The Celestial Movie Imagine that a special camera pointed at the sky was set up where you live. For a whole year the camera recorded the movements of objects in the sky on one continuous video. If you play the video back on your T.V. in very fast motion you would see… Clouds - various shapes and shades, moving across the sky in the same direction as the daily wind. Clouds would form and break up rapidly. And you may notice that clouds are not very far away. (clouds may be more earthly than celestial).

The moon is much further away and it’s motion is much more regular. It travels in a westward path across the sky. It appears to be changing shape – waxing from a thin crescent, through a half circle, to a full moon, then waning to a sliver again. A moon is a type of satellite, a celestial object that travels around a planet in a closed path, called an orbit. The moon is non- luminous and we can only see it when sunlight reflects off its surface.

The sun is much more dependable. The sun has no phases and it seems to rise at nearly the same time and place as the previous day. If you were to keep careful track though, you would see that it rises a bit later in the winter months, but earlier in the summer months. The sun is a star – an enormous ball of hot, glowing gases. Compared to other stars, the sun is about average in size, but does have a mass x that of earth! The sun appears so much brighter than other stars in the sky because if its proximity to the earth.

Most stars, constellations and asterisms rise in the east and set in the west, but not all. Some remain visible all night long, everyday of the year, never rising or setting. Careful observation would also show that the stars rise four minutes earlier each night, getting out of sink with the sun. But by getting out of step with the sun, the stars in the night sky reveal seasonal patterns.

While the stars are generally unchanging, you may notice five other objects that appear to be wandering through the constellations. The Greek word for wonderer is planets, and the five wondering planets are Mercury, Venus, Mars, Jupiter, and Saturn. A planet is a large celestial object that travels around a star. There are eight planets travelling around our sun. Each planet differs for the others in size, composition, atmosphere.

Mercury, Venus, Mars, Jupiter, and Saturn slowly change position from night to night, two of them staying near the sun (Venus and Mars). When they are visible, they are best seen in the early evening or early morning. The four planets closest to the sun are called the terrestrial planets, with hard, rocky surfaces. The next four planets are composed mainly of gases and liquids and are known as the gas giants

Planets are non- luminous, which means they do not produce their own light. They reflect light from luminous objects such as the sun and other stars.

Position of Mars during a period of retrograde motion. Each point represents the planet’s new position every 10 days over the retrograde period. Mars, Jupiter, and Saturn seem to wonder eastward through the night. About once a year they loop back briefly in retrograde motion before continuing eastward. Looking up at the night sky, you can see as many as 2000 stars. Some of the brighter pinpoints of light are planets

Within the universe are huge collections of stars, gas, dust, and planets. These are called galaxies and there are billions of galaxies scattered throughout the universe! The earth is part of the Milky Way galaxy, with more than 200 billion stars (including the sun).

Modeling Celestial Motion Early people’s search for an explanation for the motion of the stars is part of the ongoing human desire to understand the world we live in. When we pay attention to what is going on around us, we begin to see patterns. The next step is to understand the patterns. So we come up with a basic idea, or theory, that leads us to develop a model. Models are useful tools to aid inquiry and test ideas.