Understanding the Universe: Stars, Constellations, Seasons, and the Moon - Prof. Robert O., Study notes of Astronomy

Download Understanding the Universe: Stars, Constellations, Seasons, and the Moon - Prof. Robert O. and more Study notes Astronomy in PDF only on Docsity! 1 Chapter 2 Discovering the Universe for Yourself 2.1 Patterns in the Night Sky • What does the universe look like from Earth? • Why do stars rise and set? • Why do the constellations we see depend on latitude and time of year? Our goals for learning: What does the universe look like from Earth? With the naked eye, we can see more than 2,000 stars as well as the Milky Way. 2 Constellations A constellation is a region of the sky. 88 constellations fill the entire sky. Thought Question The brightest stars in a constellation… A. All belong to the same star cluster. B. All lie at about the same distance from Earth. C. May actually be quite far away from each other. The Celestial Sphere Stars at different distances all appear to lie on the celestial sphere. Ecliptic is Sun’s apparent path through the celestial sphere. 5 Angular Measurements • Full circle = 360º • 1º = 60′ (arcminutes) • 1′ = 60″ (arcseconds) Thought Question The angular size of your finger at arm’s length is about 1°. How many arcseconds is this? A. 60 arcseconds B. 600 arcseconds C. 60 × 60 = 3,600 arcseconds Angular Size angular size = physical size × 360 degrees 2π × distance An object’s angular size appears smaller if it is farther away 6 Why do stars rise and set? Earth rotates west to east, so stars appear to circle from east to west. Our view from Earth: • Stars near the north celestial pole are circumpolar and never set. • We cannot see stars near the south celestial pole. • All other stars (and Sun, Moon, planets) rise in east and set in west. Celestial Equator Your Horizon A circumpolar star never sets This star never rises Thought Question What is the arrow pointing to? A. the zenith B. the north celestial pole C. the celestial equator 7 Why do the constellations we see depend on latitude and time of year? • They depend on latitude because your position on Earth determines which constellations remain below the horizon. • They depend on time of year because Earth’s orbit changes the apparent location of the Sun among the stars. Review: Coordinates on the Earth • Latitude: position north or south of equator • Longitude: position east or west of prime meridian (runs through Greenwich, England) The sky varies with latitude but not longitude. 10 Thought Question TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. What causes the seasons? Seasons depend on how Earth’s axis affects the directness of sunlight Direct light causes more heating. 11 Axis tilt changes directness of sunlight during the year. Sun’s altitude also changes with seasons Sun’s position at noon in summer: higher altitude means more direct sunlight. Sun’s position at noon in winter: lower altitude means less direct sunlight. Summary: The Real Reason for Seasons • Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun. • Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct. • AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth. 12 Why doesn’t distance matter? • Variation of Earth-Sun distance is small — about 3%; this small variation is overwhelmed by the effects of axis tilt. How do we mark the progression of the seasons? • We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox We can recognize solstices and equinoxes by Sun’s path across sky: Summer solstice: Highest path, rise and set at most extreme north of due east. Winter solstice: Lowest path, rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west. 15 Phases of Moon • Half of Moon is illuminated by Sun and half is dark • We see a changing combination of the bright and dark faces as Moon orbits Phases of the Moon Moon Rise/Set by Phase 16 Phases of the Moon: 29.5-day cycle new crescent first quarter gibbous full gibbous last quarter crescent waxing • Moon visible in afternoon/evening. • Gets “fuller” and rises later each day. waning • Moon visible in late night/morning. • Gets “less” and sets later each day. } } Thought Question A. First quarter B. Waxing gibbous C. Third quarter D. Half moon It’s 9 am. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? We see only one side of Moon Synchronous rotation: the Moon rotates exactly once with each orbit That is why only one side is visible from Earth 17 What causes eclipses? • The Earth and Moon cast shadows. • When either passes through the other’s shadow, we have an eclipse. Lunar Eclipse When can eclipses occur? • Lunar eclipses can occur only at full moon. • Lunar eclipses can be penumbral, partial, or total. 20 2.4 The Ancient Mystery of the Planets • What was once so mysterious about planetary motion in our sky? • Why did the ancient Greeks reject the real explanation for planetary motion? Our goals for learning: Planets Known in Ancient Times • Mercury – difficult to see; always close to Sun in sky • Venus – very bright when visible; morning or evening “star” • Mars – noticeably red • Jupiter – very bright • Saturn – moderately bright What was once so mysterious about planetary motion in our sky? • Planets usually move slightly eastward from night to night relative to the stars. • But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion 21 We see apparent retrograde motion when we pass by a planet in its orbit. Explaining Apparent Retrograde Motion • Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus laps us) • But very difficult to explain if you think that Earth is the center of the universe! • In fact, ancients considered but rejected the correct explanation Why did the ancient Greeks reject the real explanation for planetary motion? • Their inability to observe stellar parallax was a major factor. 22 The Greeks knew that the lack of observable parallax could mean one of two things: 1. Stars are so far away that stellar parallax is too small to notice with the naked eye 2. Earth does not orbit Sun; it is the center of the universe With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1) because they did not think the stars could be that far away Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems. What have we learned? • What was so mysterious about planetary motion in our sky? – Like the Sun and Moon, planets usually drift eastward relative to the stars from night to night; but sometimes, for a few weeks or few months, a planet turns westward in its apparent retrograde motion • Why did the ancient Greeks reject the real explanation for planetary motion? – Most Greeks concluded that Earth must be stationary, because they thought the stars could not be so far away as to make parallax undetectable