1	Chapter 1 Our place In The Universe
2	Student information Sheets
3	How To Access on-line Materials
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5	A Modern View of the Universe The sun, moon, planets and stars appear to circle around the earth. We cannot feel the earth move, although it does. It seems natural to place the earth at the center of everything (Geocentric model). Today, we know that the earth is one of nine planets which orbits the Sun.
6	Our Cosmic Address
7	Our Cosmic Origins How did we come to be?
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10	"Within galaxies," Within galaxies, gravity causes the collapse of clouds of gas and dust, forming stars and planets. When stars die, they release much of their content back into interstellar space. Galaxies function as cosmic recycling plants. All elements heavier than H, He, and Li were manufactured by stars through nuclear fusion.
11	Images of Time We study the universe by studying light from distant stars and galaxies. The speed of light is 300,000 km/s. This is fast enough to circle the earth 8 times in 1 second. Light from stars can take many years to reach us. We measure distances to the stars in Light-Years. (ly) 1 ly is the distance light can travel in one year.
12	"1 ly = 9.46 x..." 1 ly = 9.46 x 10¹²km (9.46 trillion km) Because it takes time for light to travel through space, the farther away we look in distance, the further back we look in time. (Look back time) The speed of light limits the portion of the universe that we can see.
13	"Any picture of a distant..." Any picture of a distant galaxy is a picture of both space and time. The Great Spiral Galaxy in Andromeda (aka M31) lies about 2.5 million light-years from the earth. A picture of the Galaxy taken now is a picture of how M31 looked about 2.5 million years ago. The diameter of M31 is ~ 100,000 ly. light from the far side of the galaxy took 100,000 years longer to reach us than the light from the near side.
14	"The Great Spiral Galaxy in..." The Great Spiral Galaxy in Andromeda (aka M31) lies about 2.5 million light-years from the earth. A picture of the Galaxy taken now is a picture of how M31 looked about 2.5 million years ago. The diameter of M31 is ~ 100,000 ly. light from the far side of the galaxy took 100,000 years longer to reach us than the light from the near side.
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17	The Sizes of the Sun and the planets (1:10¹⁰)
18	Onward To The Stars The nearest star system to our own is called Alpha Centauri. It is approximately 4.4 ly from us.
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20	The Milky Way Galaxy and Beyond How many stars are there in the visible universe?
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25	Seasons The combination of Earth’s axial tilt, axial rotation and its revolution around the Sun explains why we have seasons.
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27	Spring (Vernal) Equinox On about March 21 each year. Both hemispheres receive equal amounts of sunlight. Beginning of Spring in the Northern Hemisphere. Beginning of Fall in the Southern Hemisphere.
28	Summer Solstice On around June 21, the northern hemisphere receives its most direct sunlight. The northern hemisphere has the longest period of daylight of any day of the year. This is usually considered the first day of summer in the northern hemisphere. The Southern Hemisphere receives its least direct sunlight. and has its shortest period of daylight of any day of the year. This is the first day of winter in the Southern Hemisphere.
29	Fall (Autumnal) Equinox Occurs around September 21. Both hemispheres receive the same amount of sunlight. but now the seasons are the reverse of the spring (Vernal) equinox. In the Northern Hemisphere we have the beginning of Fall. In the Southern Hemisphere we have the beginning of Spring.
30	Winter Solstice Around December 21. We have the reverse situation to that of the summer solstice. It is usually considered the first day of winter for the northern hemisphere and the first day of summer for the southern hemisphere.
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32	Precession The axis of the Earth will remain pointed toward Polaris throughout our lifetimes. However, this has not always been the case and the direction will change again in the distant future. The reason is a that the rotation axis of the Earth precesses - like a spinning top.
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34	Traveling in the Milky Way Galaxy The local solar neighborhood is only a tiny portion of the Milky Way Galaxy. The stars in the local solar neighborhood move quite fast relative to our solar system. (~ 70,000 km/h)
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36	The Entire Milky Way Galaxy Rotates Our Sun and solar system are located about 28,000 ly from the galactic center. At this distance, each orbit around the galactic center takes about 230 million years.
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38	Edge-On View of the Milky Way Galaxy. Most visible stars reside within the galaxy’s thin disk. Careful study of galactic rotation shows that most of the mass lies in the galactic halo. Because this mass emits no light that we have detected, we call it dark matter.
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40	The Expanding Universe When we look outside the local group of galaxies (remember, this is the group to which the Milky Way belongs.) we find two astonishing facts. Virtually every galaxy outside the Local Group is moving away from us. The more distant the galaxy, the faster it appears to be moving away from us. à The entire universe is expanding
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42	"End" End of Section