38
3 Astronomy Around 1875
onds of arc. James Bradley (1693–1762), third Astronomer Royal, improved
the accuracy to just a few seconds of arc. His 60,000 observations of 300
bright stars are remained unreduced (corrected for observational effects),
but they were later used by Friedrich Wilhelm Bessel (1784–1846) to published a catalog in 1818. The state of the art around 1875 was the work
of Friedrich Wilhelm August Argelander (1799–1875) from the Bonner
Sternwarte. He undertook the Bonner Durchmusterung (BD), for which he
observed a total of some 325,000 stars between 1852 and 1862. He determined their positions with an accuracy of one-tenth of a minute of arc and
brightnesses to within 0.1 magnitude. This was a complete sample up to
about magnitude 9, but it also contained many stars of magnitude 10. But
again, it concerned only stars in the northern hemisphere. Under his successor Eduard Schönfeld (1828–1891) an extension to the south (but from
Europe) was made between 1875 and 1881 which went as far as southern stars can still be seen from Bonn (the Südliche Durchmusterung, with
133,000 stars). The plan was then to further measure the southern sky from
the Córdoba Observatorio Astronómico in Argentina, but that only started in
1892. In the meantime, the use of meridian circles (see Fig. 3.6), telescopes
that can only move around one axis and can only observe in the meridian,
had become common practice.
Stars do not stand still with respect to each other in the sky, although
over a lifetime the change is imperceptible with a few exceptions. Already
in 1718 Edmond Halley noticed that the bright stars Sirius, Arcturus and
Aldebaran had changed their position by more than half a degree compared
to the catalog of Hipparchus. As we already saw, this motion in the sky
(usually expressed in seconds of arc per year or per century) is called the
‘proper motion’. The fact that the Sun does not stand still in space either, is
shown by the systematics of the proper motions of stars across the sky. Think
of a straight road in a landscape in which two trees stand in the distance on
either side of the road. When you drive a car on that road towards the trees,
you see them first close together, but then their separation increases faster
and faster until they are opposite each other at the moment you pass them.
Eventually, they will get back together behind you. If all stars except the
Sun would stand still, the proper motions of all stars in the sky would be
directed towards the point where the Sun (and the Earth) comes from, and
would move out from the point where the Sun moves towards (the Apex).
Of course, this pattern is less clear in the case of space motions by the
stars themselves, but it would still be possible to find it as a systematic pattern superimposed on a presumably random distribution of proper motions.
This was confirmed by William Herschel as early as 1783 on the basis of
3 Astronomy Around 1875
onds of arc. James Bradley (1693–1762), third Astronomer Royal, improved
the accuracy to just a few seconds of arc. His 60,000 observations of 300
bright stars are remained unreduced (corrected for observational effects),
but they were later used by Friedrich Wilhelm Bessel (1784–1846) to published a catalog in 1818. The state of the art around 1875 was the work
of Friedrich Wilhelm August Argelander (1799–1875) from the Bonner
Sternwarte. He undertook the Bonner Durchmusterung (BD), for which he
observed a total of some 325,000 stars between 1852 and 1862. He determined their positions with an accuracy of one-tenth of a minute of arc and
brightnesses to within 0.1 magnitude. This was a complete sample up to
about magnitude 9, but it also contained many stars of magnitude 10. But
again, it concerned only stars in the northern hemisphere. Under his successor Eduard Schönfeld (1828–1891) an extension to the south (but from
Europe) was made between 1875 and 1881 which went as far as southern stars can still be seen from Bonn (the Südliche Durchmusterung, with
133,000 stars). The plan was then to further measure the southern sky from
the Córdoba Observatorio Astronómico in Argentina, but that only started in
1892. In the meantime, the use of meridian circles (see Fig. 3.6), telescopes
that can only move around one axis and can only observe in the meridian,
had become common practice.
Stars do not stand still with respect to each other in the sky, although
over a lifetime the change is imperceptible with a few exceptions. Already
in 1718 Edmond Halley noticed that the bright stars Sirius, Arcturus and
Aldebaran had changed their position by more than half a degree compared
to the catalog of Hipparchus. As we already saw, this motion in the sky
(usually expressed in seconds of arc per year or per century) is called the
‘proper motion’. The fact that the Sun does not stand still in space either, is
shown by the systematics of the proper motions of stars across the sky. Think
of a straight road in a landscape in which two trees stand in the distance on
either side of the road. When you drive a car on that road towards the trees,
you see them first close together, but then their separation increases faster
and faster until they are opposite each other at the moment you pass them.
Eventually, they will get back together behind you. If all stars except the
Sun would stand still, the proper motions of all stars in the sky would be
directed towards the point where the Sun (and the Earth) comes from, and
would move out from the point where the Sun moves towards (the Apex).
Of course, this pattern is less clear in the case of space motions by the
stars themselves, but it would still be possible to find it as a systematic pattern superimposed on a presumably random distribution of proper motions.
This was confirmed by William Herschel as early as 1783 on the basis of