which the astronomers call the celestial sphere (fig.2). The extension of the earth’s axis to the celestial sphere is the celestial axis, and its ends mark the north and south celestial poles. The projection of the earth’s equator is the celestial equator. There are also circles on the celestial sphere analogous to earth’s meridian called hour circles. The observer’s celestial meridian is the projection of the observer’s terrestrial meridian. The projection of a latitude circle is a diurnal circle and represents the daily apparent path of a celestial body. As the earth rotates, the celestial sphere appears to turn about the earth’s axis. It is, therefore, convenient to imagine that the celestial sphere moves, making one turn about every 24 hours, while the earth remains at rest. Because the earth maintains the tilt of its equatorial plane with respect to its orbital plane (around the Sun) at a nearly constant angle of 23.5° and because the direction of the earth’s rotation axis is nearly constant (one cycle requires 26000 years), the celestial poles appears to remain fixed in the same place. The north celestial pole is marked approximately by the pole star, but the south pole occurs in a rather barren patch of sky and does not have a conspicuous marker. The stars retain the same relative position from night to night, to all intents and purposes, from century to century. There only obvious movement is their diurnal passage across the sky, and this comes as a result of the earth’s rotation about its axis. Since the earth spins from west to east, the stars appear to revolve in the opposite direction, from east to west. The motion of the earth about the Sun (its revolution) means that if an observer imagines the celestial sphere oriented with respect to the earth, the Sun will move across the sphere completing one trip around the celestial sphere each year (fig. 2). The apparent path of the Sun along the celestial sphere is called the ECLIPTIC. Its name derives from the fact that eclipses can occur only when the moon crosses this apparent path. Since the earth’s equator is tilted with respect to its orbit at an angle of 23.5°, the Sun’s apparent path around the celestial sphere is inclined to the equator at this critical angle, crossing it at two points, which like the celestial poles appears as fixed points on the celestial sphere. One of these two points, known as the VERNAL EQUINOX, is used as a reference point for one of the celestial co-ordinates.

The co-ordinates RIGHT ASCENSION (R.A.) and DECLINATION (Dec) which are analogous to LONGITUDE and LATITUDE, respectively on the terrestrial sphere (fig.2), determine the position of a celestial body on the celestial sphere. Right ascension is the angle measured from the hour circle that passes through the vernal equinox (the intersection of the celestial equator and the ecliptic) eastward along the celestial equator, to the hour circle that passes through the body. This angle varies from 0 to 360°, but is measured in hours from 0 to 24 hours.

Declination is the angle measured from the celestial equator, north or south along an hour circle, to the diurnal circle that passes through the body. This angle varies from 0° to 90° north or south. Thus the position of the star in fig 2. is 3h, 40° N.