Outline of lecture 1 (Ch 1)

1. Outline of lecture 1 (Ch 1) Our modern View of the Universe Survey of the universe and powers of ten The scale of the Universe Astronomical distances Spaceship Earth Motions of Earth, Sun, Galaxies

2. Review today at 4:30 in HEC 119by Tony and Zoe

3. Earth 1 AU Sun. DISTANCE SCALES • ASTRONOMICAL UNIT (AU): • The average distance between the Earth and the Sun. • Units used in our solar system

4. KUIPER BELT AND OORT CLOUD ASTEROID BELT MERCURY NEPTUNE EARTH JUPITER URANUS SATURN VENUS PLUTO MARS A TOUR OF THE SOLAR SYSTEM Distance (AU) .4 .7 1.0 1.5 5 10 20 30 40 NOT TO SCALE!!!!

5. SCALE OF THE SOLAR SYSTEM • Let’s view it to scale • say the Sun is the size of a large grapefruit (13.9 cm) in UCF • then the nearest star would be in California

6. DISTANCE SCALES • LIGHT-YEAR (LY): • The distance light can travel in one year. • NOTE: this is a distance not a time!

7. “Celestial Bodies” • Star - A large, glowing ball of gas that generates heat and light through nuclear fusion • Planets - A moderately large object which orbits a star; it shines by reflected light. Planets may be rocky, icy, or gaseous in composition. • Moons - an object which orbits a planet • Asteroids - A relatively small and rocky object which orbits a star. • Comets -A relatively small and icy object which orbits a star. • Nebula - An interstellar cloud of gas and/or dust • Galaxy - A great island of stars in space, all held together by gravity and orbiting a common center

8. SOLAR (STAR) SYSTEM A star and all the material which orbits it, including its planets and moons

9. THE UNIVERSE The sum total of all matter and energy; that is, everything within and between all galaxies

10. AGE OF UNIVERSE Age of Universe: about 14 billion years Age of Solar System: about 4.6 billion years

11. Where do we come from? • The first (and simplest) atoms were created during the Big Bang. • More complex atoms were created in stars. • When the star dies, they are expelled into space…. to form new stars and planets! Most of the atoms in our bodies were created in the core of a star!

12. SPEED OF LIGHT • The speed of light in the vacuum of space is constant! All light travels the same speed! c = speed of light = 300,000 km/sec = 3 x 106 km/sec (no need to memorize)

13. Looking back in time • Light, although fast, travels at a finite speed. • It takes: • 8 minutes to reach us from the Sun • 4.3 years to reach us from our nearest star, Alpha Centauri • 1,500 years to reach us from the Orion Nebula • Two million years to reach us from the Andromeda galaxy • The farther out we look into the Universe, the farther back in time we see!

14. 1.1 A Modern View of the Universe Our goals for learning: • What is our physical place in the Universe? We are on a planet, orbiting a star, in a galaxy • Describe our cosmic origins and why we say that we are “star stuff.” The universe started with an explosion called the “Big Bang” at that time hydrogen and helium were created, all the other elements were “cooked” (created) inside stars • Why does looking into space mean looking back in time? Because of the time it takes for light to travel from large distances back toward us on Earth. For example, the Andromeda galaxy is 2 million light years away

15. EXPANSION (of the Universe) • We say that the universe is expanding because the average distance between galaxies is increasing with time. • NOTE: individual galaxies and star systems are not expanding within themselves!

16. EXPANSION • Mostly all galaxies appear to be moving away from us. • The farther away they are, the faster they are moving. • Just like raisins in a raisin cake; they all move apart from each other as the dough (space itself) expands.

17. Outline of Ch 2 Patterns in The Sky: Stars and constellations Celestial coordinates: Celestial sphere, poles, equator, ecliptic, right ascension, declination Seasons: Tilt in Earth’s axis (23.5 degrees) Equinoxes and solstices, precession The Moon and Eclipses Lunar and Solar Eclipses Ancient Mystery of the Planets: Apparent Retrograde motion of planets

18. What is a constellation? • A constellation is a region of the sky. The sky is divided into 88 official constellations. Constellation: Orion

19. What is the celestial sphere? • An imaginary sphere surrounding the Earth upon which the stars, Sun, Moon, and planets appear to reside.

20. The Celestial Sphere North & South celestial poles the points in the sky directly above the Earth’s North and South poles celestial equator the extension of the Earth’s equator onto the celestial sphere ecliptic the annual path of the Sun through the celestial sphere, which is a projection of ecliptic plane

23. Measuring the Sky • Full circle = 360º • 1º = 60 arcmin • 1 arcmin = 60 arcsec We measure the sky in angles, not distances.

24. Measuring Angles in the Sky Moon = 0.5° Sun = 0.5°

25. The Local Sky zenith the point directly above you horizon all points 90° from the zenith altitude the angle above the horizon meridian due north horizon zenith due south horizon

26. Coordinates on the Celestial Sphere (not in book) • Latitude: position north or south of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England) • Declination: position north or south of celestial equator (in degrees) • Right Ascension: distance (in hours, 0 to 23h 59 min.) East of vernal equinox • (vernal equinox: where the sun crosses the celestial equator going North)

28. The Daily Motion • As the Earth rotates, the sky appears to us to rotate in the opposite direction. • The sky appears to rotate around the N (or S) celestial poles. • If you are standing at the poles, nothing rises or sets. • If you are standing at the equator, everything rises & sets 90 to the horizon.

29. Annual Motion • As the Earth orbits the Sun, the Sun appears to move eastward with respect to the stars. • The Sun circles the celestial sphere once every year.

30. Annual Motion • The Earth’s axis is tilted 23.5° from being perpendicular to the ecliptic plane. • Therefore, the celestial equator is tilted 23.5° to the ecliptic. • As seen from Earth, the Sun spends 6 months north of the celestial equator and 6 months south of the celestial equator. • Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the Sun!

31. Annual Motion ecliptic the apparent path of the Sun through the sky equinox where the ecliptic intersects the celestial equator solstice where the ecliptic is farthest from the celestial equator

32. Annual Motion Vernal Equinox (~March 21) When Sun crosses celestial equator going North Autumnal Equinox (~September 21) When Sun crosses celestial equator going South Summer Solstice (~June 21) When Sun is farthest North (23.5 degrees) from celestial equator Winter Solstice (~Dec. 21) When Sun is farthest South (23.5 degrees) from celestial equator

33. 2.4 Precession • What is the Earth’s cycle of precession?

34. Precession of the Earth’s Axis • The Earth’s axis precesses (wobbles) like a top, once about every 26,000 years. • Precession changes the positions in the sky of the celestial poles and the equinoxes. • Polaris won't always be the north star. • However the tilt in the axis is the same (23.5 degrees) as the Earth’s axis precesses

35. 2.5 The Moon, Our Constant Companion • Why do we see phases of the Moon? • What conditions are necessary for an eclipse?

36. Lunar Motion Phases of the Moon’s 29.5 day cycle • new • crescent • first quarter • gibbous • full • gibbous • last quarter • crescent waxing waning

39. Eclipses • The Earth & Moon cast shadows. • When either passes through the other’s shadow, we have an eclipse. • Why don’t we have an eclipse every full & new Moon?

40. Eclipses When the Moon’s orbit intersects the ecliptic (node): at new moon solar eclipse you must be in Moon’s shadow to see it at full moon lunar eclipse • everyone on the nighttime side of Earth can see it

41. Solar Eclipse

42. Lunar Eclipse

43. What have we learned? • Why do we see phases of the Moon? • At any time, half the Moon is illuminated by the Sun and half is in darkness. The face of the Moon that we see is some combination of these two portions, determined by the relative locations of the Sun, Earth, and Moon. • What conditions are necessary for an eclipse? • An eclipse can occur only when the nodes of the Moon’s orbit are nearly aligned with the Earth and the Sun. When this condition is met, we can get a solar eclipse at new moon and a lunar eclipse at full moon.

44. 2.6 The Ancient Mystery of the Planets • Why do planets sometimes seem to move backwards relative to the stars?

45. Retrograde Motion • Planets usually appear to move eastward relative to the stars. • But as we pass them by in our orbit, they move west relative to the stars for a few weeks or months. • Noticeable over many nights

46. Explaining Apparent Retrograde Motion • Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus lap us) • But very difficult to explain if you think that Earth is the center of the universe!

47. Why did the ancient Greeks reject the notion that the Earth orbits the sun? • Mainly because ancient Greeks knew that we should see stellar parallax if we orbited the Sun – but they could not detect it.

48. Parallax Angle Apparent shift of a star’s position due to the Earth’s orbiting of the Sun The nearest stars are much farther away than the Greeks thought. So the parallax angles of the star are so small, that you need a telescope to observe them.

49. Possible reasons why stellar parallax was undetectable: • Stars are so far away that stellar parallax is too small for naked eye to notice • Earth does not orbit Sun; it is the center of the universe Unfortunately, with notable exceptions like Aristarchus, the Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1)… Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.

50. Ch. 3 Ch 3 (Histrory of Astronomy) “Does the World Turn?” Archeoastronomy Astronomy of Greece: good records & more rational approach Thales and Pithagoras: roots of science Thales: Universe is rational Pithagoras: nature is governed by musical (mathematical) principles Plato and Aristotle: Geocentric universe Geocentric universe Earth is imperfect Heavens are perfect “uniform circular motion” is perfect. Hence, heavenly bodies must follow uniform circular motion Ptolemy: mathematical models of geocentric views The Copernican revolution “The Church Strikes back” The Nature of Science