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Introduction to Astronomy. Physics 103. Dr. Julie A Rathbun Dr. Tyler E Nordgren. Topics: The Night Sky …. … from the Solar System…. … to the Stars…. … to the Milky Way …. … and to the Universe Beyond. Our Goal. Understand: The sky above “What’s that bright red star to the south?”
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Introduction to Astronomy Physics 103 Dr. Julie A Rathbun Dr. Tyler E Nordgren
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.
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!
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.
Grading • Labs: 18% • Observing notebook: 18% • Homework assignments: 18% • 2 In-class exams: 24% • Final: 22%
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?
Stars are at different distances. But we see them projected on to a plane. The Projected Sky
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.
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.
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.
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. The Sun and Zodiac
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).
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.
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.
Lunar Eclipse • Sometimes the Moon passes through the Earth’s shadow as cast by the Sun.
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.
Types of Solar Eclipses • Partial • Total • Annular
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.
Angles • The sky is 360 arc degrees around. • 60 arcminutes = 1 arc degree • The Full Moon is about half an arc degree = 30 arcminutes. • 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!
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
