Physics 4 Thought header

Astronomy - The Thread to Gravity

After about 2,000 years of accepting Aristotle thought experiment ideas, the door on gravity started to open using actual observation and experiments.

Nicolaus Copernicus (1473 - 1543)

In 1514 Copernicus gave his close acquaintances a short six-page, hand-written document in which he Copernicus described the ideas of a heliocentric solar system rather than an earth centered solar system. He continued to develop his work and after many years he was finally convinced to publish. Copernicus received the first copy of his printed book the day he died.

Tycho Brahe (1546 - 1601) Johannes Kepler (1571 - 1630)

Copernicus’s work suggested to Tycho Brahe that progress in astronomy could not be made without better observations. King Frederick II of Norway and Denmark was impressed with Tycho's early observations and financed the construction of Tycho’s observatories. Tycho liked and built large instruments always adding improvements and constructing instruments bigger than any others, even designing and building entirely new instruments to improve observations.

Tycho and his sister Sophia conducted systematic and rigorous observation, spending many nights for years using the highest quality instruments that Tycho could build. All of Tycho’s instruments were based on unaided eye observations. It is of note that Tycho’s observatories were the last major observatories based on making unaided eye measurements. Even so Tycho’s measurements of planetary parallax were precise attaining arcminute accuracy better than data from any other source and for this reason Tycho jealously hoarded all his measurements.

King Frederick died and the new king, Christian IV and Tycho disagreed, so Tycho moved to Prague in 1599 and a new sponsore Rudolf II, who built an observatory at a castle near Prague.

In1600, Johannes Kepler met Tycho Brahe near Prague at the new observatory. Kepler spent a couple of months analyzing some of Tycho's observations of Mars. Tycho jealous of his data permitted Kepler only limited access. Kepler impressed Tycho, and soon Kepler was permitted to use a little more Mars data. Kepler estimated it would take two years to finish his analysis of that data. But, Tycho would not let Kepler copy his data and the ensuing argument resulted in Kepler leaving Prague. Kepler and Tycho reconciled so Kepler came back to Prague.

Tycho died unexpected in October 1601, and Kepler was appointed his successor. Kepler was to complete Tycho’s work. Kepler appropriated Tycho's data resulting in disagreements with heirs, leading to delays in publishing Tycho’s work. With all the Mars data in Keplers possession he began to analyze that data and simultaneously began the process of building Tycho’s tables.

Kepler worked exclusively with the Mars data eventially he tried an elliptical orbit which fit the Mars data almost exactly agreeing with observations to within the two arc-minutes average error in Tycho’s data. Kepler concluded that “all planets move in elliptical orbits with the sun at one focus.” Kepler’s first law of planetary motion.

Using Tycho’s measurements of the aphelion and perihelion of Mars and Earth Kepler developed a formula where a planet's motion is inversely proportional to its distance from the Sun. Kepler reformulated this formula to state that “a planets will sweep out equal areas over equal time intervals.” Kepler’s second law of planetary motion.

Playing with harmony of the spheres, Kepler developed his third law of planetary motion, "The square of a planets period is proportional to the cube of the planets mean distances from the sun."

Galileo Galilei (1564 - 1642)

Modern work on gravitational theory based on earthly observations began with Galileo. Galileo started by experimenting with different weighted small trolleys rolling down different inclined planes and using these to make very careful measurements. Then to demonstrate his finding he dropped balls of different weight from the Tower of Pisa. Galileo experiments demonstrated that gravitation accelerated objects of different weight at the same rate. This was diametrically opposite to what Aristotle's had declared to be fact or law.

Galileo used a refracting telescope for which he had made several improvements to study the heavens. Galileo’s primary targets of study were Earth’s moon and Jupiter’s moons.

Newton invented and used a reflecting telescope to study the heavens.

Sir Isaac Newton (1643 - 1727)

Kepler’s and Galileo's work set the stage for the formulation of Newton's theory of gravity.

Newtons great leap was to realize that an object falling to earth and the moon revolving around the earth were both the result of the same force. Using Kepler third law of planetary motion Newton derived the inverse square relationship of gravity. All this culminated when Isaac Newton's published his Principia Mathematica in 1687. Newton used the word “gravitas” Latin for weight, for the effect that would eventually become anglicized to gravity, and defined the law of universal gravitation.

In 1687, Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation.

Newton's gravity theory’s greatest success was when it was used to predict the existence of Neptune based on orbital perturbation or anomalies in the motion of Uranus. It was reasoned that these could only be accounted for by the existence of another planet. Seperate Calculations by Urbain Le Verrier and John Couch Adams both predicted the approximate position of an undiscovered planet. It was the calculations of Le Verrier's that provided Johann Gottfried Galle with the information for his discovery of Neptune.

Albert Einstein (1879 - 1955)

However, it was the discrepancy in another planet's orbit, Mercury, that identified potential flaws in Newton's theory of gravity. Measurements indicated anomalies in the orbit of Mercury that could not be accounted for using Newton's theory of gravity. No other planet existed so another explination needed to be found. A solution was proposed in 1915 by Albert Einstein's in his General Theory of Relativity. The perturbation in Mercury’s orbit could be explained using Einstein’s equation.

Although Newton's theory seemed to have been superseded, non-relativistic gravitational calculations still use Newton's theory of gravity, because it is much simpler, easier to use and gives accurate results for all the celestial mechanic applications needed by NASA.