1In 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 pyramids Astronomical 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 angles2Angular 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 ly3The 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 above4Eighty-eight constellations officially cover the entire sky • Ancient peoples looked at the stars and imagined groupings – Pictures in the sky– Different cultures different pictures• We still refer to many of these groupings• Astronomers call them constellations (from the Latin for “group of stars”)– Parts are asterismsModern Constellations• On modern star charts, the entire sky is divided into 88 regions– Each is a constellation• Most stars in a constellation are nowhere near one another• They only appear to be close together because they are in nearly the same direction as seen from Earth– Need to think in three dimensions The appearance of the sky changes during the course of the night and from one night to the next• Stars appear to rise in the east, slowly rotate about the Earth and set in the west.• This diurnal or daily motion of the stars is actually caused by the 24-hour rotation of the Earth.Annual Motion• The stars also appear to slowly shift in position throughout the year• This is due to the orbit of the earth around the sun• If you follow a particular star on successive evenings, you will find that it rises approximately 4 minutes earlier each night, or 2 hours earlier each month It was common to imagine that the stars are located on a celestial sphere• The celestial sphere is an imaginary object that has no basis in physical reality• However it is still a model that remains a useful tool of positional astronomy• Landmarks on the celestial sphere are projections of those on the Earth•Celestial equatordivides the sky into northern and southern hemispheres•Celestial poles are where the Earth’s axis of rotation would intersect the celestial sphere •Polaris is less than 1° away from the north celestial pole, which is why it is called the North Star or the Pole Star.•Point in the sky directly overhead an observer anywhere on Earth is called that observer’s zenith.– Nadir is directly opposite the zenith5The Reason for the SeasonsThe seasons are caused by the tilt of Earth’s axis of rotation• The Earth’s axis of rotation is not perpendicular to the plane of the Earth’s orbit• It is tilted about 23½° away from the perpendicular• The Earth maintains this tilt as it orbits the Sun, with the Earth’s north pole pointing toward the north celestial pole • The Sun appears to trace out a circular path called the ecliptic on the celestial sphere tilted at 23 ½ degrees to the equator• The ecliptic and the celestial equator intersect at only two points• Each point is called an equinox•