Ancient astronomy & celestial sphere

1. Ancient astronomy & celestial sphere ASTR 1010 Lecture 3

2. In this lecture, you will learn 2-1 The importance of astronomy in ancient civilizations around the world 2-2 That regions of the sky are divided around groups of stars called constellations 2-3 How the sky changes from night to night 2-4 How astronomers locate objects in the sky

3. Naked-eye astronomy Although physical properties of celestial objects cannot be obtained from naked-eye astronomy, questions like • why are there seasons? • why the night sky changes over time? • why the night sky looks different at different places on Earth? can be addressed and studied with our bare eyes.

4. Ancient positional astronomy Positional astronomy : the study of the positions of objects in the sky. • Stonehenge – ancient British • Sun Dagger – ancestral Puebloans • Machu Picchu • Egyptian pyramids (3000BC) • tomb of Shih Huang Ti (~250BC) Mayan Astronomy : Could measure the period of Venus (584 days versus 583.92 days)! • Venus being associated with wars, and need to predict the best dates for attack. Lead into the divisions of the night sky into constellations. Sun Dagger at Chaco Canyon : a band of light strikes the center of the spiral on the first day of summer (about 1000AD)

5. Constellations • On a clear night, with 20/20 vision, one can see about 3,000 stars at a given time. • Ancient astronomers traced out “pictures” using groups of these stars. “Group of Stars” in Latin constellations

6. Constellations • International Astronomy Union (IAU) divided the entire night sky into 88 constellations. • Helpers to find a way around the sky. • Connection to the ancient astronomy, and good tool to naming stars (e.g., alpha Orioni the brightest star in Orion) • stars in a constellation only appear to be close in the projected sky. In reality, most of these stars are not related each other (not even close in 3D).

7. Diurnal Motion = daily motion of celestial objects • At any moment, half of Earth is illuminated by the Sun  day-side and night-side. • Seen from North Pole, Earth rotates from West to East making one complete rotation in 24 hours.  for a person on Earth, the sky appears to rotate from east to west • “west to east” rotation is not a good expression. Seen from the North Pole, Earth rotates counter-clockwise!

8. Diurnal Motion (animation) • Stars do not move! We are the one that move! • This image… • day in Asia • America & Europe = night • For a person in California, Cygnus is directly overhead

9. Diurnal Motion (animation) • 4 hours later… • since Earth rotated 60 degrees, now a different constellation (Andromeda) is directly overhead now.

10. Yearly Motion • As Earth orbits around the Sun, the nighttime side of the Earth gradually turns toward different parts of the sky. Hence, the particular stars that you see in the night sky are different at different times of the year. • Winter constellation = Orion • Spring constellation = ? • Summer constellation = • Autumn constellation = ?

11. North Star and Big Dipper

12. Winter Triangle

13. Summer Triangle Try… Stellarium stellarium.org Free planetarium software

14. Celestial Sphere Ancient people believed that all stars are at the same distance from Earth and they are “attached” to the hollow sphere called “celestial sphere” and this celestial sphere rotates once a day. In truth, the diurnal motion is due to the Earth rotation. Visible stars to naked-eye range from 4.2 Ly to ~1,000 Ly. Celestial sphere is an imaginary sphere in the sky which has an infinite radius. celestial poles celestial equator

15. Celestial Sphere Ancient people believed that all stars are at the same distance from Earth and they are “attached” to the hollow sphere called “celestial sphere” and this celestial sphere rotates once a day. In truth, the diurnal motion is due to the Earth rotation. Visible stars to naked-eye range from 4.2 Ly to ~1,000 Ly. Celestial sphere is an imaginary sphere in the sky which has an infinite radius. celestial poles celestial equator zenith horizon

16. Motions of the Celestial Sphere For an observer at the northern hemisphere… stars sufficiently close to the the celestial north pole never set (i.e., always stay above the horizon). These stars are known as circumpolar stars. Some stars sufficiently close to the celestial south pole will never rise above the horizon. Apparent motion of stars can tell use about the latitudes of the observer (and the direction of the observation).

17. Apparent motion of Stars Mid-latitude, looking North

18. Star trails… mid-latitude looking south Star trails over the Gemini Northern Telescope dome. The glow from the eruption of the Kilauea volcano is seen on the left. The trail of a meteor is visible in this photograph.

19. Star trails…

20. Star tails at middle northern latitudes Looking West Looking East

21. In summary… Important Concepts Important Terms Celestial sphere Celestial North Pole Celestial South Pole Celestial Equator Horizon Zenith • Ancient positional astronomy • Constellation • Celestial Sphere • Diurnal Motion • Seasonal stars • Star trails • Chapter/sections covered in this lecture : sections 1.1, 1.2, 1.4, 2.1, 2.3