1 General Astronomy Physics 103 Dr. Tyler E. Nordgren Dr. Julie A. Rathbun

1 Topics: The Night Sky …

1 … from the Solar System…

1 … to the Stars…

1 … to the Milky Way …

1 … and to the Universe Beyond.

1 Our Goal Understand: –The sky above “What’s that bright yellow star over head?” How can I tell what time it is by where the Sun is? –News reports “Scientists discover giant black hole. Should I be afraid?” –Popular entertainment “Armageddon”, “Enterprise”, etc Learn how science works and how we know what we know.

1 Organization Lecture two days a week (Tues. and Thurs.): –1 hour and 20 minutes Lab one day a week (Mon. or Wed.): –2 hours and 50 minutes in Hornby 2 Evening observing (4 times): 7:30 pm – 9:30 pm Check the syllabus and web site for exact dates!

1 Grading Homework: 20% Quizzes: 10% Labs: 20% Exams: 30% (15% each) –3 exams, lowest grade dropped Final: 20%

1 Naked-Eye Astronomy The second oldest profession

1 Goals How does astronomy affect your daily life? The Sun: Time without clocks The Seasons: Dates without calendars The ancients did it, so can you!

1 The Sun The Sun is a star. We live by its warmth. We die by its extreme heat or frigid absence. It is the source of nearly all our energy. –How? It is the source of all our food. –How?

1 The Day The Earth is a sphere that turns on its axis. –How long does it take to turn once on its axis? Light from the sun lights only one half of Earth. As the Earth turns the sun “rises” and “sets.” So does the moon. So do the stars.

1 The Earth orbits the Sun. –How long does it take to go once around the Sun? The orbit is nearly circular, but is actually an ellipse. The Year An exaggeration.

1 Group Question Why are there seasons? Hint: It has nothing to do with the distance from the Sun.

1 The Seasons The Earth is tilted on its axis. During half the year, the northern hemisphere is tilted more towards the Sun. –Summer in California. –Winter in Australia During the other half of the year the southern hemisphere is tilted towards the Sun. –Winter in California –Summer in Australia

1 Summer During summer: –Hemisphere is tilted towards Sun. –Sun is high overhead. –Sunlight is more direct. –Daytime is longer. Why? N S

1 Winter During winter: –Hemisphere is tilted away from Sun. –Sun is low on horizon. –Sunlight is less direct. –Daytime is shorter. Why? S N

1 Time Q: So, how can we use the Sun to tell time? A: Sundials

1 Time Sun rises in the East. –If the Sun is towards the East, it is before noon. Sun sets in the West. –If the Sun is towards the West, it is after noon. At noon? –For most of northern hemisphere: Sun is due south. –For most of northern hemisphere: Sun is due north. –Near equator, Sun is overhead.

1 Date During northern summer, Sun high over head. –When Sun casts the smallest shadow at noon  Summer solstice. During northern winter, Sun low to the south. –When Sun casts the longest shadow at noon  Winter solstice. During northern spring and fall, Sun in between. Sun also rises due east, sets due west. –When Sun casts shadow due west at sunrise  spring or fall equinox.

1 In Summer, the sun is high. In winter, the sun is low. The height of the sun (at a given time) can tell you the date.

1 The Constellations We may also use the stars to tell time. We see stars projected on the sphere of the sky. They make patterns: constellations. –No physical association We see those constellations opposite the Sun from Earth.

1 Seasonal Constellations As the Earth orbits the Sun, different stars are up at night. –There are summer and winter constellations! “The Dog Days of Summer”

1 Ancient Astronomy Stonehenge (~3000 BC):

1 Ancient Astronomy Chaco Canyon (“Anasazi” ~1200 AD):

1 Fajada Butte