The New Star and the Telescope

In 1604, an event happened that would cause Galileo to take the risk and publicly argue that Copernicus's heliocentric theory was right and Ptolemy's geocentric theory was wrong. A new star appeared in the constellation Serpentarius, one that would come to be known as "Kepler's supernova," even though several persons had witnessed it, including Clavius in Rome. Using parallax observa-

PARALLAX EFFECT

Six months later

Background stars

Six months later

Background stars

Initial observation

Apparent location of star

Apparent location of star in six months

Initial observation

Apparent location of star

Apparent location of star in six months

^ Closer objects have a larger parallax

^ Objects farther away have a smaller parallax

Initial observation

Initial observation

Six months later

Six months later

A parallax is the apparent motion of an object against a background due to the motion of the viewer. The object is located closer than the background. In astronomy, stars observed as having a larger parallax were known to be closer to Earth than stars that had a lesser parallax. This method of observation provides a limited but initial piece of information about the distance to stars.

tions, Galileo confirmed that the "new star" must exist very far away and could not possibly exist within the sublunary region (the boundary between the Earth and Moon) of Earth, as was the common belief at the time. No one believed his conclusions, and, wisely, he chose not to push the issue.

A few years later he learned something amazing that would assist him in proving that the Earth truly was in motion around the Sun. In May 1609, Galileo received a letter from a friend who reported that while in Venice he saw a Dutchman named Jan Lippershey showing off his invention of a monocular, or a spyglass, made of lenses that could cause objects at great distances to appear closer. Shortly after making many celestial discoveries with this new instrument, Galileo wrote Sidereus nuncius (The starry messenger). A quotation from Galileo in Sidereus nuncius, as translated by author and historian Stillman Drake in his book Discoveries and Opinions of Galileo, states:

About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby. Of this truly remarkable effect several experiences were related, to which some persons gave credence while others denied them. A few days later the report was confirmed to me in a letter from a noble Frenchman at Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to inquire into the means by which I might arrive at the invention of a similar instrument. This I did shortly afterwards, my basis being the theory of refraction.

By using nothing more than his friend's written description of these lenses, Galileo was able to construct practically overnight a refracting telescope, and owing to his skills and knowledge of mathematics, it was at once greatly improved over Lippershey's version.

Using materials at hand, Galileo's first telescope had a magnification factor of only four, but after he experimented with grinding his own lenses, he finally produced a telescope with a magnification factor of about nine. He eventually took his creation to the Venetian Senate, which bought from him the rights to manufacture it. Within two months after producing his telescope, Galileo had made more startling new discoveries than any astronomer before

GALILEO'S TELESCOPE

Galileo's telescope worked by allowing light to enter at the skyward end (a) then pass though a plano convex—an outward-bulging lens (b)—which bent the light rays. He then placed a plano concave lens (c) ahead of the focal point—the point where the light rays would cross—producing the magnified virtual image. Galileo's best telescope magnified at 32 power (32X). Average modern pair binoculars magnify between 10X and 20X.

Galileo's telescope worked by allowing light to enter at the skyward end (a) then pass though a plano convex—an outward-bulging lens (b)—which bent the light rays. He then placed a plano concave lens (c) ahead of the focal point—the point where the light rays would cross—producing the magnified virtual image. Galileo's best telescope magnified at 32 power (32X). Average modern pair binoculars magnify between 10X and 20X.

him. In Sidereus nuncius, he recorded discoveries such as that the Milky Way was made of tiny distant stars, that there were mountains on the Moon, and that rings existed around Saturn. He also discovered the occurrence of sunspots, on which he reported in his Discourse on Floating Bodies (1612) and Letters on the Sunspots (1613).

On January 7, 1610, he discovered three bright stars near Jupiter and then, six days later, a fourth. These he determined to be Jupiter's moons, orbiting the planet in obvious regularity. This was very important toward proving the theory of a heliocentric solar

GALILEO'S JOVILABE

Four tables calculate the average motion of Jupiter's four moons.

In the face of Galileo's instrument, two rotating discs of differing diameters are connected in such a way as to be used in determining the position of the Sun and the Earth's Moon in relationship to the positions of Jupiter's moons.

system. He named the moons of Jupiter the Medicean stars and sent the grand duke of Tuscany, Cosimo de Medici II, a telescope of his own. He did this in the hopes of impressing the duke enough to obtain a new job. His efforts worked, and in 1610, one month after publishing Sidereus nuncius, he took a position at the University of Pisa as head mathematician. His only duties were to act as the grand duke's personal mathematician and philosopher.

It was around this time that Galileo invented the Jovilabe, a device used to predict the positions of Jupiter's moons. He developed this instrument in an attempt to solve the problem of determining longitude at sea by using the clockwork movements ofJupiter's moons, for in order to calculate longitude, one needed an accurate means of keeping time. (The English inventor John Harrison developed the first accurate marine chronometer in 1735.) It became impossible, however, for Galileo to produce a Jovilabe that would work well from the rocking deck of a ship; therefore, his instrument could be used only on land and was useless as a navigational aid for seafarers. Still, it was a remarkable piece of work and a fine example of his skill as a mathematician and astronomer.

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