Renaissance Astronomy

Renaissance Astronomy

Prior to the Renaissance • The lack of observable stellar parallax combined with the sensation that the Earth was stationary led to an Earth-centered view of the Universe.

Prior to the Renaissance • The Greek astronomer Ptolemy (85-165 A.D.) successfully created a model that explained the complex observed planetary motion in the context of the dogmatic restrictions of the time. • All motions were perfect circles. • The rate of motion was constant and unchanging. Explain this????

Prior to the Renaissance • Ptolemy solved the problem of retrograde motion by postulating spheres embedded within spheres (more than 40) all turning at uniform speed.

Prior to the Renaissance • Alfonso X, King of Castille in the 13th century noted, “If the Lord Almighty had consulted me before embarking on the creation, I should have recommended something simpler.”

Prior to the Renaissance • This statement captures the essence of “Occam's Razor” - the simplest explanation for a phenomenon is usually right.

A Simpler Suggestion: The Heliocentric Model • Interestingly, 300 years before Ptolemy the Greek astronomer Aristarchus was suggesting a heliocentric model. • Using the fact that the quarter Moon doesn't happen when the Moon and Sun are at a right angle – he estimated the relative distances and thus sizes of the Sun and Moon. • Knowing the Sun was much much bigger than the Moon and Earth he reasoned, correctly, that it was the center of the Solar System.

A Simpler Suggestion: The Heliocentric Model • Interestingly, 300 years before Ptolemy the Greek astronomer Aristarchus was suggesting a heliocentric model. • The Greeks of this era even had a good sense of the distances and true sizes of the various bodies via the measurement of the size of the Earth by Eratosthenes.

Copernicus • It wasn't until more than a thousand years later that the Heliocentric model was revived. • Copernicus asserted that things would be much simpler with the Sun in the center.

Copernicus • Specifically, the Copernican model made the explanation of retrograde motion simple and obvious. http://www.astro.illinois.edu/projects/data/Retrograde/ http://www.astro.ubc.ca/~scharein/a310/SolSysEx/retro/Retrograde.html

Validating the Copernican Model • Copernicus was no better than Ptolemy in asserting that his view was right. Observational evidence was required. • Tycho Brahe was THE observer of the pre-telescopic era. • Using simple tools he recorded precise planetary and stellar positions over more than 20 years.

Tycho and Kepler • Ironically, Tycho Brahe was an adherent to the Earth-centered universe. His assistant, mathematical genius, and mystic Johannes Kepler, eagerly waited for the opportunity to analyze Tycho's data (which Tycho kept close and secret). Kepler discovered remarkable properties of planetary orbits: 1. All planetary orbits were ellipses with the Sun at one focus. 2. Planets moved faster when closer to the Sun in a way that a line between the Sun and planet swept out equal area in equal time. 3. The orbital period of a planet was related to its average distance from the Sun. P2=a3

Kepler's First Law – Elliptical Orbits • An astounding discovery – Planets followed detailed mathematical relationships. • All planetary orbits were ellipses with the Sun at one focus. • A circle is a special case of an ellipse where the foci are on top of each other. • The more separated the foci the more eccentric the ellipse.

Kepler's Second Law – Equal Areas • Another quantitative relationship.... • Planets move faster when they are closer to the Sun in such a way that a line between the planet and the Sun sweeps out an equal area in the same time interval. • TheLaw of Equal Areas

Kepler's Third “Harmonic” Law • Relates • A planet's average distance from the sun - “a” • specifically ½ of the long axis of the elliptical orbit • for a circular orbit this “semi-major axis” is just the radius of the circle • The time it takes the planet to orbit the Sun – the orbital period, “P” P2=a3 • This equation works as written if P, the orbital period, is expressed in years, and a, the semi-major axis, is expressed in astronomical units. • The Earth's orbital period is 1 year and it is 1 A.U. from the Sun. • 12=13 .... it works.

An Example Using Kepler's Third Law • Consider an asteroid discovered in a circular orbit 4 A.U. from the Sun. How long does it take this asteroid to orbit the Sun? • – a = 4 A.U. P2 = a3 ?2 = 43 = 4 x 4 x 4 = 64 82 = 64 • This asteroid takes 8 years to complete an orbit around the Sun. Kepler's Laws in Motion

Galileo • Began a longstanding tradition of quick application of new technology to enable astounding astronomical discoveries. • He put the telescope to astronomical use within a couple of years of its invention – just over 400 years ago

Galileo • Began a longstanding tradition of quick application of new technology to enable astounding astronomical discoveries. • These observations were the first to provide strong direct observational support for the Copernican heliocentric mode.

Galileo: Craters on the Moon, Spots on the Sun • The Heavens weren't perfect, in fact they could be downright terrestrial. Sunspot movie

Galileo: Phases of Venus • Venus showed phases that changed over time and that were consistent with the Copernican model.

Galileo: Jupiter's Moons • Galileo discovered four satellites in orbit around Jupiter, and obeying Kepler's Harmonic Law. I should disclose and publish to the world the occasion of discovering and observing four Planets, never seen from the beginning of the world.... I noticed three little stars...near the planet...arranged exactly in a straight line... When I turned again to look [a few nights later]... I found a very different state of things. I therefore concluded... that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun.

Galileo: Jupiter's Moons • Any object could serve as the center of motion – so why not the Sun. Jupiter was an example of the Solar System in miniature.

Galileo and Neptune • Another notebook page shows an object that moved with respect to the background stars over a few nights. • Galileo made a note of it but did not follow up. • It was Neptune...

Galileo and Supernovae • Galileo, Tycho, and Kepler all observed supernovae and recognized that, because they didn't show parallax, these objects were part of a changing celestial sphere.

Galileo and the Church • Galileo died a prisoner, having been under house arrest imposed by the Inquisition for a decade. • Although characterized by many as a conflict between science and religion, the details are much more complex. • Many in the Church accepted Galileo's conclusions and even confirmed the observations for themselves. • Many refused to look through a telescope and were compelled to enforce the longstanding dogma of a stationary Earth. • Galileo had both friends and enemies. He was a stubborn, irascible, and outspoken character who was successful and popular. His enemies were thus well-motivated to plot his demise.

I have been told by a friend of mine, a priest who is very fond of you, that a gang of ill-disposed men, who are envious of your virtue and merits, met at the residence of the Archbishop of Florence, and put their heads together in a mad quest for some means by which they could damage you, either with regard to the motion of the Earth or otherwise. One of them asked a preacher to state from the pulpit that you were asserting outlandish things. The priest, seeing the animosity against you, replied as a good Christian and a member of a religious order ought to do. I write this that your eyes may be open to the envy and malice of these evildoers. In 1992, Pope John Paul II expressed formal regret for the handling of the Galileo affair and acknowledged that the church had made errors.

Galileo's Trial and Confinement Galileo's tomb at Santa Croce

Renaissance Astronomy