Programska knjižica - Hrvatsko filozofsko društvo

eports. Bošković’s third valuable source for the topic of aurora borealis was
the Moravian astronomer Christian Mayer, because the Ragusan in his treatise
also applied Mayer’s method for the determination of the aurora height from a
single position. Lastly, young Bošković shaped a scientific instruction “what
to observe in these phenomena in future”. (IM)
Four of the five propositions of Bošković’s dissertation deserve scientific
commentary. In the second proposition Bošković depicts a method for the calaculation
of the height of Earth’s atmosphere: taking that first solar rays are
seen when the Sun is 18º under the horizon, he constructs the path of light,
which, with two “reflections”, leads to atmospheric height of 20 km. Although
Bošković’s result is acceptable in comparison with the contemporary measured
results, it is rather the fruit of pure chance than of the sound model, since
the passage of light through the atmosphere is much more complex than in
Bošković’s model.
The third proposition contains the main task of Bošković’s dissertation:
determination of the aurora height. Based on the data collected by observing
the phenomenon in Rome and in Paris on 19 October 1726, and with the help
of planimetry and planar trigonometry, Bošković calculated the aurora height
as being 1080 km.
The fourth proposition solves the same problem of the aurora as observed
on 16 December 1738. This time Bošković evaluated its height as being 1220
km. The present results show that polar light appears in the atmospheric layer
between 100 and 1000 km. Therefore Bošković’s result can be understood as
a slightly exceeded value of the upper limit of the phenomenon. Inaccurate
determination of the height for several hundreds of kilometers was obviously
possible due to complex structure of the phenomenon. One should bear in
mind that the regular geometrical shape of the polar light is actually a crude
approximation.
By using Mayer’s method, Bošković solved the theoretical problem in the
fifth proposition: how to determine the height of auroral oval which is centred
on the geographical pole and whose plane is orthogonal to the Earth’s rotation
axis.
In the sixth, last proposition of his dissertation Bošković adhered to
Mairan’s explanation of the physical cause of polar light: Solar rotation, documented
by the motion of solar spots, can lead to deformation of the solar
atmosphere, which therefore obtains convex shape and is extended to ecliptic
and flows into Earth’s atmosphere. By analysis and commentary of the
conditions under which rarefied solar atmosphere mixes with the high rarefied
layer of Earth’s atmosphere, Bošković elaborated Mairan’s hypothesis and
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