Astronomy by eye: Motions in the Sky

1. Astronomy by eye: Motions in the Sky The Celestial Sphere Intrinsic and Reflex Motion Reflex Motion from Earth’s Rotation

2. Recap • Astronomy and science • Science is data driven • Models, theories and the scientific process • Science in society: • Importance of science in society • judging “quality” of scientic discussion • pseudoscience

3. Astrology • What is astrology? • Astrology is something which purports that the position of the planets and the stars at the time of your birth determines the course of your future life • Is wondering about this idea in the first place non-scientific? • Yes • No • Given the hypothesis of astrology, what would be the next step in analyzing it scientifically?

4. Observations/data relevant to astrology • There is no evidence that astrology actually works. • Note that the predictions of astrology may work sometimes; almost certainly, some of these predictions will work sometimes by chance! Certainly, astrology is not a fully deterministic theory; if it claimed to be so, even a single example of a failed astrological prediction would invalidate the theory. • Other flaw: not all astrologers even predict the same thing! • Even as a statistical theory, astrology fails to be validated by experiment. • A link to some studies on the predictions of astrology: http://www.astrosociety.org/education/resources/pseudobib.html#1

5. Theoretical basis for astrology? • Is it surprising that astrology doesn’t work? Are there successful scientific theories that make predictions about astrology? • Current theoretical understanding, which has been well supported by observation, says there are four basic forces in nature: • Gravity • Electromagnetic force • Strong force • Weak force • When quantitatively calculated, the force arising from planets and stars at the moment of your birth from any of these sources is much smaller than the force contribution from many other objects! So in no way would it be expected that the position of astronomical objects would have an impact

6. Part II Astronomy by eye : motions in the sky

7. Observing the night sky How many objects were you able to identify in the sky: Saturn, Venus, North Star, Vega, Arcturus, others? • 0 • 1 • 2 • 3 • 4 or more

8. Observing the night sky Which best describes your observations? • I was unable to detect any motion of any objects across the sky • I was able to detect the motion of objects, but was unable to judge whether they were all moving at the same rate • I was able to detect the motion of objects, and found they all moved at the same rate • I was able to detect the motion of objects, and found that they moved at different rates • I was unable to measure anything or did not complete the assignment

9. Observing the night sky Which best describes the observations? • The data suggest there is no motion of any objects across the sky • The data suggest that objects move, but we are unable to judge whether they were all moving at the same rate • The data suggest that all obects move at the same rate • The data suggest that objects moved at different rates • We cannot understand these data • Data obtained from measurements taken two hours apart. • Note the “wrap” of the map

10. Testing the model Observing that stars move across the sky during the course of a night • Proves that the Earth rotates on its axis • Proves that the Earth revolves around the Sun • Proves that the Sun revolves around the Earth • Rejects the model that the Sun is the only object that is moving in space • Rejects the model that the Earth rotates on its axis

11. Class Section II:Motions in the Sky: Astronomy by Eye • What astronomical objects do we see with our naked eyes? • Sun • Moon • Planets • Nearby stars • Many of these are relevant for everyday life: motion of the Sun, phases of the Moon, etc. • Want you to come out of astronomy class understanding “everyday” astronomy!

12. Positions in the Sky • How can we describe where astronomical objects are located in the sky? • Since we can’t immediately infer distances of astronomical objects by just looking at them, all we can describe is what direction they are in • Imagine that stars can be described by their location on an imaginary sphere centered on the earth: the celestial sphere • On this sphere, we can describe locations the way we describe locations on the surface of the Earth, which is also a sphere • Astronomical longitude (right ascension) and latitude (declination) • Froma given location on Earth, you can’t see the whole sphere, only the half of it that is above the horizon • Note that local coordinates that we have been using (azimuth / altitude) depend on where you are located on earth, and thus aren’t ideal for a coordinate system that everybody can refer to)

13. Constellations • We notice patterns of stars that are seen in different directions and call these constellations • Different cultures have come up with different constellations • Constellations are nice to know for finding your way around the sky, but are not really associated “astronomically”, because stars in a given constellation may be at very different distances! • Constellations are not usually clusters! (although sometimes they are!) • viewed from some other location in the Galaxy, constellations would be totally different!

14. Motions of astronomical objects • Looking at astronomical objects, one quickly recognizes that they move across the sky • Apparent motion of objects can come from • intrinsic motion of the object: motion that appears because object itself is moving • Reflex motion: motion that appears because Earth is moving • For distant objects (stars), essentially all observed motion comes from reflex motion • Although stars definitely have intrinsic motion, they are so far away that we don’t see much apparent motion over the course of our lifetimes • As we will discuss later, we can measure radial motion of objects, but this doesn’t give apparent sideways motion

15. Reflex motion from Earth’s rotation • Earth spins around an imaginary axis, once per day • As a result, the celestial sphere appears to spin around the Earth • Stars that happen to be located in the direction of the Earth’s rotation axis appear to stand still • There is such a star above the North pole : Polaris, the North Star • There is NOT such a star above the South pole! • Stars near these directions travel in small circles around the sky • Stars far from these directions travel in large circles around the sky

16. What’s the best description of Earth’s rotation and motion of stars? A. stars appear to move E to W because the Earth rotates from E to W B. stars appear to move E to W because the Earth rotates from W to E C. stars appear to move W to E because the Earth rotates from E to W D. stars appear move W to E because the Earth rotations from W to E E. stars don’t appear to move

17. To do • Canvas homework on science • Look up on web: celestial sphere, daily motion of the stars and Sun, yearly motion of the Sun