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Galileo and the telescope

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Telescopes are devices which use a number of lenses to supply magnified pictures of distant objects. It’s unclear who invented the primary telescope: lenses had been extensively utilized in Europe to right poor eyesight for the reason that fourteenth century and I count on that, over time, the telescope was truly invented many occasions by completely different people, who found that combining completely different lenses might produce a magnified picture.

In 1608 a spectacle maker referred to as Hans Lippershey utilized to the Dutch authorities  for a patent for a tool for seeing at a distance. His utility was refused and, within the ensuing publicity, the Italian astronomer Galileo Galilei (1564-1642) turned conscious of the machine. Galileo refined the early telescopes to supply devices with higher magnification and in 1609 he took the primary recorded astronomical observations with a telescope.  Certainly, the primary use of the phrase telescope, which is constructed from the Greek phrases ‘tele’ that means ‘far’ and ‘skopos’ that means ‘seeing’, is related to Galileo’s instrument.


Galileo Galilei – picture from Wikimedia Commons

How the Galilean telescope works

There are numerous combos of lenses which can be utilized to amplify distant objects, however the easiest is the one which was utilized by Galileo when constructing his telescope.  Telescopes of this design are referred to as Galilean telescopes and to grasp how they work it’s vital to grasp a little bit about lenses.

There are two principal kinds of lens:

  • a converging lens, proven within the high of the diagram above, causes parallel gentle rays from a distant object, proven in crimson, to bend in order that they converge at level generally known as its focus. The focal size of the lens is the gap between its centre and its focus.
  • a diverging lens, proven within the backside of the diagram, causes parallel gentle rays from a distant object to bend and unfold out so they seem to have come from its focus. Like a converging lens, the focal size is the gap between its centre and the main target. By conference the focal size of a diverging lens is unfavourable.

The diagram under, which if you happen to’ve studied physics at highschool you’ll recall is known as a ‘ray diagram’, reveals that when the rays of sunshine from a distant object move by means of a converging lens, they type an inverted picture, which is shrunk in comparison with the item.

The rays of sunshine, colored crimson, from the item (A) are targeted by the converging lens to supply an inverted picture (B).

The ray diagram under reveals that when rays of sunshine from a distant object move by means of a diverging lens they unfold out, in order that they seem to return from a picture which is nearer to the lens and shrunk in comparison with the item. That is referred to as a digital picture, as a result of the rays of sunshine don’t truly type a picture.

The rays of sunshine, colored crimson, from the item (A) seem to diverge from the digital picture B.

A Galilean telescope consists of two lenses: a big converging lens of lengthy focal size (generally known as the target) and an eyepiece which is a diverging lens of a brief focal size. Curiously, because the diagrams above present, each of those lenses on their very own produce a smaller picture of a distant object. I’ll clarify subsequent how they work collectively to supply a picture which is enlarged.

If we think about a distant object, comparable to that proven under, then its obvious measurement is how massive it seems when considered by an observer.  For bigger astronomical objects the obvious measurement is usually measured in levels. For instance, the obvious diameter of the Moon is roughly zero.5 levels. A telescope makes the obvious measurement bigger.

Within the diagram above, the blue line reveals a ray of sunshine from the highest of a distant object. The crimson line reveals a ray of sunshine from the underside of the item. The obvious measurement (a) is the angle between the 2 rays.

If we put a converging lens in entrance of the distant object then it can focus the sunshine rays and produce an inverted picture, which will likely be positioned as proven under.


The diagram above reveals plenty of gentle rays from the distant object.

  • A lightweight ray from the high of the item hanging the high of the lens (labelled 1) and a lightweight ray from the high of the item hanging the center of the lens (labelled 2) are each delivered to a spotlight.
  • A lightweight ray from the backside of the item hanging the high of the lens (labelled three) and a lightweight ray from the backside of the item hanging the center of the lens (labelled four) are each delivered to a spotlight.

If we put a diverging lens with a brief focal size able the place it intercepts the sunshine rays earlier than they’re delivered to a spotlight, then the sunshine rays are bent by the diverging lens and comply with the trail under.


Key to diagram above

As you’ll be able to see from the diagram above. the obvious measurement of the distant object is elevated to b, which a lot greater than the unique worth a. The magnification of the telescope (M) is outlined as:

M= obvious measurement of picture divided by obvious measurement of object

For extra element on this see the notes on the finish of this put up.

Galileo’s discoveries

Utilizing his telescope, Galileo made plenty of vital discoveries which revolutionised astronomy. He found the 4 brightest moons of Jupiter which at the moment are referred to as the Galilean moons.

The 4 brightest moons of Jupiter – picture from Wikimedia commons

He studied the best way that the Moon was lit and the way this modified over time and appropriately deduced that this was on account of shadows of lunar mountains and craters. Galileo turned his telescope to the Milky Means and found that it consisted of an unlimited variety of stars, every too faint to be seen individually with the bare eye. When considered from Earth these stars are so carefully packed collectively they seem like clouds. Nonetheless, the invention which had the best influence on his life was the phases of Venus, which I’ll speak about subsequent.

Venus’s phases 

As seen from the Earth,  Venus goes by means of a full set of phases in the same method to the Moon.  Nonetheless, as a result of Venus seems so small, these are solely seen by means of a telescope. Galileo was the primary individual to see them.

Venus Phases

At level A within the diagram above, when Venus is between the Earth and the Solar, the sunlit a part of Venus faces away from us making the planet nearly invisible. The quantity of the sunlit a part of Venus we will see will get bigger or waxes by means of to a crescent part (B), to a half Venus (C)  after which to a full Venus at level D, when the entire sunlit facet dealing with the Earth is illuminated.  It then will get smaller or wanes again to a half Venus (E) , then to a crescent (F) after which lastly again to being nearly invisible again at level A.

As readers of a previous post will know, in 1543, simply earlier than his loss of life, Nicolas Copernicus (1473-1543) had revealed the concept of heliocentrism which states that the planets orbit the Solar. Nonetheless, in Galileo’s time, the educating of the Catholic church favoured geocentrism, the extensively held view that the Earth was the centre of the Universe and the celebrities, planets, the Solar and the Moon had been in orbit round it. Certainly sure verses of the bible could possibly be interpreted as supporting that viewpoint, comparable to Psalm 104:5:

‘the Lord set the earth on its foundations; it could actually by no means be moved’

Nonetheless, the phases of Venus which Galileo had seen can solely be defined by Venus orbiting the Solar, not the Earth.  Subsequently, Galileo concluded that the geocentric concept was incorrect.   Sadly for Galileo, in 1616 the Catholic church declared heliocentrism to be heresy. Heliocentric books had been banned and Galileo was ordered to chorus from holding, educating or defending heliocentric concepts.

Regardless of this ruling Galileo continued to defend heliocentrism, and in 1633 the Roman Inquisition discovered him ‘vehemently suspect of heresy’, sentencing him to indefinite imprisonment. Galileo was saved below home arrest till his loss of life in 1642.

Enchancment to telescope design

Galileo’s telescope might solely amplify objects 30 occasions earlier than the picture turned distorted. It additionally had a slim subject of view. In 1610 Johannes Kepler started investigations into the best way that completely different combos of lenses might work collectively to supply a magnified picture. He invented a brand new kind of telescope with a converging lens because the eyepiece This new design turned generally known as the Keplerian telescope. It  allows the next magnification with much less distortion than a Galilean telescope, though it produces an the other way up picture, which this doesn’t actually matter for astronomy.  At this time the Galilean telescope design is just utilized in low-cost low energy binoculars.

Keplerian telescope – picture from Wikimedia Commons


Keplerian and Galilean telescopes are each instance of refractors the place lenses are used to gather and focus gentle. These days just about all massive telescopes are reflectors the place curved mirrors, moderately than lenses, are used.  Reflectors have a number of benefits over refractors.  One in all them is that reflectors don’t endure from chromatic aberration. This occurs in refractors as a result of completely different colors of sunshine are bent very barely in a different way as they move by means of the lens, which leads to a blurred picture.  Chromatic aberration may be overcome by utilizing achromatic lenses, which encompass two or extra lenses made out of various kinds of glass joined collectively to type a compound lens, however that is costly and technically tough when setting up bigger lenses. The primary benefit of reflectors is that it’s a lot simpler to supply a big mirror than a big lens. A big lens many metres in diameters could be very thick, very heavy and tough to fabricate to the standard wanted in a telescope. It might additionally are likely to sag, turning into deformed below its personal weight,  producing a blurred picture.

For these causes the most important refractor utilized in skilled astronomy is the one at Yerkes Observatory. It has an goal lens which is 1 metre in diameter. All telescopes bigger than this are reflectors. Yerkes Observatory is predicated at Williams Bay, Wisconsin and it operated by the College of Chicago.  Sadly, it is because of shut this 12 months after a formidable 120 12 months historical past.

Yerkes Observatory – Picture from College of Chicago


To calculate the magnification of a Galilean telescope, we divide the focal size of the target by the focal size of the eyepiece. So, if the focal size of the target is 200 cm and the focal size of the eyepiece is 10 cm, the magnification of the telescope could be 20.

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