3.1 Astronomy of the Solar System
29
such, undoubtedly because of his primitive optics. Even more ironic is that
John Frederick William Herschel (1792–1871), the son of William Herschel, observed Neptune in 1830, according to his notes, but also failed to
recognize it as a planet in his telescope, despite the better quality of his
images. So we very closely missed the interesting situation that the only
two planets in our Solar System that are not visible to the naked eye would
have been, both accidentally, discovered by a father and son.
On the first day of the nineteenth century, January 1, 1801, the first of
a series of small planets or asteroids was observed, which we now know
to be a swarm of objects, situated between the orbits of Mars and Jupiter.
In 1802, 1803 and 1807 three more were found, and after a long interlude
from 1845 onward even more of them. At the end of 1850 the number had
risen to thirteen; at the end of 1870 the number was 157 and at the close of
the nineteenth century, on 31 December 1900, it was 463. From a certain
apparent regularity in the distances between the planets and the Sun one
could expect that in the relatively large space between Mars and Jupiter
there should be another planet. Planets form because dust particles fuse
together to form smaller debris, called planetesimals. This then produced
eventually the planets, but the gravitation of the largest planet Jupiter, which
is close to the asteroid belt, must have disturbed this process.
In the nineteenth century people were already familiar with comets. These
are objects that move in highly elliptical orbits. They can therefore get far
from the Sun, at which time they are not visible for us; but they must
return regularly. Edmond Halley (1656–1742) suggested after noting the
close similarity in their orbits that the comet visible in 1682 may have
been the same as that of 1607 and 1531, and predicted that it would return
again in 1758/9. And several scientists such as Immanuel Kant (1724–1804)
had already concluded that comets should consist of volatile materials that
evaporated because of sunlight, which causes the spectacular tails of comets
when they come close to the Sun.
From the positions in the sky of objects in the Solar System, it is possible
to calculate with great precision the orbits of these objects relative to each
other in space; these were therefore also known in the nineteenth century.
But because only positions and no distances could be measured, the result
was a good map of the Solar System, on which the orbits of the celestial
bodies are accurately represented relative to each other, but the scale of
that map would remained unknown. For this it is necessary to measure the
distance between one of these objects and the Earth at a certain point in time.
At first astronomers used the planet Mars for that, the first planet outside
the orbit of the Earth. when Mars and the Earth are on one line with the
29
such, undoubtedly because of his primitive optics. Even more ironic is that
John Frederick William Herschel (1792–1871), the son of William Herschel, observed Neptune in 1830, according to his notes, but also failed to
recognize it as a planet in his telescope, despite the better quality of his
images. So we very closely missed the interesting situation that the only
two planets in our Solar System that are not visible to the naked eye would
have been, both accidentally, discovered by a father and son.
On the first day of the nineteenth century, January 1, 1801, the first of
a series of small planets or asteroids was observed, which we now know
to be a swarm of objects, situated between the orbits of Mars and Jupiter.
In 1802, 1803 and 1807 three more were found, and after a long interlude
from 1845 onward even more of them. At the end of 1850 the number had
risen to thirteen; at the end of 1870 the number was 157 and at the close of
the nineteenth century, on 31 December 1900, it was 463. From a certain
apparent regularity in the distances between the planets and the Sun one
could expect that in the relatively large space between Mars and Jupiter
there should be another planet. Planets form because dust particles fuse
together to form smaller debris, called planetesimals. This then produced
eventually the planets, but the gravitation of the largest planet Jupiter, which
is close to the asteroid belt, must have disturbed this process.
In the nineteenth century people were already familiar with comets. These
are objects that move in highly elliptical orbits. They can therefore get far
from the Sun, at which time they are not visible for us; but they must
return regularly. Edmond Halley (1656–1742) suggested after noting the
close similarity in their orbits that the comet visible in 1682 may have
been the same as that of 1607 and 1531, and predicted that it would return
again in 1758/9. And several scientists such as Immanuel Kant (1724–1804)
had already concluded that comets should consist of volatile materials that
evaporated because of sunlight, which causes the spectacular tails of comets
when they come close to the Sun.
From the positions in the sky of objects in the Solar System, it is possible
to calculate with great precision the orbits of these objects relative to each
other in space; these were therefore also known in the nineteenth century.
But because only positions and no distances could be measured, the result
was a good map of the Solar System, on which the orbits of the celestial
bodies are accurately represented relative to each other, but the scale of
that map would remained unknown. For this it is necessary to measure the
distance between one of these objects and the Earth at a certain point in time.
At first astronomers used the planet Mars for that, the first planet outside
the orbit of the Earth. when Mars and the Earth are on one line with the