Earth’s Motion Earth is also revolving around the Sun. The time it takes to complete on revolution is called a year. At any given time, only half the sky is visible at night. Fall 2013Astro 103April, 2009 You can see the constellations on this side only.
Conversely The sun appears in a constellation on the “other side” –Zodiac: Belt around sky, ~18° wide, centered on ecliptic, w/in which we find & planets Root of Zodiac same as Zoo, means collection of animals Pattern of ’s w/in zodiac belt reminded ancients of animals Fall 2013Astro 10319
Three Views of Orion Eighty-eight constellations cover the entire celestial sphere Fall 2014Astron 120
Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups. Fall 2014Astron 121
Constellations (2) They were believed to represent great heroes and mythological figures. Each culture has its own set of constellations, usually pertaining to local beliefs. Fall 2014Astron 122
Constellations (3) Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions. Fall 2014Astron 123
Constellations (4) The stars of a constellation only appear to be close to one another. Usually, this is only a projection effect: The stars of a constellation may be located at very different distances from us. Fall 2014Astron 124
Constellations (5) Stars are named by a Greek letter ( ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Orion Betelgeuse Rigel Rigel = Orionis Betelgeuse = Orionis Fall 2014Astron 125
Constellations (6) Some examples of easily recognizable constellations and their brightest stars Fall 2014Astron 126
Apparent Motion of The Celestial Sphere Some constellations around the Celestial North Pole never set. These are called “circumpolar”. The circle on the celestial sphere containing the circumpolar constellations is called the “circumpolar circle”. Fall 2014Astron 127
Eratosthenes (c. 276-194 B.C.) If the Earth is round... –How big is it? In Syene (modern day Aswan, Egypt) the Sun casts no shadow at Summer Solstice. In Alexandria, Egypt Sun is ~7 o from Zenith. Fall 2014Astron 128 The angle is 7 degrees
Eratosthenes’ Findings Distance between Alexandria and Syene ~ 5,000 stadia Angular between Alexandria and Syene ~ 7 degrees R ~ 40,000 stadia ….. probably within 15% Fall 2014Astron 130 Circumference of a circle
Precession (1) The Sun’s gravity is doing the same to Earth. The resulting “wobbling” of Earth’s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession. At left, gravity is pulling on a slanted top. => Wobbling around the vertical.
Precession (2) As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. It will be closest to Polaris ~ A.D. 2100. There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, the celestial north pole will be close to Vega in the constellation Lyra.
Review Topics How are constellations used in modern astronomy? How many constellations are recognized by modern astronomers? Name three constellations. What is the concept of ‘projection’? And, how does it relate to constellations? What are circumpolar constellations? Fall 2014Astron 1April, 2009
Review Topics Why do the constellations we see at night change throughout the year? What is precession? What causes it? Fall 2013Astro 103April, 2009