Notes & Reminders

1. Notes & Reminders Homework #1 is due Thursday, September 6. Tip: No need to carry seven digits in your calculations! Only carry 2-3 digits in most instances (occasionally more, depending on the accuracy of your input quantities)

2. The Local Sky An object’saltitude (above horizon) and azimuth(along horizon) specify its location in your local sky. The meridian connects East to West through the zenith (90 degrees overhead).

3. Why do stars rise and set? Earth rotates from west to east, so stars appear to circle from east to west. Altitude of north celestial pole gives your latitude on Earth.

4. The sky varies as Earth orbits the Sun • As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. • At midnight, the stars on our meridian are opposite the Sun in the sky.

5. How does the orientation of Earth’s axis change with time? • Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years. • Polaris won’t always be the North Star. • Positions of equinoxes shift around orbit; e.g., spring equinox, once in Aries, is now in Pisces astrological birth signs are wrong!

6. Summary of Celestial Motion Observable Time Scale Cause Stars, Sun, planets rise in 1 day Earth rotates the east, set in the west on its axis Sun drifts through the 1 year Earth revolves 12 (13) zodiacal around the Sun constellations North celestial pole ~26,000 years Earth’s orbit changes its position precesses

7. Astronomy Myth #1 Seasons are caused by the changing distance of the Earth from the Sun during the year. Obviously wrong, since we have summer when Australia has winter. 23.5° tilt of Earth’s axis is responsible for the different hemispheres receiving different amounts of daily sunlight throughout the year.

8. What causes the seasons? Seasons depend on how Earth’s axis affects the directness of sunlight. Northern Summer – north pole points toward Sun Northern Winter – north pole points away from Sun

9. How do we mark the progression of the seasons? • We define four special points (Northern hemisphere only!): • summer (June 21) solstice – longest daylight • winter (December 21) solstice – shortest daylight • spring (March 21) equinox - 12 hours daylight • fall (September 21) equinox – 12 hours daylight solstice = “sun stops” equinox = equal day and night

10. We can recognize solstices and equinoxes by Sun’s path across sky: Summer (June) solstice: highest path; rise and set at most extreme north of due east Winter (December) solstice: lowest path; rise and set at most extreme south of due east Equinoxes: Sun rises precisely due east and sets precisely due west.

11. Sun’s altitude also changes with seasons Sun’s position at noon in summer: Higher altitude means more direct sunlight. The figure-eight path is called the analemma. Sun’s position at noon in winter: Lower altitude means less direct sunlight.

12. Why doesn’t distance matter? • variation of Earth–Sun distance is small-about 3%; this small variation is overwhelmed by the effects of axis tilt. • distance does matter for some other worlds, notably Mars (19%) and Pluto (66%).

13. Astronomy Myth #2 Lunar phases are caused by Earth’s shadow. Wrong. This is the most common astronomy misconception, by a wide margin. Instead, lunar phases are a consequence of the Moon’s 27.3 day orbit around Earth.

14. Phases of the Moon: 29.5-day cycle • Waxing • Moon visible in afternoon/evening • Gets “fuller” and rises later each day • Waning • Moon visible in late night/morning • Gets “less full” and sets later each day

15. Phases of the Moon • Half of Moon is illuminated by Sun and half is dark at all times. • The phase of the Moon depends on what fraction of the lit half is visible from Earth. • There is no “dark side of the Moon” – this is a misnomer • There is a “back side of the Moon” which never faces Earth

16. 27.3-day or 29.5-day cycle?

17. 27.3-day or 29.5-day cycle? The Moon orbits the Earth once every 27.3 days – this is the sidereal period (P)of the Moon. The Moon completes one complete phase rotation (i.e., full moon to full moon) every 29.5 days – this is the synodic period (S)of the Moon. In general, if E is the revolution period of Earth (365 days): 1/S = abs(1/E – 1/P) Time units do not matter as long as you are consistent

18. Astronomy Myths #3 and #4 The Moon does not rotate, and the unseen side of the Moon is always dark. Wrong. These are also very common misconceptions. The Moon MUST rotate in order for it to always keep the same face toward us. As half the Moon is always lit by the Sun, the unseen side MUST be seeing as much sunlight as the near side over the course of a month. Synchronous rotation

19. What causes eclipses? • The Earth and Moon cast shadows. • When either passes through the other’s shadow, we have an eclipse.

20. When can lunar eclipses occur? • Lunar eclipsescan occur only at full moon. • Lunar eclipses can be penumbral, partial, or total.

21. Total Lunar Eclipse of 2000 July 16 (Lahaina, Maui)

22. Why lunar eclipses often appear blood red. Earth’s atmosphere refracts sunlight, bending a little bit of (preferably red) light into the Earth’s shadow. If Earth had no atmosphere  Moon would disappear completely in Earth’s shadow!

23. When can solar eclipses occur? • Solar eclipses can occur only at new moon. • Solar eclipses can be partial, total, orannular. • Total eclipses last no more than 7 ½ minutes at any location on Earth.

24. When can solar eclipses occur? • Solar eclipses can occur only at new moon. • Solar eclipses can be partial, total, orannular. • Total eclipses last no more than 7 ½ minutes at any location on Earth.

25. Why don’t we have an eclipse at every new and full moon? • The Moon’s orbit is tilted 5° to ecliptic plane. • So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon. Most months, no eclipses happen.

26. Summary: Two conditions must be met to have an eclipse: • It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). AND • The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes) – in other words, the line-up in geometry of the Sun-Moon -Earth system must be nearly perfect. This explains why eclipses are rare.

27. Predicting Eclipses • Eclipses recur with the 18-year, 11 1/3-day Saros cycle, but type (e.g., partial, total) and location may vary.

28. Start Planning for April 8, 2024 Eclipse!

30. Planets Known in Ancient Times • Mercury • difficult to see; always close to Sun in sky • Venus • very bright when visible; morning or evening “star” • Mars • noticeably red • Jupiter • very bright • Saturn • moderately bright • Planet means “wanderer” in Greek.

31. What was once so mysterious about planetary motion in our sky? • Planets usually move slightly eastwardfrom night to night relative to the stars. • But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion. Uranus Mars at 5-8 day intervals

32. We see apparent retrograde motion when we pass by a planet in its orbit. Easy for us to explain: occurs when we “lap” another planet. But very difficult to explain if you think that Earth is the center of the universe! In fact, ancients considered but rejected the correct explanation.

33. Why did the ancient Greeks reject the real explanation for planetary motion? Their inability to observe stellar parallaxwas a major factor. Highly exaggerated, but you get the point With the naked eye, stars would have to be at a distance of ~0.05 light years or closer for parallax to be detectable (almost 100 times closer than the closest star) Parallax: apparent shifting of position of a foreground object relative to background objects (think: finger in front of clock)

34. The Greeks knew that the lack of observable parallax could mean one of two things: • Stars are so far away that stellar parallax is too small to notice with the naked eye. • Earth does not orbit the Sun; it is the center of the universe. With rare exceptions such as Aristarchus, the Greeks rejected the correct explanation (1.) because they did not think the stars could be that far away. Thus, the stage was set for the long, historical showdown between Earth-centered and Sun-centered systems.

35. Chapter 3The Science (and History) of Astronomy

36. How did astronomical observations benefit ancient societies? • Keeping track of time and seasons • for practical purposes, including agriculture • for religious and ceremonial purposes • aid to navigation However, few cultures used the scientific method to learn about the nature of the Universe (Greeks were an exception), thus they were not truly studying astronomy.

37. Ancient people of central Africa (6500 BC) could predict seasons from the orientation of the crescent Moon.

38. England: Stonehenge (completed around 1550 B.C.)

39. England: Stonehenge (1550 B.C.)

40. Southwest United States: “Sun Dagger” marks summer solstice

41. Scotland: 4,000-year-old stone circle; Moon rises as shown here every 18.6 years.

42. Macchu Pichu, Peru: Structures aligned with solstices.

43. South Pacific: Polynesians were very skilled in art of celestial navigation.

44. "On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star." China: Earliest known records of supernova explosions (1400 B.C.) "On the Xinwei day the new star dwindled." Bone or tortoiseshell inscription from the 14th century B.C.

45. France: Cave paintings from 18,000 B.C. may suggest knowledge of lunar phases (29 dots)

46. Days of week were named for the Sun, Moon, and visible planets.

47. Why does modern science trace its roots to the Greeks? • Greeks were the first people known to make models of nature. • They tried to explain patterns in nature without resorting to myth or the supernatural. Greek geocentric model (c. 400 B.C.)

48. Astronomy Myth #5 Educated people thought the world was flat just 500 years ago. Wrong! The ancient Greeks were well aware that the Earth was round over 2000 year ago. Greeks noticed: (1) ships disappeared over the horizon (2) during a lunar eclipse, the shadow of Earth over the Moon’s surface was curved

49. Eratosthenes Measures Earth (c. 240 B.C.) Measurements: Syene to Alexandria distance ≈ 5000 stadia angle = 7° Calculate circumference of Earth: 7/360  (circum. Earth) = 5000 stadia  circum. Earth = 5000  360/7 stadia ≈ 250,000 stadia Compare to modern value (≈ 40,100 km): Greek stadium ≈ 1/6 km  250,000 stadia ≈ 42,000 km

50. How did the Greeks explain planetary motion? Underpinnings of the Greek geocentric model: • Earth at the center of the universe • Heavens must be “perfect”: Objects moving on perfect spheres or in perfect circles  big wrong assumption! Aristotle Plato