Astronomy Principles and Practice Fourth Edition.pdf

152 Geocentric planetary phenomena
System but could be looked upon as merely a simpler mathematical device. It is probable, however,
that Copernicus himself believed it to be a physical reality. Certainly, it aroused deep psychological
and religious opposition, displacing as it did the Earth, the home of mankind, from the centre of the
Universe to the status of a satellite.
There were indeed two scientific objections that could be made, illustrating that a little learning is
a dangerous thing.
Opponents of the Copernican theory pointed out that if the Earth moved round the Sun (a) the
Moon would be left behind, (b) the brighter, and, therefore nearer stars, would show a parallactic
yearly shift against the background of the fainter and presumably more distant stars. Neither of these
events was observed.
The vindication of the Copernican theory had to await the work of three men, Tycho Brahé (1546–
1601 AD), Johannes Kepler (1571–1630 AD) and Galileo Galilei (1564–1642 AD). We will consider
Galileo’s work first of all.
12.4 The astronomical discoveries of Galileo
In 1609, Galileo built his first telescope and initiated a revolution in astronomy. Among his discoveries
with that telescope and improved instruments he constructed in later years were the following:
• He saw the Milky Way resolved into stars.
• He discovered sunspots. From their movements he measured the period of rotation of the Sun to
be close to 27 days.
• When he turned his telescopes to the Moon he observed mountains, craters and plains. He also
measured the Moon’s apparent wobble on its axis, enabling us to see more than half of the lunar
surface.
• The planets showed disks of appreciable size.
• Venus showed phases like the Moon.
• He discovered Jupiter had four moons of its own.
• Saturn was seen to have two blurred appendages which Galileo was quite unable to interpret.
• The Pleiades, the star group containing six or seven stars as seen with the unaided eye, had at least
36 stars when viewed with the telescope.
• The three nebulous stars in Praesepe became 40 in number.
Two discoveries, in particular, gave Galileo tremendous satisfaction, enabling him to dismiss the
Ptolemaic System in favour of the Copernican one.
To those who said that the Moon would be left behind if the Earth moved, he could point to
Jupiter’s satellites. Even in the Ptolemaic System, Jupiter moved, yet its four moons were not left
behind.
The phases of Venus were even more convincing as an argument in favour of the heliocentric
theory.
Undoubtedly, the planets shone by the reflected light of the Sun. Their phases, as observed from
the Earth, would depend upon whether or not the Earth was the centre of the Solar System.
In the Ptolemaic System (figure 12.3(a)), Venus could never lie on the other side of the Sun from
the Earth (in superior conjunction as we would now put it). In the telescope, it would always remain
crescent in phase, and its apparent angular size would not change greatly.
In the Copernican System (figure 12.3(b)), Venus could be in superior conjunction, V 4 . Before
it became lost in the Sun’s glare, it would show an almost full phase, rather like full Moon. When it
reappeared some days later on the other side of the Sun, having passed behind the Sun, its phase would
again be almost full but would be waning, instead of waxing. As the planet moved round in its orbit
from V 4 to V 2 via V 1 , not only would the phase wane until a thin crescent was seen just before V 2 but