ASTRONOMY

1. ASTRONOMY B.Miller

2. Contents • History of Astronomy • Earth • Moon • Survey of Solar System • Inner Planets • Outer Planets • Asteroids, Meteors and Comets • The Sun • Measuring Properties of Stars • Stellar Evolution • Life in the Universe

3. HISTORY OF ASTRONOMY

4. HISTORY OF ASTRONOMY • Our picture of the universe has been assembled bit by bit from many separate discoveries- from many people, from many parts of the world • We divide the history of Western Astronomy into 4 main periods….

5. 4 main periods of Western Astronomy • Prehistoric (before 500 B.C.) • People observed daily & seasonal motions of the sun, moon and stars • Learned cyclic motions for keeping time and determining direction • Classical (500 B.C. – A.D. 1400) • Began to make measurements of heavens • Constructed models to account for the motions of the heavens using geometry

6. Renaissance (1400-1650) • Reassessed geometrical models, found problems • Constructed new models • Benefited from the development of telescope • Modern (1650 – Present) • Began research for physical laws (gravity) • Huge technological advances (lenses, computers) • Better math (Calculus)

7. Prehistoric Astronomy • Many of the astronomical phenomena well known to ancient people, are not nearly as known to people today. • We do not notice the heavens because of lights, smog and changes in lifestyle • Structures like stonehenge were built around the movements of the sun during each season • Why would it be important to understand when the seasons would change?

8. The Celestial Sphere • We imagine this sphere as a gigantic dome that stretches over head and all the way around the Earth • HORIZON- the visual line where the sky meets the ground • The celestial sphere “dome” is just a way to visualize the heavens, therefore is just a model. The stars are not really all contained in this dome.

9. Constellations • We humans seek order in what we look at, therefore ancient people noticed patterns and forms in the night sky – CONSTELLATIONS • We see virtually the same constellations as ancient people • The names given were probably mnemonic devices to help people remember the seasons and help in navigation.

10. Motions of the Sun & Stars • Daily or Diurnal Motion- • Stars move across the sky from East to West just as our sun seems to move • This is why ancient people believed the Earth was not moving and the celestial sphere moved around us • 2 points on the celestial sphere DO NOT move. These are the North and South Celestial Poles • Another useful sky marker is the Celestial Equator • Both the celestial poles and celestial equator lie directly above these same points on the Earth

11. Annual Motion • If you watch the sky over several months you would notice new constellations appearing in the East and some disappearing behind the Western horizon • This knowledge helped ancient people measure the passage of time for each season “How many days before we need to harvest the crops?” -for example • We see this change in the constellations because of our motion around the sun.

12. The Ecliptic • This is the path that the sun takes across the celestial sphere (or the sky over head)

13. Seasons • Many people believe that we have seasons because the Earth’s orbit is elliptical, however we (N.Hemisphere) are nearest the sun in January when the we experience winter ????? • THEREFORE the elliptical orbit IS NOT the reason for the seasons • The tilt on our axis IS what causes the seasons • During part of our orbit around the sun the Northern hemisphere is tilted slightly (23.5 degrees)

14. The seasons

16. Solstices and Equinoxes • Due to the tilt of the Earth on its axis, we see a change in the position and path the sun appears to take across the sky. • Example: In the northern hemisphere, in the summer the sun rises in the northeast and sets in the northwest, however in winter the sun rises in the southeast and sets in the southwest

17. Astronomers give names to different positions of the sun • Spring (Vernal) Equinox = near March 21 • Fall (Autumnal) Equinox = near Sept 23 • Summer Solstice = near June 21 • Winter Solstice = near Dec. 21 • Spring begins and Autumn begins when the sun crosses the celestial equator. • Days and Nights are of equal length during Equinoxes

18. Summer solstice: • Time when the sun has reached its northern most position in the sky • Longest day of the year • Winter solstice: • Time when the sun has reached its southern most position in the sky • Shortest day of the year

19. Solstice’s sun positions

20. The sun’s rays are more direct during the summer months, which warms the surface of the planet more than the angular rays during the winter. • The changes in sun position were obvious to ancient people, which can be seen in the buildings and structures that orient along these astronomical alignments • Ex: Egyptian temples & pyramids, Mayan pyramids, and Stonehenge in England

21. The Planets & The Zodiac • Early cultures noticed objects in the sky that seemed to move across the sky at regular intervals • These were planets, and they seemed to move within a very narrow band in the celestial sphere. This band is called the Zodiac. • Zodiac means “circle of animals” which is the circle where the constellations are located • Signs of the Zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces

22. Eclipses • Solar Eclipse: • When the moon passes between the sun and the earth

23. Lunar Eclipse: • When the Earth passes between the Sun and the Moon

24. Early Ideas of the Heavens: Classical Astronomy • Greek astronomers were the 1st to try to explain the workings of the heavens in a careful, systematic manner, using observations & models

25. The Shape of the Earth • 560-480 B.C. Pythagoras, a mathematician, was teaching that the Earth was a sphere. • Not because he calculated it, but because he believed a sphere was a perfect shape according to the Gods. • By 300 B.C. Aristotle was presenting arguments for a spherical Earth through observations • Viewing Earth’s shadow during eclipses • Observing that a traveler seemed to disappear into the horizon as they got further away

26. The Size of Earth • Eratosthenese (276-195 B.C.) • Made the first measurements of Earth • His estimate was a sphere with a circumference of 25,000 miles • This was amazingly close to today’s measurements!

27. Distance & Size of Sun & Moon • Before Eratosthenes, Aristarchus estimated the relative sizes of Earth, Moon and Sun, and the distances between them • He was not very accurate, but the ratios of distance were not too far off

28. Motion of the Planets • The main thread of astronomical research for nearly 2,000 years centered on the motion of the planets • Early ideas of the Earth being at the center with all the planets and stars moving around it. Called the GEOCENTRIC model by Eudoxus

29. Ptolemy • Unfortunately the Geocentric Model did not explain “Retrograde motion”, which is the apparent backward movement of some planets at some times • Ptolemy tried to explain why planets would briefly change direction, then continue on their orbits • He proposed something called Epicycles, which is a smaller circle in side of a larger orbit

30. Epicycles • There was a problem with Ptolemy’s epicycle theory • For an object to orbit in a circle, they must have something with mass holding them in that patter at the center of the orbit, therefore, support failed.

31. Astronomy in the Renaissance • Copernicus (1473-1543) • Could not make sense of the geocentric model, therefore investigated further on the Heliocentric model • HELIOCENTRIC = “sun centered”

32. Tycho & Kepler • Tycho was making some very accurate measurements about the locations and paths of planets • It was not until Kepler helped make mathematical sense of Tycho’s notes that the idea of Elliptical orbits emerged • An Ellipse is sort of oval in shape, with 2 foci at the center.

33. Ellipse shape • This is the general idea behind how an ellipse gets his oval shape. • We now know this is the orbit shape of the planets.

34. Kepler’s 3 Laws • Planets move in elliptical orbits, with the sun at one focus • Orbital speed varies so the line joining the sun and the planet sweeps over equal area in equal time intervals • The amount of time it takes to orbit the sun is related to its orbit size

35. Galileo • Galileo Galilei (1564-1642) Italian scientist • Was not only interested in the motion of the heavenly bodies, but of all objects • He is often given credit for developing the telescope, however a Dutch spectacle maker made it and Galileo was the 1st to use it to look at the stars and planets

36. Galileo’s Contributions: • Saw mountains on the moon • Saw sunspots on the sun and noted their changes • Figured out that the sun rotated • Discovered the 1st 4 moons of Jupiter • Saw the rings of Saturn • Galileo’s finding & writings got him into trouble with the church. He was vocal about his ideas of a sun-centered universe. Because of his view he was placed under house arrest for the rest of his life. He also went blind from his extensive viewing of the sun.

37. Isaac Newton (1642-1727) & The Birth of Astrophysics • Born the year Galileo died • Arguable the greatest scientist of all time • Made contributions in math, physics & astronomy • Invented Calculus • Determined a way to calculate the gravitational force from the mass of an object • Invented milling of coins (process of putting grooves in the coin edges to recognize if someone had dried to shave gold from it)

38. Newton Continued… • Most of Newton’s findings are in the foundations of Astronomy today • The math and physics that Newton applied to solving questions in astronomy have developed into a very logical term called ASTROPHYSICS.

39. EARTH

40. THE EARTH • Earth is a hug, rocky sphere spinning in space and hurtling around the sun • Many other planets share similar properties, but not in the right mix to allow for life.

41. Shape & Size of Earth • Radius of 6400 km (4000 miles) • Sphere shaped, with an equitorial bulge, which is caused by gravity • Objects with a radius greater than 350 km will be pulled into a sphere by gravity • Objects with a radius less than 350 km will retain their irregular shape

42. A sphere with a bulge is called an oblate spheroid • The bulge is caused from our rotation on the axis. This spinning pushed outward against the force of gravity • All points on the Earth take 1 day to rotate, however, points closer to the equator move much faster and go a greater distance.

43. Composition of Earth • Rock = minerals and chemical elements • Most common elements in surface rocks • Oxygen • Silicon • Aluminum • Magnesium • Iron • Olivine (Much of Earth’s interior is composed of this greenish iron-magnesium silicate)

44. Density of Earth • Density = mass/volume • Earth = 5.5 g/cubic cm (average density) • Comparison: • Ordinary rocks = 3 g/cubic cm • Iron =8 g/cubic cm • Water = 1 g/cubic cm

45. Earth’s Interior • We have only drilled 12 km of 6400 km total radius of Earth • Most of what we know about Earth’s interior is because of Earthquakes • These seismic waves are recorded around the world • Tells us about which layers are liquid or solid

46. Seismic Waves P-Waves S- Waves • Pass through solids and liquids • Pushes and pulls matter by compressing • Only move through solids • Moves up and down in an S- like shape • Also occur on the surface, and cause the most damage

47. Differentiation • Causes the most dense materials to settle to the center of a sphere and the lightest materials to move to the outer surface.

48. Heating of Earth’s Core • As we move deeper into the Earth, the temperature rises 2K for every 100 meters you descend. • At this rate the core would be 120,000K, however much of this heat escapes from the surface. • Current estimates = 6,500K core temp (hotter than the sun)

49. Why is the Core so Hot? • Pressure from Gravity • Radioactive elements in the core decay and release energy

50. Age of Earth • The oldest rocks dated show Earth to be approx. 4 Billion years old • If we find older rocks then we would realize that Earth is even older