Transcript The Sky
Introduction to Astronomy Physics 103 Dr. Julie A Rathbun Dr. Tyler E Nordgren 1A Topics: The Night Sky … 1A … from the Solar System… 1A … to the Stars… 1A … to the Milky Way … 1A … and to the Universe Beyond. 1A Our Goal • Understand: – The sky above • “What’s that bright red star to the south?” – News reports • “Scientists discover giant black hole.” – Popular entertainment • “Armageddon”, “Mission to Mars”, etc • Learn how science works and how we know what we know. 1A Organization • Lecture one night a week for 3 hours: – 2 lectures plus outdoor viewing • Lab one night a week (Wed. or Thurs.) – Meet in lab room about EVERY OTHER WEEK. • Check the Syllabus for exact dates! 1A How the Class Works • • • • Combine 3 hour Lecture with outdoor viewing. Homework assigned in Lecture. Formal Labs about every other week. Weeks without Lab we will assign small Observing projects. • Record ALL observations in an Observing Notebook. • 2 one-hour in-class exams. • One final. 1A Grading • • • • • Labs: 18% Observing notebook: 18% Homework assignments: 18% 2 In-class exams: 24% Final: 22% 1A The Sky: What do YOU see? • • • • • • A sphere of stars. Are they spaced uniformly? Do they move? Do they move relative to each other? Some do: How do they move? What are those two bright things (one of which is always up during the day)? • How do they move and change? 1A The Projected Sky • Stars are at different distances. • But we see them projected on to a plane. 1A A Sphere of Stars • We see the entire Universe projected on to a sphere. • The stars are scattered randomly on this sphere (except for the Milky Way). • In this randomness, we see pictures: Constellations. 1A Constellations 1A As the World Turns … • As the Earth turns on its axis, the stars (and everything else) move from east to west. • Only the North Star (Polaris) appears to stand still. • There is no Southern Star. 1A The Fixed Stars • The stars are fixed relative to one another. • The constellations you see today are the ones that ancient peoples saw long ago. • Over thousands to millions of years even these constellations will change as stars move through the galaxy. • On time scales of weeks to years, however, five ``stars’’ are seen to move relative to the others. – The Wanderers: the 5 naked-eye Planets. 1A Venus 1A The Sun and Zodiac • As the Earth orbits the Sun, the Sun “appears” to move through some constellations. • These constellations are the Zodiac. • The path the Sun follows is called the Ecliptic. • The Sun’s light always blots out the stars behind it. • Result: Seasonal constellations. 1A Seasons • The North Pole of the Earth is not perpendicular to the plane in which it moves around the Sun. • The result is Seasons. • Northern Summer: When the Northern hemisphere is tilted towards the Sun (Southern winter). • Northern Winter: When the Northern hemisphere is pointed away from the Sun (Southern Summer). 1A Solstices, Equinoxes, Oh My…. • Summer Solstice – June 21: The longest day of the year. The Sun reaches its highest point in the sky. • Winter Solstice – December 21: The shortest day of the year. The Sun is at its lowest. • Spring Equinox – March 21: The days and nights are EQUAL. • Fall Equinox – September 21: It’s the same then too. 1A The Moon and Phases • The Moon takes 27.3 days to orbit the Earth. • The Moon takes 27.3 days to turn on its axis. • Result: We always see the same face. • There is no “dark” side of the Moon. 1A Lunar Eclipse • Sometimes the Moon passes through the Earth’s shadow as cast by the Sun. 1A Composite photo of the January 2001 lunar eclipse. 1A Solar Eclipse • Sometimes the Moon passes between the Sun and the Earth (and casts a shadow on the Earth). August 1999 eclipse. Photo from crew of Mir. 1A Composite photo of August 1999 solar eclipse. 1A Types of Solar Eclipses • Partial • Total • Annular 1A Angular Size • The Sun is a million times larger than the Earth. • The Moon is a fourth the size of the Earth. • The distance from the Earth determines their ANGULAR SIZE. • Angular size: How big does something look as viewed from the Earth. 1A Angles • The sky is 360 arc degrees around. • 60 arcminutes = 1 arc degree – The Full Moon is about half an arc degree = arcminutes. 30 • 60 arcseconds = 1 arcminute – Mars is about 2 arcminutes now. • 1000 milliarcsecond = 1 arcsecond – Polaris is 46 milliarcseconds in diameter – An astronaut on the Moon is 2 milliarcseconds tall! 1A Angular vs. Linear (True) Size • How big an object LOOKS from the Earth depends upon its TRUE size (Linear Size) and distance from Earth • Angle = Linear Size / Distance 1A 1A