In the years after 1500, Copernicus came to realize that a heliocentric
model has several advantages beyond providing a natural
explanation of retrograde motion. In the Ptolemaic system, the
arrangement of the planets—that is, which are close to the Earth
and which are far away—was chosen in large part by guesswork.
But using a heliocentric model, Copernicus could determine the
arrangement of the planets without ambiguity.
Copernicus realized that because Mercury and Venus are always
observed fairly near the Sun in the sky, their orbits must be
smaller than the Earth’s. Planets in such orbits are called inferior
planets. The other visible planets—Mars, Jupiter, and
Saturn—are sometimes seen on the side of the celestial sphere opposite
the Sun, so these planets appear high above the horizon
at midnight (when the Sun is far below the horizon).
When this happens, the Earth must lie between the Sun and these planets.
Copernicus therefore concluded that the orbits of Mars, Jupiter,
and Saturn must be larger than the Earth’s orbit. Hence, these
planets are called superior planets.
Uranus and Neptune, as well as Pluto and a number of other
small bodies called asteroids that also orbit the Sun, were discovered
after the telescope was invented (and after the death of
Copernicus). All of these can be seen at times in the midnight sky,
so these also have orbits larger than the Earth’s.
The heliocentric model also explains why planets appear in
different parts of the sky on different dates. Both inferior planets
(Mercury and Venus) go through cycles: The planet is seen in the
west after sunset for several weeks or months, then for several
weeks or months in the east before sunrise, and then in the west
after sunset again.
When Mercury or
Venus is visible after sunset, it is near greatest eastern elongation.
(The angle between the Sun and a planet as viewed from Earth is
called the planet’s elongation.) The planet’s position in the sky is
as far east of the Sun as possible, so it appears above the western
horizon after sunset (that is, to the east of the Sun) and is often
called an “evening star.” At greatest western elongation, Mercury
or Venus is as far west of the Sun as it can possibly be. It then
rises before the Sun, gracing the predawn sky as a “morning star”
in the east. When Mercury or Venus is at inferior conjunction, it
is between us and the Sun, and it is moving from the evening sky
into the morning sky. At superior conjunction, when the planet is
on the opposite side of the Sun, it is moving back into the evening
sky.
A superior planet such as Mars, whose orbit is larger than
the Earth’s, is best seen in the night sky when it is at opposition.
At this point the planet is in the part of the sky opposite the Sun
and is highest in the sky at midnight. This is also when the planet
appears brightest, because it is closest to us. But when a superior
planet like Mars is located behind the Sun at conjunction, it is
above the horizon during the daytime and thus is not well placed
for nighttime viewing.
Also Read: Copernicus and his work
What do you think about the out of the box thinking of copernicus in that era?
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