3.2 Higher Parts of the Heavens
33
the south point ‘S’) and the altitude the height above the horizon. This of
course changes all the time, and is not useful for a catalogue, but important
to set a telescope at a given time from a given location on an astronomical
object.
Therefore astronomers use a coordinate system that rotates along with
the sky, i.e. relative to the poles and the equator 90 ◦ away from these (righthand panel in Fig. 3.2). One of the two coordinates is comparable to the
geographical latitude and is called Declination; this is the angle between
the star and the projection on the equator. So it is also 90 ◦ minus the angle
from the star to the North Pole (South Pole in the southern hemisphere;
the declination is then negative). The other coordinate, equivalent to the
geographical longitude is called it Right Ascension (in Fig. 3.1 indicated
by R.A.). This is the angle from the projection of the position of that star
on the equator to the point (the ‘vernal equinox’) where the Sun crosses the
equator in the northern spring (i.e., on 20 or 21 March). This is indicated in
Fig. 3.1 with the symbol ‘à’.
Figure 3.1 shows the situation with respect to the horizon and zenith. In
the sky, the North Pole (’NP’) is the point directly above the Earth’s North
Pole. As seen from any position on Earth, the sky rotates around its axis
once a day; the celestial poles stand still. In the northern hemisphere, the
star Polaris happens to be close to the North Pole (currently about twothirds of a degree from it). Figure 3.3 shows the diurnal rotation on a long
exposure. The position of the pole above the horizon can be determined
with circumpolar stars, which are so close to the pole that they do not set.
One then measures the height above the horizon when such a star passes
through the meridian above and below the pole; the pole is then exactly
midway between the two measurements.
In Fig. 3.1 the time is chosen such that the vernal equinox just crosses the
meridian. The sky rotates and the vernal equinox, like the stars, rises every
day in the east and sets in the west. Accurate measurement of the position
of the Sun is necessary to know where this vernal equinox is located relative
to the stars at any given moment.
The easiest way to determine these coordinates is to take (in the northern
hemisphere, but for the southern hemisphere the procedure is similar) the
moment at which the star passes through the meridian. One then measures
the angle above the horizon and the exact time. The declination follows
from the first measurement, because it is known how far above the horizon
the North Pole is at one’s location of observation. From the second, that
of the exact time of the meridian passage of the star, you deduce the right
ascension. Because the latter is a time measurement, right ascension is
33
the south point ‘S’) and the altitude the height above the horizon. This of
course changes all the time, and is not useful for a catalogue, but important
to set a telescope at a given time from a given location on an astronomical
object.
Therefore astronomers use a coordinate system that rotates along with
the sky, i.e. relative to the poles and the equator 90 ◦ away from these (righthand panel in Fig. 3.2). One of the two coordinates is comparable to the
geographical latitude and is called Declination; this is the angle between
the star and the projection on the equator. So it is also 90 ◦ minus the angle
from the star to the North Pole (South Pole in the southern hemisphere;
the declination is then negative). The other coordinate, equivalent to the
geographical longitude is called it Right Ascension (in Fig. 3.1 indicated
by R.A.). This is the angle from the projection of the position of that star
on the equator to the point (the ‘vernal equinox’) where the Sun crosses the
equator in the northern spring (i.e., on 20 or 21 March). This is indicated in
Fig. 3.1 with the symbol ‘à’.
Figure 3.1 shows the situation with respect to the horizon and zenith. In
the sky, the North Pole (’NP’) is the point directly above the Earth’s North
Pole. As seen from any position on Earth, the sky rotates around its axis
once a day; the celestial poles stand still. In the northern hemisphere, the
star Polaris happens to be close to the North Pole (currently about twothirds of a degree from it). Figure 3.3 shows the diurnal rotation on a long
exposure. The position of the pole above the horizon can be determined
with circumpolar stars, which are so close to the pole that they do not set.
One then measures the height above the horizon when such a star passes
through the meridian above and below the pole; the pole is then exactly
midway between the two measurements.
In Fig. 3.1 the time is chosen such that the vernal equinox just crosses the
meridian. The sky rotates and the vernal equinox, like the stars, rises every
day in the east and sets in the west. Accurate measurement of the position
of the Sun is necessary to know where this vernal equinox is located relative
to the stars at any given moment.
The easiest way to determine these coordinates is to take (in the northern
hemisphere, but for the southern hemisphere the procedure is similar) the
moment at which the star passes through the meridian. One then measures
the angle above the horizon and the exact time. The declination follows
from the first measurement, because it is known how far above the horizon
the North Pole is at one’s location of observation. From the second, that
of the exact time of the meridian passage of the star, you deduce the right
ascension. Because the latter is a time measurement, right ascension is