Apparent motions of the Celestial Objects

1. Apparent motions of the Celestial Objects

2. Background Definitions (do not write) • Celestial sphere: Imaginary sphere encircling Earth on which all object in the night sky appear • Celestial object: Object in the sky outside Earth’s atmosphere; the sun, moon, stars, planets • Terrestrial: Objects that are a part of Earth, such as rocks, oceans, and clouds

3. Motions • The sun, moon, planets, stars or any distant object seems to rise in the east and set in the west- this is an illusion: the Earth is the object that is moving • The apparent motion of objects occurs at a rate of 15 degrees per hour, which is the rate that earth rotates • Daily Motion: the movement of a celestial object over a 24-hour period • Day: Average interval of time during which the sun passes from its highest point on one day to its highest point on the next; ~24 hours

4. Changes in the Altitude of the Sun at Noon • The sun is always highest at local solar noon • Only between the latitudes of 23.5 degrees North and 23.5 degrees South can the noon sun be directly overhead

5. Path of the Planets • Planets do not stay in place- they change their position from night to night • Each planet has its own unique motion • Sometimes planets appear to drift west, other times east • “Planet” means “Wanderer”

6. The Moon • The moon revolves around Earth in about 27 days • The apparent rising and setting of the moon is also due to our rotation • The moon rises about 50 minutes later each evening

7. On handout… • Zenith: The point on the celestial sphere that is directly above the observer.

8. From Different Perspectives • Mid-Latitudes: Stars appear to move from east to west in circular paths • North Pole: Stars appear to move in a circle • Facing South: Stars appear to move in a low arc across the horizon

10. Star's Path Through the Sky • SIDE NOTE! • The Earth rotates, causing the stars to appear to rise and set. Remember that the Sun is a star. It rotates/spins in place but does not revolve/orbit

11. Did you know??? (do not write) • Sundials were used to tell time using the apparent movement of the sun • The passing of hours is indicated by the movement of the shadow • Days are short during the winter because we are pointed away from the sun so the sun is below the horizon most of the day • Sundials • More on sundials

14. Apparent Motions of the Sun • The sun appears to rise in the east, travel in an arc across the sky and set in the west • The greater the length of the sun’s path over an area, the more hours of daylight • In the spring and fall, the sun rises due (exactly) east and sets due west • In the summer, the sun rises north of east and sets north of west • In the winter, the sun rises south of east and sets south of west