3.2 Higher Parts of the Heavens
35
The most famous classical catalog is that of Hipparchus of Nicaea
(c.190—c.120 BC). It was he who, as far as we know, first used the magnitude scale. He made a remarkable discovery, namely that the position of the
zero point of the right ascension changes over time. This point, called vernal (spring) equinox because then day and night are equally long, changes
because the rotation axis of the Earth does not stand still in space. Compare
this to a spinning top. When it is inclined, the attraction of the Earth will
want to pull it down, but that is kept in check by its rotation. The axis of the
spinning top then rotates while maintaining the angle to the Earth’s surface.
The same goes for the Earth. The axis makes an angle of about 23 ◦ with
a line perpendicular to its orbital plane and because the Earth is slightly
flattened, the Sun applies a torque or rotational force to it, pulling the axis
of the Earth towards that plane. But the rotation of the Earth, just like with
the top, ensures that the rotation axis swings around, always maintaining
the same angle with the orbital plane. This so-called precession involves a
slow circular motion of the North and South Poles in the sky around the
poles of the ecliptic; a complete 360-degree revolution, after which the poles
return to the same point on the sky, takes about 26,000 years. As a result
of the precession, the equinox shifts more than 50 seconds of arc per year,
so the exact value of the equinox is of great importance. Right ascension
and declination change slowly with time and you then have to allow for this
when comparing the positions of a star at different times. The amount of the
change of the equinox was not yet known very precisely in the middle of
the nineteenth century; it was not until around 1900 that Simon Newcomb,
whom we already met in connection with the solar parallax, determined a
value that was accurate by modern standards.
Claudius Ptolemæus (Ptolemy) (c.90–c.168) used Hipparchus’ catalog,
supplemented with his own observations, to publish a new catalog in his
famous book Almagest, which summarized the astronomy of his time. This
catalog, which has been preserved, contains over a thousand stars with
positions accurate to about 10 arc minutes (one third of the diameter of the
full Moon in the sky).
Star catalogs became more and more important for navigation, and more
accurate star positions were required. An early method to determine the
positions of stars was using a quadrant or sextant (see Fig. 3.4). As with
the legs of a compass, one looked along two lines at two different stars, or
at a star and the horizon. On the scale along the quadrant (rectangular) or
sextant (60 ◦ -construction) the angle was then read off. Later there were wall
quadrants, which were often pointed at the meridian, i.e. in the direction
to the south or north. The Dane Tycho Brahe (1546–1601) was a famous
35
The most famous classical catalog is that of Hipparchus of Nicaea
(c.190—c.120 BC). It was he who, as far as we know, first used the magnitude scale. He made a remarkable discovery, namely that the position of the
zero point of the right ascension changes over time. This point, called vernal (spring) equinox because then day and night are equally long, changes
because the rotation axis of the Earth does not stand still in space. Compare
this to a spinning top. When it is inclined, the attraction of the Earth will
want to pull it down, but that is kept in check by its rotation. The axis of the
spinning top then rotates while maintaining the angle to the Earth’s surface.
The same goes for the Earth. The axis makes an angle of about 23 ◦ with
a line perpendicular to its orbital plane and because the Earth is slightly
flattened, the Sun applies a torque or rotational force to it, pulling the axis
of the Earth towards that plane. But the rotation of the Earth, just like with
the top, ensures that the rotation axis swings around, always maintaining
the same angle with the orbital plane. This so-called precession involves a
slow circular motion of the North and South Poles in the sky around the
poles of the ecliptic; a complete 360-degree revolution, after which the poles
return to the same point on the sky, takes about 26,000 years. As a result
of the precession, the equinox shifts more than 50 seconds of arc per year,
so the exact value of the equinox is of great importance. Right ascension
and declination change slowly with time and you then have to allow for this
when comparing the positions of a star at different times. The amount of the
change of the equinox was not yet known very precisely in the middle of
the nineteenth century; it was not until around 1900 that Simon Newcomb,
whom we already met in connection with the solar parallax, determined a
value that was accurate by modern standards.
Claudius Ptolemæus (Ptolemy) (c.90–c.168) used Hipparchus’ catalog,
supplemented with his own observations, to publish a new catalog in his
famous book Almagest, which summarized the astronomy of his time. This
catalog, which has been preserved, contains over a thousand stars with
positions accurate to about 10 arc minutes (one third of the diameter of the
full Moon in the sky).
Star catalogs became more and more important for navigation, and more
accurate star positions were required. An early method to determine the
positions of stars was using a quadrant or sextant (see Fig. 3.4). As with
the legs of a compass, one looked along two lines at two different stars, or
at a star and the horizon. On the scale along the quadrant (rectangular) or
sextant (60 ◦ -construction) the angle was then read off. Later there were wall
quadrants, which were often pointed at the meridian, i.e. in the direction
to the south or north. The Dane Tycho Brahe (1546–1601) was a famous