Download Episode 14: Planetary paths-2

Episode 14: Planetary paths-2
[Note for script writer: This brief has been prepared keeping the average non-specialist
listener in mind. The attempt has been to develop the subject in a logical sequence that could
bring out excitement of the process of making new discoveries and relating them to the
gradual understanding of the nature of the universe. It is designed to be a popular account of
the development of astronomy as a scientific discipline and its impact on our understanding
of the cosmos. Since the treatment is not rigorous, many details are not included. The script
has to be developed along the outline suggested using simple colloquial language as far as
possible. The dialogues and conversations need to be lively without any suggestion of
pedagogy.]
Points to be covered:
- Works of Tycho Brahe and Johannes Kepler
- Kepler’s laws of planetary motion
- Biographies of Tycho Brahe and Kepler
Outline of content:
When Copernicus placed the Sun at the centre with the planets going around it his ideas were
not accepted at first. But at the same it made a deep impact on the development science and
scientific thought, especially on development of astronomy. Yet there were serious flaws in
the Copernican model, which assumed the orbits of the planets to be circular, whereas in
reality they are elliptical, or slightly elongated which makes the distance of the planets from
the Sun vary as they go about their respective orbits. Also, the planets do not move with
uniform speed around the Sun, as was believed by Copernicus; they move faster as they come
closer to the Sun and slow down when moving away.
How did astronomers come to know about these facts? It was not a simple task. It took years
of meticulous observation and analysis to find out the truth. And the credit for this goes to
two astronomers – Tycho Brahe of Denmark and Johannes Kepler of Germany.
At the age of 14, Tycho Brahe was a student of law at the University of Copenhagen when
two natural events turned his interest from law to astronomy. The first was a total eclipse of
the Sun, which had been predicted for 21 August 1560. When the eclipse actually occurred
on the predicted date, Tycho was impressed. He was so moved that he wanted to learn more
about astronomy.
Once his interest in astronomy was aroused, Tycho was eager to learn more. He persuaded
his uncle, with whom he lived, to send him to Leipzig University in Germany where he could
study under the leading astronomers of his time.
The second important event in Tycho’s life occurred in August 1563 when the planets Jupiter
and Saturn came close in the sky – a phenomenon known as a ‘conjunction’. Tycho made his
first recorded observation of the conjunction. After his observation, when he looked up the
then existing almanacs, which gave the predicted positions of the planets on different dates of
the year, he found they had lots of errors. Tycho’s observed positions of Jupiter and Saturn
did not match with the positions given in the almanacs; they were off by several days. This
was a turning point in Tycho’s life; he decided to take upon himself the task of making
accurate observations of the sky to correct the existing tables. And that involved years of
dedicated observation from an observatory Tycho built in 1571.
An important discovery made by Tycho was that of a ‘nova’ or a ‘new star’ in the
constellation of Cassiopeia in November 1572. This was a faint star that had suddenly
brightened up and appeared brighter than the planet Venus and was visible during the day.
For the first time a star had been seen to change in brightness so dramatically, and it went
against the earlier held belief that the stars were permanent and unchanging.
By the time Tycho died in 1601, he had collected massive records, charts and notebooks
crammed with astronomical data that he had recorded himself painstakingly over several
decades. He left his rich collection to his assistant Johannes Kepler who had joined him in
1600. Kepler used Tycho’s data to arrive at his famous laws of planetary motion.
After Tycho’s death, Kepler set about working out the planetary orbits with accuracies never
known before. Apart from being a skilled mathematician, he was also a meticulous observer.
He started working with the piles of data that Tycho had left behind for him.
It took Kepler five years to work out his first planetary orbit – that of Mars. After detailed
analysis he found two deficiencies in the Copernican system. He found that the orbit of Mars
was not circular, as presumed by Copernicus, but was elliptical. The distance of Mars from
Sun did not remain the same but varied as the planet went round the Sun. Kepler also found
that Mars did not move with uniform velocity in its orbit, but sped up as it came near the Sun
and slowed down as it moved away. Kepler made these discoveries in 1605, but did not
publish them till 1609. Almost a decade later, Kepler came up with another relationship
linking the distances of planets with their orbital periods.
The three discoveries were formulated as Kepler’s three laws of planetary motion: (1) All
planets move in elliptical orbits, with the Sun at one the foci; (2) the velocity of a planet
varies in such a way that the line joining the planet to the Sun sweeps out equal areas in equal
lengths of time; (3) the squares of the orbital periods of the planets are directly proportional
to the cubes of their mean distances from the Sun.
By the end of the 17th century, thanks to the works of astronomical giants like Copernicus,
Tycho and Kepler, astronomy had been transformed from a religious dogma into a perfect
science. It was now possible to explain the observed astronomical facts with mathematical
accuracy. But it was still not known what made the planets go round the Sun. That had to
wait another eight decades before another giant of science Isaac Newton came up with the
answer.