In his paper, Bergman summarised the observations and explanations of twilights back to the days of Johannes Kepler and Tycho Brahe and provided a
geometrical description accompanied by figures.
A long paper on the altitude of aurora borealis, based on his own and other’s
observations was published in two parts in 1764 [12] with an addition in 1766 [13].
During the period 1645–1715, the so-called Maunder minimum, a period of
anomalously low solar activity, auroras had been very rare phenomena. In the
eighteenth century the solar activity increased, and the solar maximum in 1761 gave
Bergman good opportunities to study auroras. In order to understand the nature of
auroras, their altitude was of great importance. If they appeared at very high altitudes, it was unlikely that they could be attributed to combustion of vapours from
the ground. Bergman described a method reported by a F C Mayer which could be
used to calculate the altitude of auroras under certain circumstances. Bergman then
described a simpler method which was based on two observations of the same
aurora. If the angle from the ground to the aurora was measured at two locations,
and the distance between the two observations was known, the altitude of the aurora
could be calculated through geometry.
3 The great draw-back was of course the need
of two measurements, but by comparing his diary with the diary of Nils Gissler
(1715–1771) in Härnösand,
4 he found that they both had observed the same auroras
and made the necessary measurements. Other pairs of measurements were taken
from literature sources. Bergman concluded that the auroras always appeared over
the clouds, usually at altitudes between 530 and 1,070 km (50–100 Swedish mil).
Auroras typically appear at 90–150 km altitude, but may occasionally stretch up to
1,000 km. After becoming professor in chemistry, it is not likely that Bergman had
much time for astronomical observations and he published no more astronomical
papers.
On June 6, 1761, planet Venus was scheduled to transit the sun (Fig. 5.1), a very
rare event. Transits of Venus occur in pairs separated by 8 years followed by a
100-year long gap to the next transit. The first transit to be observed was in 1639,
some 30 years after the invention of the telescope, followed by transits in 1761,
1769, 1874, 1882, 2004, and 2012. The transit gives a unique opportunity to study
planet Venus, and one of the objectives was to determine whether Venus had an
atmosphere or not.
Thus, astronomers had waited a long time for this opportunity and it was perhaps
the first international scientific collaboration on a large scale, with astronomers
distributed over the area were the transit would be visible. The Royal Swedish
Academy of Sciences considered sending Bergman to Torneå in northern Finland
(at the time still a Swedish province) as an observer [3]. After some consideration, it
was decided that Kajaneborg would be a better location for observations, and
Anders Planman (1724–1803), who was born in Hattula in Finland and spoke
3
Bergman does not give a reference and seems to have developed this method by himself. Whether
he was the first astronomer to use the method is unknown to me.
4
Gissler was physician and lecturer in Härnösand. He had studied physics for Anders Celsius and
Klingnestierna, chemistry for Wallerius and finally medicine for Linneaus and Rosén.
5.1 Astronomical Research
59
geometrical description accompanied by figures.
A long paper on the altitude of aurora borealis, based on his own and other’s
observations was published in two parts in 1764 [12] with an addition in 1766 [13].
During the period 1645–1715, the so-called Maunder minimum, a period of
anomalously low solar activity, auroras had been very rare phenomena. In the
eighteenth century the solar activity increased, and the solar maximum in 1761 gave
Bergman good opportunities to study auroras. In order to understand the nature of
auroras, their altitude was of great importance. If they appeared at very high altitudes, it was unlikely that they could be attributed to combustion of vapours from
the ground. Bergman described a method reported by a F C Mayer which could be
used to calculate the altitude of auroras under certain circumstances. Bergman then
described a simpler method which was based on two observations of the same
aurora. If the angle from the ground to the aurora was measured at two locations,
and the distance between the two observations was known, the altitude of the aurora
could be calculated through geometry.
3 The great draw-back was of course the need
of two measurements, but by comparing his diary with the diary of Nils Gissler
(1715–1771) in Härnösand,
4 he found that they both had observed the same auroras
and made the necessary measurements. Other pairs of measurements were taken
from literature sources. Bergman concluded that the auroras always appeared over
the clouds, usually at altitudes between 530 and 1,070 km (50–100 Swedish mil).
Auroras typically appear at 90–150 km altitude, but may occasionally stretch up to
1,000 km. After becoming professor in chemistry, it is not likely that Bergman had
much time for astronomical observations and he published no more astronomical
papers.
On June 6, 1761, planet Venus was scheduled to transit the sun (Fig. 5.1), a very
rare event. Transits of Venus occur in pairs separated by 8 years followed by a
100-year long gap to the next transit. The first transit to be observed was in 1639,
some 30 years after the invention of the telescope, followed by transits in 1761,
1769, 1874, 1882, 2004, and 2012. The transit gives a unique opportunity to study
planet Venus, and one of the objectives was to determine whether Venus had an
atmosphere or not.
Thus, astronomers had waited a long time for this opportunity and it was perhaps
the first international scientific collaboration on a large scale, with astronomers
distributed over the area were the transit would be visible. The Royal Swedish
Academy of Sciences considered sending Bergman to Torneå in northern Finland
(at the time still a Swedish province) as an observer [3]. After some consideration, it
was decided that Kajaneborg would be a better location for observations, and
Anders Planman (1724–1803), who was born in Hattula in Finland and spoke
3
Bergman does not give a reference and seems to have developed this method by himself. Whether
he was the first astronomer to use the method is unknown to me.
4
Gissler was physician and lecturer in Härnösand. He had studied physics for Anders Celsius and
Klingnestierna, chemistry for Wallerius and finally medicine for Linneaus and Rosén.
5.1 Astronomical Research
59