1. Celestial Motions of Planets & Stars Dr. Charles Ophardt EDU 370

2. Celestial Sphere • Coordinate System For Universe

3. Perspective From Earth

4. Earth Perspective

5. Previous Slide • See the local sky "dome" placed on the Earth at Chicago • Celestial Sphere drawn. • Visible half of the sky is shown as a smaller dome • As the Earth rotates towards the east, parts of the sky just below the eastern horizon will rise in the east. • Those above the western horizon will set.

6. Apparent Motions of Stars • The Apparent Paths of objects are parallel to the Celestial Equator. • Their orientation depends on your latitude: • At Equator: perpendicular to the horizon • At Poles: parallel to the horizon • Mid-Latitudes: Tilted by (90º–Latitude)

7. Star Map

8. Apparent Motions of Stars • A good illustration is the apparent daily motions of the stars: • The stars appear to rise in the east and set in the west, returning to the same place in your local sky a little under 24 hours later (precisely, 23h 56m 04s = 1 sidereal day). • In Chicago: we are at 41.5° N, so the paths are tilted by (90°-41.5°)=48.5° from our local horizon.

9. Apparent Motions of Stars • Apparent Motions of Big Dipper • Movement shown with green arrows

10. Apparent Motions of Stars • Proper Motion depends on the Distance • The amount of proper motion shown by a star depends on its distance. • More distant stars tend to have smaller Proper Motions

11. Apparent Motions of Stars • Stars moving exactly towards or away from us will show no proper motions!

12. Distances of Stars • Why are Distances Important? • Distances are necessary for estimating: • Total energy released by an object (Luminosity) • Masses of objects from orbital motions (Kepler's third law) • Physical sizes of objects • The problem of measuring distances • Question: How do you measure the distance of something that is beyond the reach of your measuring instruments?

13. Distances of Stars • The Method of Trigonometric Parallaxes • Nearby stars appear to move with respect to more distant background stars due to the motion of the Earth around the Sun. • This apparent motion (it is not "true" motion) is called Stellar Parallax.

14. Distances of Stars • The line of sight to the star in Dec. is different than in June, Earth is on other side of orbit. • As seen from the Earth, the nearby star appears to sweep through the angle shown. Half of this angle, is the parallax, p.

15. Distances of Stars • As the distance to a star increases, its parallax angle decreases. Upper figure, the star is about 2.5 times nearer than the star in the lower figure, parallax angle is 2.5 times larger.

16. Retrograde Motion • In general, planets move eastward relative to the "fixed" stars. • Called "Direct Motion". • Motion is non-uniform (not at the same speed). • Sometimes planets appear to • Slow down & stop! • Start moving westward, or RETROGRADE, • Slow down & stop again, • Resume moving eastward again.

17. Retrograde Motion of Planets • Plot of Mars motion against star back-ground

18. Retrograde Motion for Mars • Retrograde motion results from the change in perspective viewed from earth

19. Four Seasons • Earth's rotation axis is tilted relative to the plane of its orbit around the Sun: Tilt is about 23.5º

20. Four Seasons • Winter Solstice - Dec. The Earth's axis tilts away from the Sun- North Pole.

21. References • http://www-astronomy.mps.ohio-state.edu/~pogge/Ast161/ • http://www.pparc.ac.uk/Ed/Notes/Bbwebpage.asp • http://www.fourmilab.ch/yoursky/