Galileo and Telescopes

Timelines of Events

  • 1543
    • Since the Earth doesn't appear to be moving, Nicolaus Copernicus' theory of a sun-centered cosmos needs to be proved.
  • 1608
    • The first telescopes were made by Dutch eyeglass manufacturers.
  • 1656
    • Christiaan Huygens, a Dutch scientist, creates ever-larger telescopes that can detect more intricate and fainter objects.
  • 1668
    • The first reflecting telescope was created by Isaac Newton, which is a device that is significantly less impacted by chromatic aberration distortion.
  • 1733
    • The first flint glass/crown glass achromatic lens is created, greatly enhancing refracting telescopes' potential for high-quality images.

Limits of the naked eye

  • The naked eye has two main limitations:
    • It cannot record in great detail, and
    • Only reasonably bright objects can be detected by it.
  • When observing a full moon, the lunar diameter spans an angle of 1/2° at the surface of the Earth.
    • This indicates that a 1/2° angle is formed at the eye of the moon where two lines extending from opposite sides of the moon meet.
    • Only objects that are separated by more than about 1/60° can be seen with the naked eye.
  • On a clear moonless night, 2,500 stars would be visible above the horizon in the Italian countryside where Galileo lived.
  • The Milky Way looks like a river of milk to our eyesight.
    • Only with the help of a telescope can we see that the Milky Way is made of stars.
  • Galileo would be among the first to recognize the true nature of this band of stars across the sky by pointing his new telescope at the night sky.

Building a telescope

  • Galileo did not invent the telescope himself.
    • Hans Lipperhey, Jacob Metius, and Sacharias Jansenn introduced the idea of combining two lenses.
  • Combining two lenses:
    • a large one at the front of a tube to collect the light, and
    • a small one at the back to magnify the image.
  • A telescope does two important things.
    • Its resolution is proportional to the diameter of the objective lens.
    • Objective Lens: The large lens at the frog that collects the light.
    • Resolution: The detail a telescope can detect.
    • It serves as a light bucket as well. When the objective lens's diameter is doubled, four times as much light is collected, allowing for detection of objects with similar light output from twice as far away.
  • Galileo wasn't happy with his first instrument because it could only magnify three times.
    • He understood that the ratio of the focal length of the objective lens to the focal length of the eyepiece was directly proportional to the magnification of a telescope.
    • It was necessary to use either a shorter-focus concave lens for the eyepiece or a longer-focus convex lens for the objective.
    • Galileo learned how to grind and polish lenses and made his own since they weren't readily available.
  • Living in northern Italy, which at the time was the world's glassmaking capital, greatly aided him.
    • Later, he created a new telescope with a 33-fold increase in magnification, and it was this improved model that allowed him to find the Jovian moons.

Three little stars

  • On the evening of January 7, 1610, Galileo made the discovery of Jupiter's moons.
    • He initially believed he was viewing distant stars, but soon realized the new bodies were orbiting Jupiter.
    • Galileo was a 45-year-old professor of mathematics at the University of Padua, which is located close to Venice, at that time.
  • When he published his pioneering telescopic observations, he wrote:

“Through a spyglass, Jupiter presented himself. And since I had prepared for myself a superlative instrument, I saw that three little stars were positioned near him—small but yet very bright. Although I believed them to be among the number of fixed stars, they nevertheless intrigued me because they appeared to be arranged exactly along a straight line and parallel to the ecliptic ….”

Repeated observations

  • Galileo was enthralled by his unexpected discovery. It quickly became evident as he continued to observe Jupiter at night after night that the new stars were not located beyond Jupiter in the faraway heavens. They moved not only alongside the planet as it made its way across the sky, but also in all directions around it.
  • Galileo discovered that there were four moons in orbit around Jupiter, traveling with it as it circled the sun, just as the moon orbits Earth once per month. The orbital periods of the farther-away moons were longer than those of the nearer moons. A scale model of the planetary system of the sun appeared in the Jovian moon system.
  • It served as evidence that not all of the universe's objects orbited Earth as previously believed in pre-Copernican times. The discovery of these four moons contributed to the advancement of the sun-centered universe theory.
  • Galileo quickly made his discovery public in his book Siderius Nuncius (The Starry Messenger).
  • Galileo dedicated the book to Cosimo II de' Medici, a former student who went on to become the Grand Duke of Tuscany, in the hopes of advancing his cause.
  • In recognition of the four regal Medici brothers, he gave the moons the name "Medicean Stars." He was appointed Chief Mathematician and Philosopher to the Medici at the University of Pisa as a result of his political astuteness.
  • Many people initially expressed skepticism, claiming that the moons were nothing more than telescope lens flaws. However, when Jupiter returned to the night sky later in 1610 after passing behind the sun, other early telescopic astronomers including Thomas Harriot, Joseph Gaultier de la Vatelle, and Nicolas-Claude Fabri de Peiresc confirmed the moon’s existence.

Refracting telescopes

  • There were two different types of early refracting telescopes: the Galilean and the Keplerian, which Johannes Kepler invented in 1611.
    • The light was collected and focused by an objective, which was a long-focus, large-diameter lens on both of them.
    • The smaller, short-focus eyepiece lens enlarged the image at the focus.
  • The instrument's magnification is equal to the objective lens's focal length divided by the eyepiece's focal length.
    • A longer focal length, lower chromatic aberration, and higher magnification for a fixed eyepiece were all benefits of a flatter convex objective lens.

Disputed priority

  • In 1614, Simon Marius, a German astronomer published Mundus Iovialis, this is where he described Jupiter’s moons, also claiming to have discovered them before Galileo even did.
  • Although Marius would later be charged with plagiarism by Galileo, it is now widely acknowledged that he independently made his discovery at roughly the same time.
  • The Roman god Jupiter's love conquests inspired Marius to name the moons Io, Europa, Ganymede, and Callisto; these names are still in use today.
  • They are now referred to as the Galilean moons collectively.

A Jovian clock

  • Galileo carefully observed how the Jovian moons' daily positions changed. He came to the conclusion that their positions could be predicted in advance, just like the planets. Galileo recognized that, if done correctly, the system would function as a global clock and be able to address the issue of determining longitude at sea.
  • It is necessary to be able to tell the time in order to determine longitude, but in Galileo's time, there were no watches that could be used on a boat.
  • A "Jovian clock" would function from anywhere on Earth because Jupiter is at least four times as far from Earth as the sun.
  • When English clockmaker John Harrison introduced precise chronometers around 1740, the longitude conundrum was finally resolved. This happened a long time before the precise orbits of Jupiter's moons were known.
  • Another intriguing outcome came from Galileo's discovery of four satellites orbiting Jupiter.
  • In the chapter on Laputa of Gulliver's Travels, which was published in 1726, Jonathan Swift made the assumption that Mars would have two moons simply because Jupiter had four and Earth had one.
  • In 1877, Asaph Hall made the accidental discovery of Phobos and Deimos, two tiny moons of Mars, while using a brand-new 26-in (66-cm) refracting telescope at the US Naval Observatory in Washington.

Support for Copernicus

  • In Galileo’s time, the ancient biblical view, in which the Earth was fixed at the center of the universe, and Copernicus's new theory, stating that the Earth was in orbit around the sun, were still the subject of a furious argument.
  • While the heliocentric (sun-centered) proposal reduced Earth to just one member of a family of planets, the geocentric (Earth-centered) theory emphasized the planet's singularity.
  • The Copernican principle is now used to refer to the presumption that Earth does not hold a special position in the cosmos.
  • Finding observations to demonstrate that one theory was true and the other false was the current challenge.
  • The discovery of moons orbiting Jupiter provided strong evidence in favor of a sun-centered system.
  • Even though it was now obvious that nothing revolved around a central Earth, many questions remained.
  • The Earth must be moving if the sun-centered system was correct. The orbital speed of Earth would have to be 30 km/sec (20 mi/sec) if it had to circle the sun once a year. Although the exact distance between Earth and the sun was unknown during Galileo's time, it was obvious that it was far enough for Earth to need to be moving quickly, which we cannot perceive.
  • Additionally, stellar parallax, a phenomenon where stars appear to swing from side to side every year, should be caused by this orbital motion. This again was not observed at the time.
  • Galileo and his contemporaries were unaware that stellar parallax is so small that it is challenging to measure because the average distance between stars in the Milky Way is about 500,000 times greater than the distance between Earth and the sun.
  • It wasn't until the middle of the 19th century that significantly better instruments allowed for the detection of this swing.
  • Despite these concerns, Galileo believed that his research had established Copernicus's theory's veracity beyond a shadow of a doubt.
  • He also made discoveries about the sun's rotation, which is indicated by the movement of sunspots, as well as Venus' phases, which are best explained by the planet's orbit around the sun.
  • In 1619, Galileo was at odds with the Church as a result of his combative defense of Copernicus, which had been deemed heretical in 1616.
  • In 1633, he presented himself to the Inquisition. His books were outlawed, and he was placed under house arrest for the final ten years of his life.

New moons

  • For 283 years, Jupiter's only known moons were four.
  • Amalthea, a fifth satellite, was found by American astronomer E. E. Barnard used a 36-inch (91-cm) refractor at the Lick Observatory in California in 1892.
  • It was the final satellite in the solar system to be found through direct observation.
  • Many smaller moons may be discovered in the future, bringing the total number of known Jupiter satellites to 67 from 12 by the mid-1950s.

Facts about Galileo

  • Galileo Galilei was born in Pisa, Italy on 15 February 1564.
  • He moved to the University of Padua in 1590 after being appointed to the Chair of Mathematics at the University of Pisa in 1589.
  • Galileo was an astronomer, physicist, mathematician, philosopher, and engineer who made significant contributions to the Scientific Revolution, a period of intellectual development in Europe.
  • He was the first to use the refractor telescope to effectively focus on the heavens.
  • He learned that the sun rotated once every roughly one month, Venus underwent phase changes, the moon was mountainous, and Jupiter had four moons.
  • He wrote a lot and made his research available to a big audience.
  • In 1610, he published The Starry Messenger.
  • In 1632, the Dialogue Concerning the Two Chief World Systems was published.
  • In 1638, The Discourses and Mathematical Demonstrations Relating to Two New Sciences was publicized.