Astronomy 100 The Solar System Tuesday, Wednesday, Thursday Tom Burbine tomburbine@astro.umass

1. Astronomy 100The Solar SystemTuesday, Wednesday, ThursdayTom Burbinetomburbine@astro.umass.edu

2. What is a galaxy?

3. HW #2 • Due tomorrow (June 10th)

4. What is a galaxy? • Is a massive, gravitationally bound system consisting of stars, gas and dust, and dark matter. Galaxies can contain between ten million and a trillion stars • Dark matter is matter that does not emit or reflect enough radiation to be seen, but whose gravitation effects can be felt http://en.wikipedia.org/wiki/Image:NGC_4414_%28NASA-med%29.jpg

5. When we are looking at stars or galaxies • We are looking into the past Light-year is the distance light travels in a year.

6. Milky Way Galaxy • Milky Way is 100,000 light years in diameter • There are ~200 billion stars in the Milky Way (estimates from 100-400 billions stars) http://www.venusproject.com/ecs/images/photos/galaxy.jpg

7. What is the Universe?

8. What is the Universe? • Sum total of all matter and energy – all galaxies and everything between them • Observable universe – portion of the universe that can be seen from Earth, probably only tiny portion of the whole universe ~93 billion Light-years wide

9. What causes seasons? • The tilt of the Earth’s axis relative to the ecliptic

10. Seasons

12. Solstices • Summer Solstice –June 21 – Northern Hemisphere receives its most direct sunlight • Winter Solstice – December 21 – Northern Hemisphere receives its least direct sunlight

13. Equinoxes • Sun shines equally on both hemispheres • Spring Equinox – March 21 – Northern Hemisphere goes from slightly tipped away from the Sun to slightly tipped towards • Fall Equinox – September 21 - Northern Hemisphere goes from slightly tipped toward from the Sun to slightly tipped away

14. Why does the orbital difference not matter?

15. Reasons • There is only a 3% difference in the distance from the Earth to the Sun at its farthest and closest point • The Earth is actually closer to the Sun in the winter than in the summer

16. Mars is now visible in the sky • http://www.post-gazette.com/pg/10025/1030378-369.stm

17. Angular size • We measure distances in the sky using angles • 180o in the observable sky

18. More precise distances • 1 degree = 60 arcminutes (symbol ´) • 1 arcminute = 60 arcseconds (symbol ´´) • So something that is 2 degrees, 10 arcminutes, 22 arcseconds would be written as • 2o 10´ 22´´

19. Terminology for looking at the sky

20. Celestial Sphere • an imaginary sphere of infinite extent on which all celestial objects appear to lie • http://www.skyandtelescope.com/s?action=login

21. Celestial Sphere

23. What is a constellation?

24. Constellations • People refer to constellations as a pattern of stars • Astronomers refer to constellations as specific regions of the sky • In 1928, the IAU (International Astronomical Union) decided there were 88 constellations • Many of the constellation names go back thousands of years

25. Constellations • The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!). • The real purpose for the constellations is to help us tell which stars are which, nothing more.

26. What is this constellation?

27. Orion Bigger the star, the brighter it is

28. Orion was the son of the god of the sea, Poseidon and a great hunter. One story is that he made an enemy of Hera who sent a scorpion to sting him. Orion was restored to health by Ophiuchus, the first doctor of medicine. Another story is that Artemis was tricked by by Apollo to shoot an arrow at Orion. When he died, Poseidon asked Zeus to put him among the stars.

33. Ursa Major • Ursa Major, the Great Bear, was identified with a bear by native American Indians of the Northeastern United States and the ancient Greeks. • The name common in Britain, the Plough,seems to have a medieval origin, • Another common name among northern European cultures is the Wain, a shortened form of wagon

34. What are the constellations named after • 14 men and women • 9 birds • 2 insects • 19 land animals • 10 water creatures • 2 centaurs • one head of hair • a serpent • a dragon • a flying horse • a river • 29 inanimate objects

35. Originally considered part of Leo’s tail

37. Named after Queen Berenice II of Egypt, wife of Ptolemy III Euergetes (246 BC - 221 BC) • Around 243 BC, the king undertook a dangerous expedition against the Syrians, who had murdered his sister. • Berenice swore to the goddess Aphrodite to sacrifice her famous long hair if her husband returned safely. • He did, she had her hair cut, and placed it in the goddess' temple. • By the next morning, the hair had disappeared. • To appease the furious king and queen (and save the lives of the temple priests), the court astronomer, Conon, announced that the offering had so pleased the goddess that she had placed it in the sky. • He indicated a cluster of stars that at the time were identified as Leo's tail, but now have been called Berenice's Hair.

38. Zodiac • The zodiac is an imaginary belt in the heavens extending approximately 8 degrees on either side of the Sun's apparent path (the ecliptic), that includes the apparent paths of the Moon and the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune.

42. Question: • Why do all the planets seem to follow the same path?

43. Answer: • The planets, the Earth, and the Sun all tend to fall in the same plane called the ecliptic

46. Why don’t all the constellations have ancient names?

47. Ancient cultures such as the Greeks and Egyptians could not see the constellations in the Southern Hemisphere

48. Question: • Why is the path of the constellations on the zodiac not on the celestial equator?

49. Answer: • The rotation axis of the Earth is inclined with respect to the ecliptic