Thursday, October 11, 2018

What is universe? - Part 5 - Under the wandering stars





The Ancient Greek astronomers, with their dedication to detailed observations and recordings, have left us a rich legacy of knowledge about the stars.
The nature of five particular stars, however, eluded them. They called them planetes, meaning ‘wanderers’, because of their movement across the sky from one night to the next, unlike all other stars which remained ‘fixed’. From their Greek name you might correctly deduce that these planetes are in fact planets – our nearest five planets, Mercury, Venus, Mars, Jupiter and Saturn, which can be seen with the naked eye. The Greeks could have no concept that these were worlds in their own right, and imagined them to be gods, or at the very least emissaries of the gods, whose influence affected the fortunes of individuals on Earth.
Two of these wandering planets, Mercury and Venus, follow orbits between Earth and the Sun. So, viewed from Earth they stay close to the Sun and are only ever seen in the twilight sky. Mars, Jupiter and Saturn orbit the Sun further out than the Earth and can be clearly seen making their slow paths through the night sky. Many early astronomers became dedicated to tracking the motion of all the wandering planets so that their future positions could be predicted. This task was seen as important because when planets drew close to one another, their influences were thought to combine and magnify. Thus, conjunctions, as they were known, were significant events that needed to be predicted in order to cast horoscopes.
With the advent of the Christian era, the opinion was widely adopted that the motion of the Heavens would always remain mysterious because the sky was God’s domain and mankind’s puny intellect could never understand His omnipotent will. This perspective began to change in the first decades of the 17th century when Johannes Kepler distilled the movement of the planets into three mathematical laws of planetary motion (see Why Do the Planets Stay in Orbit?). This proved that the Universe could not only be measured, but understood.
At the same time, in Italy, Galileo Galilei was making discoveries that sparked our fascination with the wider Universe. In 1609, he raised his telescope and pointed it at the misty band of light that stretches across the night sky, known as the Milky Way. Through his basic telescope, tiny by today’s standards, Galileo could see that the Milky Way was composed of a multitude of faint stars. This was a revelation to all, because it had been believed that the entire Universe contained only what could be seen with the naked eye. Now, however, Galileo had shown that there was far more that lay beyond unaided vision. This realization was the start of the centuries-long fascination, with each generation of astronomers developing larger and larger telescopes to see fainter and fainter objects, which continues to this very day. The largest optical telescopes in use now are fully 10 metres across, some 500 times larger than Galileo’s original telescope.

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