40
3 Astronomy Around 1875
the Sun in the positions of stars in the sky. Each star will describe an ellipse
in the course of the year as a result of the changing direction of observation
from the moving Earth. The further away the star, the smaller the ellipse.
For stars that are relatively close by, this motion should be measurable by
comparing the position of a nearby star to stars in the background that are
further away. The semi-major axis of the ellipse is called the ‘parallax’ and
that is thus also a measure for the distance.
Now, despite detailed attempts, parallaxes could not be measured and
so the heliocentric model was either incorrect, or the distances of the stars
were too large. The latter, of course, is the case. Finally Friedrich Bessel
succeeded in 1838 in measuring the parallax of the star 61 Cygni in the constellation Cygnus (0.31 seconds of arc); Thomas James Henderson (1798–
1844) from Scotland did the same in 1839 for α Centauri. The latter—in
the southern constellation Centaur, see Fig. 3.5—is the closest star to our
Solar System. It is at 4.35 lightyears away from us, where a lightyears is the
distance that the light travels in a year. At 300,000 km/s, a lightyear comes
out as 9.5 trillion (9.5 × 10 12 ) km. Astronomers usually use the unit parsec
(pc); that is the distance of a star with a parallax of 1 second of arc, and that
amounts to 3.26 lightyears. The parallax of α Centauri, which is at 1.3 pc,
is therefore 0.75 seconds of arc (indicated as 0 .75) and that is the largest
parallax of all stars (except for the Sun of course). We now know, thanks to
accurate parallax measurements from space by the ESA satellite Hipparcos,
that there are only 271 stars with a parallax greater than 0 .1; most of them
are so faint that in the middle of the nineteenth century astronomers had not
even attempted to measure their parallaxes.
3.3 The Construction of the Heavens
It was the aforementioned William Herschel who was the first to attack the
problem of the spatial distribution of the stars. He called this the Construction of the Heavens. He built his own telescopes, the largest and best of his
time, using mirrors of speculum, an alloy of copper and tin in a ratio of two
to one, which he himself ground into the desired shape. He did this at his
house in Slough, over 30 km west of London and not far from Bath. His
largest telescope was the so-called 40-foot telescope (see Fig. 3.7). This 40
ft (just over 12 m) refers to the focal length of the primary mirror; this mirror
had a diameter of 48 inch. (1.20 m). That was huge and unprecedented at
the time. He carried out his first study with a predecessor; that was a 20-foot
telescope, which looked a lot like the 40-foot telescope depicted. He kept
3 Astronomy Around 1875
the Sun in the positions of stars in the sky. Each star will describe an ellipse
in the course of the year as a result of the changing direction of observation
from the moving Earth. The further away the star, the smaller the ellipse.
For stars that are relatively close by, this motion should be measurable by
comparing the position of a nearby star to stars in the background that are
further away. The semi-major axis of the ellipse is called the ‘parallax’ and
that is thus also a measure for the distance.
Now, despite detailed attempts, parallaxes could not be measured and
so the heliocentric model was either incorrect, or the distances of the stars
were too large. The latter, of course, is the case. Finally Friedrich Bessel
succeeded in 1838 in measuring the parallax of the star 61 Cygni in the constellation Cygnus (0.31 seconds of arc); Thomas James Henderson (1798–
1844) from Scotland did the same in 1839 for α Centauri. The latter—in
the southern constellation Centaur, see Fig. 3.5—is the closest star to our
Solar System. It is at 4.35 lightyears away from us, where a lightyears is the
distance that the light travels in a year. At 300,000 km/s, a lightyear comes
out as 9.5 trillion (9.5 × 10 12 ) km. Astronomers usually use the unit parsec
(pc); that is the distance of a star with a parallax of 1 second of arc, and that
amounts to 3.26 lightyears. The parallax of α Centauri, which is at 1.3 pc,
is therefore 0.75 seconds of arc (indicated as 0 .75) and that is the largest
parallax of all stars (except for the Sun of course). We now know, thanks to
accurate parallax measurements from space by the ESA satellite Hipparcos,
that there are only 271 stars with a parallax greater than 0 .1; most of them
are so faint that in the middle of the nineteenth century astronomers had not
even attempted to measure their parallaxes.
3.3 The Construction of the Heavens
It was the aforementioned William Herschel who was the first to attack the
problem of the spatial distribution of the stars. He called this the Construction of the Heavens. He built his own telescopes, the largest and best of his
time, using mirrors of speculum, an alloy of copper and tin in a ratio of two
to one, which he himself ground into the desired shape. He did this at his
house in Slough, over 30 km west of London and not far from Bath. His
largest telescope was the so-called 40-foot telescope (see Fig. 3.7). This 40
ft (just over 12 m) refers to the focal length of the primary mirror; this mirror
had a diameter of 48 inch. (1.20 m). That was huge and unprecedented at
the time. He carried out his first study with a predecessor; that was a 20-foot
telescope, which looked a lot like the 40-foot telescope depicted. He kept