The Origin of Modern Astronomy

1. The Origin of Modern Astronomy

2. Essential Questions • What scientists have contributed to our understanding of astronomy? • How have these scientists improved our current understanding? • What are Kepler’s Laws of Planetary motion? • What are Newton’s Laws of Motion and Gravity?

3. What scientists have contributed to our understanding of astronomy? A timeline

4. The Babylonians • 3000 BCE • Identified constellations • Developed a calendar tied to the recurrence of certain astronomical events • 1792-1750 BCE • Venus Tablet was inscribed • They believed that the movements and positions of planets with respect to the constellations could influence the fate of Kings and nations

5. The Chinese • 2500 BCE • Developed the concept of a 365-day year based on what appeared to be the Sun’s annual journey across the background stars • They believed an eclipse took place when a dragon attacked the Sun, trying to consume it • They would then gather people to scare the dragon • This lead to the development of the Saros cycle where the Sun, Moon, and Earth are aligned every 18 years and 11.3 days

6. The Egyptians • The Pyramids • Contain prayers which reference the pharaoh joining the circumpolar stars, which neither rise nor set, and therefore live eternally • Archaeologists speculate they are observatories

7. Stonehenge • Built between 2800 – 1550 BCE • Believed to be • An astronomical observatory • A computer of astronomical phenomena

8. The Greeks • Anaximander – 610–545 BCE • Theorized • The world and everything in it were derived from an imperceptible substance apeiron (unlimited) • Earth floated freely in space at the center of the universe • The stars were fiery jets • The sun was a chariot wheel whose rim was hollow and filled with fire

9. Anaximenes of Miletus • Theorized • Aer (air or vapor) was the most basic form of matter and was also the substance that formed the life spirit of animals, the soul of humankind, and the divine essence of the gods • Pythagorus – 580-500 BCE • Theorized • Earth was a globe fixed within a sphere that held the stars

10. Anaxagoras – 500-428 BCE • Theorized • The Sun was a large, red-hot body • The moon was much like Earth, complete with mountains and ravines • Solar eclipses were caused by the passage of the moon between the Sun and Earth

11. Plato – 427-347 BCE • Proposed that celestial bodies moved about Earth at a constant speed and followed a circular motion with Earth at the centre - geocentric model • Asserted that heavenly motion must be in perfect circles and that heavenly objects reside on perfect spheres • Eudoxus – 408-355 BCE • A student of Plato • Attempted to explain the RETROGRADE MOTION of the wandering stars (planets) • Charted the northern constellations • Created a model that used nested spheres

12. Aristotle – 384-322 BCE • Put the Earth at the centre of the universe • Expanded on the idea of the spheres put forward by Eudoxus • Aristarchus of Samas – 310-230 BCE • Theorized • Earth is not at the center of the universe, but that it orbits the Sun while also rotating – heliocentric model • This theory however was rejected

13. Hipparchus – 190-120BCE • Sorted the stars into 6 orders of brightness • Along with Aristarchus, measured the Earth-Moon distance by timing lunar eclipses. • Claudius Ptolemaeus – 127 – 145 CE • Known as Ptolemy • Added to the geocentric model a system of complex deferents and epicycles • Deferents – large circular orbits centered on Earth • Epicycles – small circles whose centers travel around the circumferences of the deferents • There were 80 deferents and epicycles

14. Ptolemaic Model

15. The Middle East • Abu ‘Abd Allah Muhammad Ibn Jabir Ibn Sinan Al-battani Al-harrani As-sabi’ – 858-929 CE • Also known as al-Battani • Refined the calculations of the length of the year and the seasons • Demonstrated that the Sun’s apogee is variable • Al-Sûfi – 903-986 CE • Meaningfully discussed the comparative brilliance of the Stars

16. The Europeans • Nicolaus Copernicus – 1473-1543 CE • 1514 • Was asked by the Catholic Church to improve the accuracy of the current calendar • Rediscovered the heliocentric model • He argued that all the motion we see in the heavens is the result of Earth’s daily rotation on its axis and yearly revolution around the Sun, which is motionless at the center of the planetary system • Held onto the idea of epicycles and constant circular motion • Proposed that stars were very far away

17. Why was this new idea considered such a big deal? It required a shift in understanding in two areas

18. The universe had to be a much bigger place than previously imagined • Stars appeared in the same positions with the same apparent brightness. Why didn’t they change? • Copernicus explained that the starry celestial sphere had to be so distant from Earth that changes simply could not be detected

19. Why do things fall? • Aristotle had explained that bodies fell toward their natural place, which was the center of the universe. Aristotle of course had believed that the Earth was the center of the universe, explaining why all things fell towards the ground. • Copernicus had no explanation to counter this argument.

20. Tycho Brahe – 1546-1601 CE • Considered the best naked-eye observer of all time. • Nov 11, 1572 observed a very bright star (SN 1572) in the constellation Cassiopeia • This star showed no daily parallax, thus it was far away • The star did not change position relative to the fixed stars over several months • This star is now classed as a Supernova

21. Was able to show that the Stars existed way beyond the distance of the moon • He was convinced that the planets must orbit the sun, but was unable to develop a satisfying model • He believed in geo-centrism, and furthered the work of Ptolemy • He developed the Tychonic system • The sun circles the Earth • The 5 planets circle the sun

22. Johannes Kepler – 1571-1630 CE • Worked for Brahe • Highly religious • Believed in the Heliocentric model • Attempted to find a physically realistic model for Mars’ orbit • This finally lead him to discard the circular orbit

23. What is Kepler’s Laws of Planetary motion?

25. Kepler’s 1st Law of Planetary Motion • Published in 1609 • the orbits of planets and other celestial bodies around the Sun are ellipses. • The above is a series of ellipses having the same major axis but different eccentricities

26. An ellipse is defined as a figure drawn around 2 points called FOCI, such that the distance from one focus to any point on the figure back to the other focus is a constant

27. This constant is the measure of the long diameter of the ellipse, the major axis. • Half of this segment is called the semimajor axis. • The short diameter, the minor axis, is a perpendicular bisector of the major axis. • Half of the minor axis is called the semiminor axis. • For planets, the Sun is at one focus, nothing is at the other.

28. Definitions • Eccentricity • the measure of an ellipse’s flatness. 0=perfect circle 1=a straight line • Numerically, it is the distance between the foci divided by the length of the major axis.

29. Kepler’s 2nd Law of Planetary Motion • Published in 1609 • a line from the Planet to the Sun sweeps over equal areas in equal amounts of time • This means that a planet orbits slower as it moves further from the Sun.

30. Kepler’s 3rd Law of Planetary Motion • Published in 1619 • deals with the length of time that it takes a planet to orbit the Sun (The Period of Revolution). P2=a3 The square of the period of revolution is equal to the cube of the planet’s average (mean) distance from the Sun. • “P” is measured in years, “a” is measured in AU • If objects are not orbiting the Sun, the equation used is: P2=ka3 Where k is dependent upon the situation

31. Galileo Galilei – 1564-1642 CE • Built a telescope in 1609 which he used to: • Explore the imperfect surface of the moon covered with craters, “seas”, and features that looked like the surface of Earth • See the surface of the sun was blemished with sun spots and that these changed position from day to day • He concluded that the sun was rotating making a complete revolution in 1 month • His observations helped solidify the idea of a heliocentric model and Kepler’s orbits, which he published in 1610 • Brought before a Catholic Church inquistion in 1616. • The church judged the work to be heretical and banned it • 1632 he published a comparison of the Ptolemaic and Copernican models where the Copernican view came out the winner • 1992 he was formally vindicated by the Pope

32. Sir Isaac Newton – 1642-1727 CE • Proposed three Laws of Motion and the Law of Universal Gravitation

33. Newton’s 1st Law of Motion • Unless acted upon by some external force, a body at rest remains at rest and a moving object continues to move forever in a straight line and at a constant speed • Inertia • The measure of an object’s inertia is its mass • This law explains why the planets move in nearly circular orbits – essentially because an external force (gravity) acts on each planet

34. Newton’s 2nd Law of Motion • The acceleration of an object is directly proportional to the force applied to the object and inversely proportional to the mass of the object

35. Newton’s 3rd Law of Motion • Forces don’t act in isolation • For every action there is an equal and opposite reaction

36. Newton’s Law of Universal Gravitation • The gravitational force between two objects is directly proportional to the product of their masses and this will decrease in proportion to the square of the distance between these two objects F=G m1m2r2 F=the force between the masses G=is the gravitation constant m1=the first mass m2=the second mass r=the distance between the masses

37. Newton’s Revisions to Kepler’s Laws of Planetary Motion: • Kepler’s 1st and 2nd Laws apply to all objects (not just planets) • 3rd Law rewritten: • 4π2 and G are just constant #s (they don’t change) • M1 and M2 are any two celestial bodies (could be a planet and Sun) • Importance: if you know period and average distance of a planet, you can find mass of Sun (2 x 1030 kg) or any planet! Mass of Sun is 2 000 000 000 000 000 000 000 000 000 000 kg Mass of Earth is 6 000 000 000 000 000 000 000 000 kg

38. Modern Astronomy

39. Edmund Halley – 1656-1742 • calculate the orbits of comets

40. John Frederick William Herschel – 1738-1822 • 1781 - discovered Uranus • catalogued about 2000 nebulae, discovered several satellites of Uranus and Saturn, studied the rotation of planets. Discovered and studied binary stars. He discovered two satellites each orbiting Uranus and Saturn. He studied the rotation period of many planets, the motion of double stars, and nebulae. He cataloged more than 800 double stars and contributed new information on the constitution of nebulae. Herschel was the first to propose that these nebulae were composed of stars. He is considered the founder of sidereal astronomy

41. James Edward Keeler – 1857-1900 • the first to observe the gap in Saturn's rings now known as the Encke Gap, using the 36-inch refractor at Lick Observatory on 7 January 1888

42. Annie Jump Cannon – 1863-1941 • organized the stellar spectral types according to stellar temperature

43. Henrietta Swan Leavitt – 1868-1921 • discovered the cepheid variable star period-luminosity relation which she further developed into the first method of measuring distance outside of our solar system

44. Ejnar Hertzsprung – 1873-1967 • 1913 - he determined the distances to several Cepheid variable stars by statistical parallax, and was thus able to calibrate the relationship discovered by Henrietta Leavitt between Cepheid period and luminosity • discovered two asteroids, one of which is the Amor asteroid 1627 Ivar • developed the Hertzsprung–Russell diagram

45. Vesto Slipher – 1875-1969 • used spectroscopy to investigate the rotation periods of planets and the composition of planetary atmospheres • 1912, he was the first to observe the shift of spectral lines of galaxies, making him the discoverer of galactic redshifts

46. Henry Norris Russell – 1877-1957 • developed the Hertzsprung–Russell diagram 1910

47. Edwin Hubble – 1889-1953 • confirmed the existence of galaxies other than our own • discovered that the degree of "Doppler shift" (specifically "redshift") observed in the light spectra from other galaxies increased in proportion to a particular galaxy's distance from Earth

48. Karl Jansky – 1905-1950 • observed that the nucleus of the Milky Way and other celestial objects are strong sources of Radio Waves in 1931. Based on radar technology developed in WWII, Radio Astronomy becomes an active field in the late 1940s

49. Clyde Tombaugh – 1906-1997 • 1930 of the dwarf planet Pluto • discovered nearly 800 asteroids • credited with the discovery of comet C/1931 AN • discovered hundreds of variable stars, as well as star clusters, galaxy clusters, and a galaxy supercluster

50. Stephen Hawking – 1942- • principal fields of research are theoretical cosmology and quantum gravity