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R. Barrett and P. P. Delsanto
This explanation of the motion of stars, sun, moon and planets held until
the Sixteenth Century, when Copernicus revived an earlier, and forgotten,
heliocentric model by Aristarchus. The troubles accompanying the Reformation, and conflict between different political powers (including the Roman
Church), made the Copernican revolution particularly dramatic, as we know
from the history of Galileo, who was forced to recant his belief in the
Copernican model.
From the Seventeenth century, modern science became pervasive in all
areas, including astronomy and cosmology. Telescopes allowed much more
powerful observations of the sky. Newton’s theory of universal gravitation
shed new light on celestial mechanics, and it was realised that planetary orbits
were elliptical, not circular, removing the need for epicycles. At the end of
the Nineteenth century, the description and interpretation of the universe in
scientific terms began.
However, even today the Ptolemaic model still finds an application. The
projectors for modern planetariums are built using gears and motors to carry
out the functions of the deferents and epicycles of Ptolemy. Their success is a
testament to the accuracy of his model at predicting the motion of heavenly
bodies, even though we now know that it has no physical basis. The planetariums perhaps provide us a warning that simply because a model describes
a set of observations, it is not necessarily a correct representation of nature.
Figure 10.2 shows a photograph of a Zeiss Klein planetarium projector.
10.3 Why is the Sky Dark at Night?
Let us begin our explorations of the universe by asking the simple question
above, which on the face of it appears naïve to the point of childishness.
Certainly the reaction of most people when first encountering this question is to reply somewhat scornfully: “because the sun is on the other side
of the earth”. The retort is usually accompanied by a few derogatory asides.
However, what on the surface appears an inane query contains within it some
deeply puzzling aspects when we rigorously apply the tool of logic, which we
have discussed in Chap. 3. So much so, that the question has been given the
name Olbers’ Paradox after the German astronomer Heinrich Wilhelm Olbers
(1758–1840), who was one of the first to raise it. 1
1 Actually, this paradox, in one form or another, had been considered since at least a couple of
centuries before Olbers, who formulated it in 1823.
R. Barrett and P. P. Delsanto
This explanation of the motion of stars, sun, moon and planets held until
the Sixteenth Century, when Copernicus revived an earlier, and forgotten,
heliocentric model by Aristarchus. The troubles accompanying the Reformation, and conflict between different political powers (including the Roman
Church), made the Copernican revolution particularly dramatic, as we know
from the history of Galileo, who was forced to recant his belief in the
Copernican model.
From the Seventeenth century, modern science became pervasive in all
areas, including astronomy and cosmology. Telescopes allowed much more
powerful observations of the sky. Newton’s theory of universal gravitation
shed new light on celestial mechanics, and it was realised that planetary orbits
were elliptical, not circular, removing the need for epicycles. At the end of
the Nineteenth century, the description and interpretation of the universe in
scientific terms began.
However, even today the Ptolemaic model still finds an application. The
projectors for modern planetariums are built using gears and motors to carry
out the functions of the deferents and epicycles of Ptolemy. Their success is a
testament to the accuracy of his model at predicting the motion of heavenly
bodies, even though we now know that it has no physical basis. The planetariums perhaps provide us a warning that simply because a model describes
a set of observations, it is not necessarily a correct representation of nature.
Figure 10.2 shows a photograph of a Zeiss Klein planetarium projector.
10.3 Why is the Sky Dark at Night?
Let us begin our explorations of the universe by asking the simple question
above, which on the face of it appears naïve to the point of childishness.
Certainly the reaction of most people when first encountering this question is to reply somewhat scornfully: “because the sun is on the other side
of the earth”. The retort is usually accompanied by a few derogatory asides.
However, what on the surface appears an inane query contains within it some
deeply puzzling aspects when we rigorously apply the tool of logic, which we
have discussed in Chap. 3. So much so, that the question has been given the
name Olbers’ Paradox after the German astronomer Heinrich Wilhelm Olbers
(1758–1840), who was one of the first to raise it. 1
1 Actually, this paradox, in one form or another, had been considered since at least a couple of
centuries before Olbers, who formulated it in 1823.