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ASTR 330: The Solar System

ASTR 330: The Solar System. Lecture 2: Finding Our Place in Space ~ and ~ A Historical Perspective on Astronomy. Dr Conor Nixon Fall 2006. “An', as it blowed an' blowed, I often looked up at the sky an' assed meself the question -- what is the stars, what is the stars?”

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ASTR 330: The Solar System

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  1. ASTR 330: The Solar System Lecture 2: Finding Our Place in Space ~ and ~ A Historical Perspective on Astronomy Dr Conor Nixon Fall 2006

  2. “An', as it blowed an' blowed, I often looked up at the sky an' assed meself the question -- what is the stars, what is the stars?” from Juno and the Paycock, by Sean O’ Casey ASTR 330: The Solar System …what sorts of things do we see? The Night Sky… Dr Conor Nixon Fall 2006 Picture credit: Wally Pacholka AMS

  3. ASTR 330: The Solar System • What do we mean by the term ‘constellation’? • A constellation (meaning ‘stars together’) is a pattern of stars on the sky, popularly recognized to form the shape of a person, animal or object: e.g. Orion the hunter, Ursa Major the Great Bear, Libra the scales, etc. • The stars comprising a constellation are only apparently clustered together in one place: they may actually be at greatly varying distances from us. Constellations Dr Conor Nixon Fall 2006

  4. ASTR 330: The Solar System Orion: The Hunter Dr Conor Nixon Fall 2006 Figure credit: wikipedi.org

  5. ASTR 330: The Solar System The Orion Nebulae Dr Conor Nixon Fall 2006

  6. ASTR 330: The Solar System “Pillars of Creation” in M42, The Great Orion Nebula (picture: Hubble Space Telescope) Nebulae In Orion M43 Horsehead Nebula (picture: Angle-Australian Telescope) Dr Conor Nixon Fall 2006

  7. ASTR 330: The Solar System (Circumpolar Stars Movie) Apparent Motion of The Sun and Stars (Sirius Diurnal Motion Movie) Movie credit: Rick Pogge, Ohio State Dr Conor Nixon Fall 2006

  8. ASTR 330: The Solar System • The Sun appears to travel from east to west across the sky each day. • The path the sun takes across the sky changes during the year: • Higherin the summer → longerdays. • Lowerin thewinter→ shorterdays. Solar ‘motions’ Dr Conor Nixon Fall 2006

  9. The Ecliptic ASTR 330: The Solar System apparent solar path on sky: Figure credit: David P. Stern, Code 695, NASA GSFC Dr Conor Nixon Fall 2006

  10. ASTR 330: The Solar System • Some important dates: • 21st June = northern summersolstice: • longest day (northern hemisphere) • shortest day (southern hemisphere) • 21st December = northern winter solstice: • shortest day (northern hemisphere) • longest day (southern hemisphere) • 21st September & 21st March … what? • equal length day and night: spring and fall equinoxes. Seasonal Changes Dr Conor Nixon Fall 2006

  11. ASTR 330: The Solar System • The extremes of solar motion occur at the poles: • 24 hours daylight in mid-summer. • 24 hours darkness in mid-winter. • What sort of seasonal variation in the length of day would we expect at the equator? Extremes of Day and Night Dr Conor Nixon Fall 2006

  12. ASTR 330: The Solar System The Sun appeared to pass through 12 of the original constellations in the sky over the course of one year, hence your ‘sun-sign’ depending on which day you were born. The 12 signs were mostly animals, hence the ‘zodiac’ from the same Greek etymology as ‘zoo’. The Zodiac Actually, the Sun now passes through 13 constellations, and at different dates from the ‘official’ astrological ones! Picture credit: outreach@cea.berkeley.edu Dr Conor Nixon Fall 2006

  13. ASTR 330: The Solar System The Moon is observed to change in appearance over the course of about 30 days: Phases Of The Moon Why? Dr Conor Nixon Fall 2006

  14. ASTR 330: The Solar System The position of the Moon, relative to the Sun (lighting) and Earth (viewer) determines whether we see the sunlit side, shadow side, or somewhere in between. Lunar Phases: Explanation Dr Conor Nixon Fall 2006 Picture credit: wikipedia.org

  15. ASTR 330: The Solar System • The Sun, Moon and Earth all lie nearly in the same plane. • Also, by coincidence, the apparent size, or angular diameterof the Moon and Sun as seen from the Earth are about the same. • As the Moon goes round the Earth, what happens when: • The Moon comes between the Earth and Sun? • The Earth comes between the Moon and Sun? Eclipses Dr Conor Nixon Fall 2006

  16. ASTR 330: The Solar System Solar Eclipse Dr Conor Nixon Fall 2006

  17. ASTR 330: The Solar System Total Solar Eclipse Dr Conor Nixon Fall 2006

  18. ASTR 330: The Solar System Below shows a sequence of shots as the eclipse unfolds… Views Of Solar Eclipses... The solar corona (‘crown’) – the outer part of the sun’s atmosphere, and normally invisible – is spectacularly revealed at totality. Many solar astronomers use this opportunity to study the corona. Dr Conor Nixon Fall 2006

  19. ASTR 330: The Solar System “Bailey’s Beads” What could be causing the ‘beading’ effect? Dr Conor Nixon Fall 2006

  20. ASTR 330: The Solar System Lunar Eclipse Dr Conor Nixon Fall 2006

  21. ASTR 330: The Solar System Lunar Eclipse Views… Why does the moon not disappear completely at totality? Dr Conor Nixon Fall 2006

  22. ASTR 330: The Solar System • Revised Office hours: Conor Nixon: 11-12 Tuesdays and 2-3 Thursdays Room CSS 0225 KwangHo Park: 11-12 Mondays and Wednesdays Room CSS 0224 • Yellow forms & enrollment. • Textbooks. • Homework #1 out today. Due back 9/12/06. Announcements 9/5/06 Dr Conor Nixon Fall 2006

  23. ASTR 330: The Solar System • The near 30-day cycle of lunar phases gives us the basic period of one ‘monath’, or month. • We can then simply divide the year (solar cycle) into 12 months (lunar cycles), with some extra days being added to some of the months. • We already know where the idea of ‘day’ comes from… … but where does the 7-dayweek come from?? The Moon and the Calendar Dr Conor Nixon Fall 2006

  24. ASTR 330: The Solar System • In the night sky, there are five points of light visible to the naked eye, which move, like the Sun and Moon, and unlike the stars in the fixed constellations. • The Greeks used the term ‘wanderer’ from which we get the word ‘planet’. • These acquired the names of different gods in different cultures: we use the Roman names: swift Mercury the messenger god, bright Venus goddess of love, red Mars god of war, kingly Jupiter, and slow-moving Saturn, god of time. Wandering Stars Dr Conor Nixon Fall 2006

  25. ASTR 330: The Solar System Over time, the seven ‘heavenly bodies’ became associated with more than just deities: Planets, Astrology, Alchemy Symbol graphics from astro.uvic.ca Dr Conor Nixon Fall 2006

  26. ASTR 330: The Solar System • The planets wandered through the same 12 constellations of the zodiac as the Sun and Moon. • Mercury and Venus stayed close to the Sun and so were always seen at dawn or dusk (hence the ‘Evening Star’). The others could be seen during the night as well. • However, unlike the Sun and Moon, these planets exhibited curious ‘retrograde motion’ – periodic reversals of direction. Motions of the planets Dr Conor Nixon Fall 2006

  27. ASTR 330: The Solar System The apparent motion of Mars, Jupiter and Saturn was erratic, showing retrograde loops in their forward motion. Picture: www.mhhe.com Retrograde Motion (Mars Retrograde Movie) Movie credit: Rick Pogge, Ohio State Dr Conor Nixon Fall 2006

  28. ASTR 330: The Solar System To the ancient Greeks, the circle was the most perfect geometric figure. The Sun, Moon and planets were thought to be perfect, unchanging bodies circling a stationary Earth – a geocentric universe. There was little reason to doubt this hypothesis. However, an explanation for retrograde motion and also the periodic variations in brightness of the planets was needed. This was provided by a system of circles within circles: known as epicycles. The triumph of ancient astronomy was the Ptolemaic system of epicycles (after Claudius Ptolemy, right, 2nd century AD) which endured for over 1000 years! Geocentric System Dr Conor Nixon Fall 2006

  29. ASTR 330: The Solar System Aristotle’s Geocentric Universe Picture credit: phys.utk.edu Dr Conor Nixon Fall 2006

  30. ASTR 330: The Solar System Ptolemy’s Epicycles Picture and animation credit: phys.utk.edu Dr Conor Nixon Fall 2006

  31. ASTR 330: The Solar System Copernicus was the reluctant revolutionary who overthrew the geocentric universe. In a book published at the end of his life, he proposed that a much simpler model of the universe was possible, if we assume that the Sun is at the center and the Earth and other planets circled around it. This heliocentric model was a heretical view in the 16th century! In fact, Aristarchus of Samos had proposed a heliocentric model around 200 BC, but Aristotle’s view won (back then)! What were the objections? Nicholaus Copernicus (1473-1543) Dr Conor Nixon Fall 2006 Picture: Univ. St. Andrews

  32. ASTR 330: The Solar System Copernican Heliocentric Universe Picture credit: phys.utk.edu Dr Conor Nixon Fall 2006

  33. ASTR 330: The Solar System Much fewer epicycles were needed. Retrograde motion in the Copernican system Animation credit: phys.utk.edu Dr Conor Nixon Fall 2006

  34. The Ecliptic: revisited ASTR 330: The Solar System the Earth’s view of the Sun changes over the year: Figure credit: David P. Stern, Code 695, NASA GSFC definition: 1 astronomical unit (AU) is the average distance from the Earth to the Sun (150 million km, 93 million miles). Dr Conor Nixon Fall 2006

  35. ASTR 330: The Solar System Kepler took Copernicanism a step further. By analyzing very precise astronomical observations made by his mentor, Tycho Brahe (1546-1601), he realized that circles were hopeless for fitting the data. Kepler’s genius was to fit the motions of Mars using an elliptical orbit, with the Sun at one focus: Johannes Kepler (1571-1630) Picture credit: phys.utk.edu Dr Conor Nixon Fall 2006 Picture: Univ. St. Andrews

  36. ASTR 330: The Solar System • Law of Orbits: Each planet moves in an elliptical orbit about the sun, with the Sun at one focus of the ellipse. Kepler’s Laws (1) a = semi-major axis e = eccentricity Ra = aphelion distance Rp = perihelion distance Picture credit: gsu.edu Dr Conor Nixon Fall 2006

  37. ASTR 330: The Solar System 2. Law of Areas: An imaginary line connecting the Sun with a planet sweeps out equal areas in equal times as the planet moves about the sun. Kepler’s Laws (2) Picture credit: gsu.edu Dr Conor Nixon Fall 2006

  38. ASTR 330: The Solar System 3. The Law of Periods: The square of the period of any planet is proportional to the cube of the semi-major axis of orbit. Kepler’s Laws (3) T2 = K a3 Picture credit: gsu.edu Dr Conor Nixon Fall 2006

  39. ASTR 330: The Solar System Galileo provided the first crucial evidence that the Copernican solar system was physical reality, not just a useful aid to calculation. • Galileo enthusiastically took to the new tool of science: the telescope, and turned it on the sky. He soon found: • The four large moons of Jupiter, a mini-solar system, in itself. • The phases of the planet Venus, similar to the moon. 3. The rings of Saturn. Galileo Galilei (1564-1642) Dr Conor Nixon Fall 2006 Picture: Univ. St. Andrews

  40. ASTR 330: The Solar System Newton was a temperamental genius who founded multiple areas of classical physics: including optics, gravitation, and mechanics. His three laws of motion (see textbook) form the basis of mechanics. Newton crucially realized that the force which holds the Moon in its orbit about the Earth is the same force causing an apple to fall to the ground. Newton was hence able to deduce the famous inverse-square law of gravity, and prove Kepler’s laws. Isaac Newton (1642-1727) Dr Conor Nixon Fall 2006

  41. ASTR 330: The Solar System “The gravitational force between two objects is proportional to product of their masses and inversely proportional to the square of the distance between them”. Newton’s Law of Universal Gravitation This centrally-acting force opposes the tendency of the planets to continue in straight line motion, and holds them in orbit about the sun. Dr Conor Nixon Fall 2006

  42. ASTR 330: The Solar System • Newton proposed a ‘thought experiment’ in which a cannon was fired from a mountaintop, at progressively greater and greater speeds. • The ball falls further and further from the mountain, and eventually ‘misses’ the Earth altogether! Today, we know it is possible to do as Newton imagined and to send objects into orbit… Newton’s Cannon and orbits Dr Conor Nixon Fall 2006

  43. ASTR 330: The Solar System Blast-Off! (STS 108 Launch Movie) Picture and movie credit: NASA KSC Dr Conor Nixon Fall 2006

  44. ASTR 330: The Solar System • Orbital velocity depends on the altitude of the orbit: • LEO – Low Earth Orbit (200 km) requires 8 km/s • GEO – Geo-stationary Earth Orbit requires about 10 km/s • At a velocity of 11.2 km/s, known as escape velocity, the spacecraft can escape the Earth’s gravity and go into solar orbit. • This is the minimum velocity required for spacecraft to reach other planets. Escape Velocity Dr Conor Nixon Fall 2006

  45. ASTR 330: The Solar System • Question 2: What is the revolution period of a hypothetical planet that orbits the Sun at half the distance of Mercury? Kepler’s third law states that T2 = Ka3, where T is the orbital period and a is the semi-major axis of orbit. If T is in years and a in AU (distance from Sun to Earth) then K=1. So T2 = a3. Now, Mercury orbits the Sun at a=0.39AU. So, a for the proposed planet is 0.195, and a3 = (0.195)3 = 7.41 x 10-3. Finally, T = √(7.41 x 10-3) = 0.086 years. • Of one that has twice the distance from the Sun as Pluto? (distance of Pluto = 39.48 AU). You do it! Answer: 702 years! Quick method: rearrange formula to T=√(a3)=a1.5 Numerical Examples from Chapter 1 Dr Conor Nixon Fall 2006

  46. ASTR 330: The Solar System • Question 5: A spacecraft on a trajectory from the Earth to Saturn follows an ellipse with perihelion at the Earth’s orbit (1 AU) and aphelion at Saturn’s orbit (9 AU). If the semi-major axis of the ellipse is 5 AU what is the time required for the trip from the Earth to Saturn? The key here is to realize that the spacecraft is in an orbit about the Sun, although an elliptical one. Kepler’s third law is applied again, to calculate the period, T = √(a3) = √(53) = 11.2 years. But, this is the time for one complete orbit. We only need the time from perihelion to aphelion, or half the total time, = 5.6 years. • Using similar reasoning, find the trip time to Mars (1.5 AU). Answer: ½ T = ½ √(a3) = ½ √(1.253) = 0.7 years. Numerical Examples from Chapter 1 Dr Conor Nixon Fall 2006

  47. ASTR 330: The Solar System • What is a constellation? • Define ecliptic and zodiac, and explain the relation between them. • What causes a solar eclipse? A lunar eclipse? • What is meant by a geocentric universe? A heliocentric one? • Which is the odd one out: 1 day, 1 week, 1 month, 1 year? • Give a major contribution to astronomy by each of the following: Aristotle, Ptolemy, Copernicus, Kepler, Galileo, Newton. • What are Kepler’s three laws of planetary motion? Quiz - Summary Dr Conor Nixon Fall 2006

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