Naked-eye (unaided-eye) astronomy had an important place in ancient civilizationsAstronomical observations led to the development of the modern calendarAstronomers use angles to denote the positions and apparent sizes of objects in the skyAngular MeasurementsPowers-of-ten notation – a useful shorthand system for writing numbersCommon Prefixes for Powers of TenAstronomical distances are often measured in astronomical units, parsecs, or light-yearsThe parallax of stars reveals their distances Barnard’s star has a parallax of 0.54 arcsec The Small Angle FormulaSmall Angle Formula ExampleiClicker QuestionEighty-eight constellations officially cover the entire sky Modern ConstellationsThe appearance of the sky changes during the course of the night and from one night to the nextAnnual MotionIt was common to imagine that the stars are located on a celestial sphereThe seasons are caused by the tilt of Earth’s axis of rotationThe Moon helps to cause precession, a slow, conical motion of Earth’s axis of rotationPrecession causes the gradual change of the star that marks the North Celestial PoleThe Moon’s rotation always keeps the same face toward the Earth – this is synchronous rotationAncient astronomers measured the size of the Earth and attempted to determine distances to the Sun and Moon To understand the universe, astronomers use the laws of physics to construct testable theories and modelsAncient astronomers invented geocentric models to explain planetary motionsNicolaus Copernicus devised a comprehensive heliocentric modelTycho Brahe’s astronomical observations provided evidence for another model of the solar systemParallax – apparent difference in position of object viewed from two different locationsJohannes Kepler proposed elliptical paths for the planets about the SunKepler’s First LawKepler’s Second LawKepler’s Third LawiClicker QuestionGalileo’s discoveries with a telescope strongly supported a heliocentric modelGeocentric Model IssuesIsaac Newton formulated three laws that describe fundamental properties of physical realityNewton’s Law of Universal GravitationNewton’s description of gravity accounts for Kepler’s laws and explains the motions of the planets and other orbiting bodies Orbital MotionOrbits follow any one of the family of curves called conic sectionsGravitational forces between two objects produce tides in distant regions of the universeUnderstanding Tidal ForcesiClicker QuestionKey WordsVocabularyVocabularyVocabularyIn The BeginningandCosmology Becomes a ScienceNaked-eye (unaided-eye) astronomy had an important place in ancient civilizations• Positional astronomy– the study of the positions of objects in the sky and how these positions change– importance in navigation• Naked-eye (unaided-eye) astronomy– the sort that requires no equipment but human vision • Extends far back in time, across all cultures– British Isles Stonehenge– Native American Medicine Wheel– Aztec, Mayan and Incan temples– Egyptian pyramidsAstronomical observations led to the development of the modern calendar• The day– based on the Earth’s rotation• The year– based on the Earth’s orbit• The month– based on the lunar cycle• None of these are exactly the same as nature so astronomers use the average or mean day and leap years to keep the calendar and time consistentAstronomers use angles to denote the positions and apparent sizes of objects in the sky• The basic unit of angular measure is the degree (°).• Astronomers use angular measure to describe the apparent size of a celestial object—what fraction of the sky that object seems to cover• The angular diameter (or angular size) of the Moon is ½° or the Moon subtends an angle of ½°.If you draw lines from your eye to each of two stars, the angle between these lines is the angular distance between these two starsThe adult human hand held at arm’s length provides a means of estimating anglesAngular Measurements• Subdivide one degree into 60 arcminutes– minutes of arc– abbreviated as 60 arcmin or 60´• Subdivide one arcminute into 60 arcseconds– seconds of arc– abbreviated as 60 arcsec or 60”1° = 60 arcmin = 60´1´ = 60 arcsec = 60”Powers-of-ten notation – a useful shorthand system for writing numbersCommon Prefixes for Powers of TenFactor Name Symbol(billion) 109 Giga- G(million) 106Mega- M(thousand) 103kilo- k(hundredth) 10-2centi- c(thousandth) 10-3milli- m(millionth) 10-6micro-μ(billionth) 10-9nano- nAstronomical distances are often measured in astronomical units, parsecs, or light-years• Astronomical Unit (AU)– One AU is the average distance between Earth and the Sun– 1.496 X 108km or 92.96 million miles• Light Year (ly)– One ly is the distance light can travel in one year at a speed of about 3 x 105km/s or 186,000 miles/s– 9.46 X 1012km or 63,240 AU• Parsec (pc)– the distance at which 1 AU subtends an angle of 1 arcsec or the distance from which Earth would appear to be one arcsecond from the Sun– 1 pc = 3.09 × 1013km = 3.26 lyThe parallax of stars reveals their distances• Distances to the nearer stars can be determined by parallax, theapparent shift of a star against the background stars observed as the Earth moves along its orbit• Parallax measurements made from orbit, above the blurring effects of the atmosphere, are much more accurate than those made with Earth-based telescopes• Stellar parallaxes can only be measured for stars within a few hundred parsecsBarnard’s star has a parallax of 0.54 arcsecThe Small Angle Formula206265dDα=D = linear size of objecta = angular size of object (in arcsec)d = distance to the objectSmall Angle Formula Example• On July 26, 2003, Jupiter was 943 million kilometers from Earth and had an angular diameter of 31.2”. • Using the small-angle formula, determine Jupiter’s actual diameter.kmkmD561043.120626510943"2.31×=××=iClicker Question• The Sun and full Moon have about the same angular size on the sky of 0.5 degrees. Why?A. the sun and moon are the same size and the same distance from Earth, but always in different parts of the sky.B. the moon is bigger than the sun and farther from EarthC. all spherical objects have the same angular size on the skyD. the moon is smaller than the sun, but closer to EarthE. none of the aboveEighty-eight constellations officially cover the entire sky • Ancient peoples looked at the stars and