Astronomy

1. Astronomy The First Science

3. Ancient Astronomy • First observed and used by cultures like the Chinese, Egyptians, and Babylonians • Used for planting crops, tracking hunting migrations • Used to track the Sun, Moon, and the 5 known planets. • Chinese tracked Halley’s comet for 10 centuries. Also observed and recorded “guest stars”. We know these as normal stars, which are usually too faint to be visible, which increases its brightness as it explosively ejects gases from its surface, a phenomenon we call a nova or supernova (novus = new).

4. Golden Age of Astronomy • Occurred in Greece from 600 BC – 150 AD • Used Geometry and Trigonometry to measure the distances between as well as the size of the Sun and Moon. • The Greeks created both geometry and trigonometry!

5. Geocentric Model • Geo = Earth • Centric = center • Stated that Earth was a motionless sphere at the center of the universe in which everything else orbited. • Earth’s rotation not demonstrated until 1851.

6. Ptolemy’s Model • Created in 141 AD. • Was a model that accounted for observable motions of celestial bodies. • Noticed planets had a retrograde motion and accounted for it with this model. • Results from the combination of the motion of the Earth and the planet’s own motion around the sun. • Mars looked like it went backwards because Earth orbits the Sun faster than Mars.

8. Ptolemy’s Model (cont.) • Used circular orbits around the Earth with smaller circular orbits following them called epicycles.

10. One step forward, two steps back… • Much of the accumulated knowledge of astronomy was lost around the 4th century during the fall of the Roman Empire when libraries were burned and destroyed.

11. Saving Science!! • After the decline of Greek and Roman civilizations the center of the study of astronomy moved to the Middle East. Specifically Baghdad. • Ptolemy’s model had been translated into Arabic. • Arabic astronomers expanded the then current star catalog and divided the sky into 48 constellations. • Became the foundation of our present day constellation system.

12. Ptolemy’s model had been translated into Arabic. • Arabic astronomers expanded the then current star catalog and divided the sky into 48 constellations. • Became the foundation of our present day constellation system. • Ptolemy’s model not reintroduced into Europe until the 18th century.

13. Modern Astronomy • Break from philosophical and religious views. • Took almost 13 centuries after Ptolemy to make any major advances in astronomy in Europe. • 5 major scientists contributed to what we know about astronomy.

14. Nicolaus Copernicus(1473-1543) Poland • The idea of Earth being a sphere was lost. • Convinced that Earth was a planet, just like the other 5 then known planets. • Daily motions of celestial bodies explained more simply by a rotation Earth.

15. Nicolaus Copernicus (cont.) • Constructed the Heliocentric model. • Helio = Sun (Greek) • Centric = center • Still used circular orbits and smaller epicycles like Ptolemy. ~~Follower Giordano Bruno was burned at the stake in 1600 for refusing to denounce Copernican theory.

16. Tycho Brahe(1546-1601) Danish • Had an observatory near Copenhagen, Denmark where he used pointers to measure locations of celestial bodies for 20 years. • Didn’t use telescopes because he didn’t have one! No one did….. • Tried to disprove Copernicus using stars. • Couldn’t observe the apparent shift shown with rotation • The apparent shift of stars is called stellar parallax, which is used to measure distances between stars.

17. Johannes Kepler(1571-1630) Czech Republic • Created 3 laws of planetary motion (1609-1619). • He used these to account for the motion of Mars. • Led him to find that the orbit of Mars is elliptical not circular and its speed varies based on its location during its orbit.

18. Kepler’s First Law of Planetary Motion • The path of each planet around the Sun, while almost circular, is actually an ellipse, with the Sun at one focus.

19. Kepler’s Second Law of Planetary Motion • Each planet revolves so that an imaginary line connecting it to the Sun sweeps over equal areas in equal intervals of time. This law or equal areas geometrically expresses the variations in orbital speeds of the planets.

20. Kepler’s Third Law of Planetary Motion • The orbital periods of the planets and the distances to the Sun are proportional. The orbital period is measured in Earth years, and the planet’s distance to the Sun is expressed in terms of Earth’s mean distance to the Sun called the astronomical unit (AU) (1 AU ≈ 150 millions or 93 million miles). The planet’s orbital period squared is equal to its mean solar distance cubed.

22. Johannes Kepler (cont.) • Kepler’s laws support Copernicus’ theory • Still didn’t determine the forces that act to produce planetary motion.

23. Galileo Galilei(1564-1642) Italy • Supported Heliocentric model. • Used telescopes for astronomy after hearing about a Dutch lens maker who had devised a system of lenses to magnify objects. • He built many of his own and was able to magnify x30

24. Galileo (cont.) • 5 Major Discoveries • 1: Jupiter’s 4 largest satellites (moons). • 2: Planets are circular disks (like Earth) rather than points of light. • 3: Venus exhibits phases like the moon showing that is must orbit around the Sun, not Earth.

25. Galileo (cont.) • 4: The moons surface is not smooth glassy sphere, instead it has craters and plains (he thought plains might be bodies of water) • 5: The sun has sunspots which were areas of darker regions caused by slightly lower temperatures. These were also tracked and estimated a rotational period of the Sun to just under a month.

26. Sir Isaac Newton(1642-1727) England • Formalized the concept of inertia, the first law of motion. • Determined the force that keeps the planets from going in a straight line out in to space; Gravity. • Formulated and tested the law of universal gravitation. • Everybody in the universe attracts every other body with a force that is proportional to their masses and inversely proportional to the square of the distance between them.

28. Earth’s Movement • Rotation = 1 day • Revolution = 1 year • Earth rotates as it revolves around the sun. • Earth’s orbit is an ellipse

29. Axial tilt is the reason for the seasons!! • Perihelion = Earth is closest to the sun. • About 91 million miles away from sun. • Northern Hemisphere pointed away from sun makes is our winter (January) • Aphelion = Earth is furthest from the sun. • About 95 million miles away from sun. • Northern Hemisphere pointed towards the sun makes is our summer (July)

30. Earth/Sun Relationships • Equinoxes and Solstices determined by the circle of illumination. • Equinoxes • 2 per year • Vernal (spring) – March 20 • Autumnal (fall) – September 24 • 12 hours of day and night at every latitude.

31. Earth/Sun Relationships (cont.) • Solstices • 2 per year • Winter Solstice (Dec 22) • Winter for northern hemisphere • Northern hemisphere tilted away from sun • Occurs during perihelion. • Summer Solstice (Jun 22) • Summer in northern hemisphere • Northern hemisphere tilted towards the sun. • Occurs during aphelion.

32. Earth/Sun/Moon Relationships • Tides – daily changes in the elevation of the ocean surface. • Easiest ocean movement to observe other than waves. • Were not explained until Sir Isaac Newton showed that there is a mutual attractive force between two bodies (ex: Earth and Moon)

34. Tides • Moon causes bulge of water on side nearest the moon because of gravitational forces. • Creates a equal sized bulge on the opposite side of the Earth. • The Sun’s tide-generating effect is only about 46% that of the moon.

35. Tides (cont.) • New and Full moons • Sun and Moon are aligned and their forces are added together. • The combined gravity causes higher high tides and lower low tides. • These are called Spring tides.

37. Tides (cont.) • First and Third Quarter moon phases. • Moon and Sun act on right angles and each partially offset the influence of the other. • Daily tidal range is less. • These are called Neap tides.