Galileo’s astronomical observations using the telescope had great impact on the field of astronomy.
Have you ever watched a crescent moon about a day or two after the new moon, in the evening?
You will find that the dark part of the moon is also lit by a dim light and if you look closely, you can easily discern the round body of the moon very clearly! Whose light is this? What makes the dark part of the moon visible to us? Could this be the Earth’s light falling on the moon! Indeed it is.
Do you know who first found an answer to this question? It was Galileo Galilei (1564-1642 CE). But it required quitea bit of reasoning and some courage on the part of Galileo to answer this question. In one of his books, Galileo writes of the Earth’s reflection of sun light,
“The solar reflection from the Earth is quite real. This fact is against those who argue that the Earth must be excluded from the dancing whirl of stars [or heavenly bodies] for the specific reason that it is devoid of motion and light. We shall prove the Earth to be a wandering body surpassing the moon in splendour, and not the sink of all dull refuse of the universe;
this we shall support by infinitude of arguments drawn from nature.” Galileo had to face a court trial, apologize in the court for writing books supporting the Copernican system (that the earth was not the centre of the solar system), and had to live under house arrest.
But Galileo got heaps of praise not only from fellow scientists and philosophers but also from poets, courtiers and painters alike after the publication of his first book on his astronomical discoveries: Sidereus nuncius. Johannes Kepler wrote in the preface to his book Dioptrics, “O telescope, instrument of much knowledge, more precious that any sceptre! Is not he who holds thee in his hand made king and lord of the work of God?” In a poem, poet Johannes Fabercompared Galileo with Columbus in the
In modern geophysics we use a unit of acceleration called gal (short form of Galileo) which is 1 centimetre/ square second, to measure acceleration due to gravity.
The poet John Donne in one of his works describes Galileo as the one “who of late hath summoned the other worlds, the Stars, to come nearer to him and give him an account of themselves”. John Milton, the author of Paradise Lost, writes that when he was in Italy he “found and visited the famous Galileo”. Besides the Earth-lit lunar night, Galileo made a number of other revolutionary astronomical discoveries.Let’s take a look at some of his observations.
“Yield, Vespucci, and let Columbus yield.
Each of these
Holds, it is true, his way through the
unknown sea...
But you, Galileo, alone gave to the
human race the sequence of stars,
New constellations of Heaven.”
 |
| Galileo was constantly in conflict with the intellectual establishment of his time (Source - Wikipedia ) |
A Rugged Moon
During Galileo’s time and earlier, there was a belief that man’s senses deceive him. The moon is eternal, perfect and absolutely spherical and the pale dark regions that we see on the moon, it was believed, are actually not there. One should not trust reason, people believed ,since man’s mind is not quite capable of grasping cosmic mysteries.But what Galileo discovered about the moon, in his own words, is that “the surface of the moon (and other heavenly bodies) is not smooth, uniform and precisely spherical as a great number of philosophers believe it to be, but is uneven, rough, and full of cavities and prominences, being not unlike the face of the Earth, relieved by chains of mountainsand deep valleys”.
Heights of Lunar Mountains
Galileo is called the father of modern physics. One of the reasons for this is that he emphasized on measurements. Wherever possible, he always measured various quantities he studied. Thus not only did he discover mountains on the moon, he even determined the heights of lunar mountains and found them to be about four miles high.
Earthshine
We have already talked about Galileo’s discovery that earthshine fails on moon and sometimes makes he dark part of the moon also visible to those on the Earth. This was an important discovery because people ten believed that the Earth cannot give light .Of Earth in fair Graceful ecchange ,pays back the illumination similar to that which it recives from her throughout nearly all the darkest gloom of night.” The Earthshine on the moon is much brighter than moonshine on the Earth. One could easily read a newspaper under this light, and even read the smallest fonts printed on papers on medicine bottles underthat light (of course, your vision must be normal .
Distinction between Stars and Planets
In the prehistoric era, people were immensely curious about the sun, moon and the stars. One question they put a lot of effort into answering was: Do all the stars make a fixed pattern in the sky, or, does the pattern keeps on changing with the passage of time? Years of observations revealed that all the ‘stars’ in the sky except five make a fixed pattern in the sky. These five move slowly in the background of the others. So these five were given a separate name ‘planets’, as, in Greek, the word ‘planet’ means ‘wanderer’.
Galileo must have been very excited to discover the following important distinction between planets and stars as seen through a telescope: planets show their discs very clearly with well-defined edges and uniform light, whereas stars look almost the same as when seen with naked eye. Thus the planets look like little moons through a telescope. This discovery indicated that the stars are at very far off distances from the earth than the planets.
Phases of Venus
To his utter surprise, Galileo found that Venus presents itself like a miniature moon with different phases at different times as seen through the telescope. This proved conclusively for the first time that the planets do not have their own light and that they are illuminated by sun’s light. This was a big discovery during Galileo’s time.
Another thing that gave Galileo immense pleasure was that the observation of the phases of Venus provided a sharp contrast between the Ptolemaic system and the Copernican system. The Ptolemaic system explains the motion of Venus by positioning it in between the Sun and the Earth such that Venus should not show a complete sequence of phases. But Galileo did observe that Venus shows a complete sequence of phases, implying that the Ptolemaic system could not be correct.
“Galileo must have been
very excited to discover the
following important distinction
between planets and stars
as seen through a telescope:
planets show their discs very
clearly with well-defined edges
and uniform light, whereas
stars look almost the same as
when seen with naked eye.”
 |
Galileo Galilei’s telescope at The Franklin Institute in
Philadelphia (Courtesy: AP Photo) |
