The Universe Around Us

• Geocentric = Earth-centered• Egocentric = Self-centered• Read Huygens quote (c. 1690) on page 3• Is there a center to the Universe?

This should be the whole course!• We have centers of motion: Solar System,

Milky Way Galaxy, clusters, etc..• Centers of motion caused by Gravity

Cosmic History

• Big Bang Cosmology is a well accepted theory (12-16 billion years ago)

• Gravity drives the formation of our Milky Way galaxy (11-14 billion years ago)

• Gravity drives the formation of stars, and nuclear fusion in stars produces elements such as carbon, nitrogen, and oxygen

• Our solar system formed 4.6 billion years ago.• Our Earth and the other planets in the solar system

formed around our star: The Sun

Light Travel Time

• Light travels at 300,000 km/s• Astronomical distances are measured in

light-years: 0.946 1013 km• Stars in our sky are many light- years away• Center of the Milky Way is 28,000

light-years away • Galaxies are millions to billions of

light-years away.• Observable universe: 12-16 billion

light-years in radius

The Earth Rotates

Relevant Geography

Earth Orbits Sun

Tilt of Spin Axis

The Equinoxes and Solstices

• Tilt of the spin axis is related to the seasons

• Effect of tilt on climate is not instantaneous

Cause of Seasons

a) The path of the Sun on Summer Solstice in the northern hemisphere

b) The path of the Sun on Winter Solstice in the northern hemisphere

Precession of the Earth’s Axis