10/17/2012. Observing the Sky. Lecture 8. Chapter 2 Opener

Transcription

1 Observing the Sky Lecture 8 Chapter 2 Opener 1

2 Figure 2.1 Figure 2.2 2

3 Figure 2.6 Figure 2.4 Annotated 3

4 The Celestial Sphere The celestial sphere is the vast hollow sphere on which the stars appear fixed. The celestial equator is defined by extending the earth s equator outward. The N & S poles of the celestial sphere correspond to the earth s poles. Lec. 2: The Night Sky I 7 The ecliptic is the apparent path of the sun through the sky. It is also the plane of the earth s orbit about the sun on the celestial sphere. Note: The ecliptic is tilted w.r.t. the earth s equatorial plane by 23.5 o. The Ecliptic Lec. 2: The Night Sky I 8 4

5 The Zodiac The zodiac is a band of celestial sphere which represents the path of the planets, the moon and the sun. Extends ~8 o to either side of the ecliptic. In astrology the zodiac is divided into 12 equal parts called signs, each bearing the name of a constellation. Lec. 2: The Night Sky I 9 5

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7 Astrology is NOT a science! Propagates the claim that a person s life is determined by the position of the sun, moon, and planets at birth. This notion is patently false, and potentially harmful. Astrology is neither a science nor a religion. Lec. 2: The Night Sky I 13 Figure 2.7B 7

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9 Figure 2.7A 9

10 Zenith The zenith is the point on the celestial sphere that is directly above the observer. Lec. 2: The Night Sky I 19 Meridian The meridian is the great circle passing through the two poles of the celestial sphere and the observer s zenith. Lec. 2: The Night Sky I 20 10

11 Equatorial Coordinates Astronomers use equatorial coordinates to locate objects on the celestial sphere. Right Ascension Notation: RA or Equivalent to longitude Declination Notation: Dec or Equivalent to latitude Lec. 3: The Night Sky II 21 Defining RA and Dec. RA is measured in hours The range is from 0 to 24 hours increasing on sky towards the east. The zero point is towards the constellation Pisces (Vernal Equinox). Dec is measured in degrees. The zero is on the equator North Pole = 90 o South Pole = -90 o Lec. 3: The Night Sky II 22 11

12 Equatorial Coordinates (Cont d) The equatorial (celestial) coordinate system is fixed on the sky. The coordinates (, ) of the stars and constellations do not change (ignoring precession). Lec. 3: The Night Sky II 23 Sun here on first day of fall Equatorial Plane 18 Autumnal Equinox o North Equinoxes 1 st day of summer 6 hr Ecliptic Plane 1 st day of winter Equinoxes at the intersection of the equatorial and ecliptic planes. 0 Vernal Equinox Sun here on first day of spring Lec. 3: The Night Sky II 24 12

13 Finding objects Circumpolar objects can be visible any time of the year For example, Polaris, the pole star. From Ithaca southerly objects are best observed during transit. Really, all objects are best observed when they transit (you look through the least atmosphere) Lec. 3: The Night Sky II 25 Using RA to find an object Given two of RA, meridian transit time, and date Find remaining one Method Always work with RA (RA mid ) on the meridian at midnight Find closest reference date (via RA or date) Sep 21 (0), Dec 21 (6), Mar 21 (12), Jun 21 (18 hr) Use fact sky changes by ~ 1 hour of RA per 2 weeks. Lec. 3: The Night Sky II 26 13

14 Figure 2.20 Unannotated Figure 2.20B Annotated 14

15 Figure 2.5 Unannotated 15

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21 The Changing Sky At the same time each night, a different RA will be on the meridian at different times of the year. For instance at midnight, the RA s on the meridian are: Sept hr, March hr Dec hr, June hr Lec. 3: The Night Sky II 42 21

22 Fluxes and Magnitudes Flux is the power per unit area received from an object e.g. f sun = 1 kw/m 2 If two stars, A and B, have fluxes, f A and f B, their magnitudes are related by m A m B 2.5log( f A / fb) Magnitude example Suppose f B / f A = 10, then using m A m B 2.5log( f A / fb) we have m A - m B = 2.5*log(10) so that m A - m B =

23 Converting between f and m We can also write our relation between magnitudes and fluxes as f f B A 10 m A m B 2.5 So if m A = 5 and m B = 0, f B /f A = 100. Flux example If m A = 5 and m B = 0, then using f f B A 10 m A m B 2.5 we have f B /f A = 10 (5-0)/2.5 so that f B /f A = 10 2 =

24 Measuring distance from fluxes If we know the luminosity of an object (such as a star) and measure its flux we can determine its distance! f L r L 2 4 r 4 f Standard Candles Objects with known luminosity are called standard candles in astronomy. They are of fundamental importance. Astronomers use standard candles to measuring distances. There are very few standard candles and it is a problem to calibrate them (determine L). 24

25 Viewed looking down from the north. 6 hr Winter 12 hr Sun Sun 0 hr Fall Midnight Sep. 21 Midnight Dec hr Lec. 3: The Night Sky II 49 Simulation 1: Yearly variation Earth moving around the sun. Why we see different parts of the sky at different times of the year. Here the observer is out at midnight each night Lec. 3: The Night Sky II 50 25

26 Simulation 2: Daily variation Rotation of earth as it moves around the sun. How we see different portions of the sky at night. Lec. 3: The Night Sky II 51 Viewed looking down from the north. 6 hr midnight 12 hr on meridian Spring 12 hr Sun 0 hr 4:00 AM 16 hr on meridian 18 hr Lec. 3: The Night Sky II 52 26

27 midnight 10/17/2012 The Changing Sky (continued) Each night a given object will pass over the meridian 4 minutes earlier. This corresponds to 2 hours earlier each month, or 24 hours in one year. Objects rise and set earlier each day. At a given time, the RA crossing the meridian increases by 4 min. per day. Lec. 3: The Night Sky II 53 1 day along orbit 4 min. Motion of earth along orbit is exaggerated. On what date does Orion appear on the meridian at midnight? Orion Nebula RA = 5.5 hr Dec = -5.5 deg RA = 6 hr transits at midnight on Dec 21. Orion transits at midnight on Dec 14. Also Orion transits at 9:00 p.m. on Jan 28. Orion transits at 3:00 a.m. on Oct. 31. Lec. 3: The Night Sky II 54 27

28 Example 1: What RA is on the meridian for a given date and time? What RA is on the meridian at 3:00 am on Feb. 21? Dec hr overhead at midnight Feb months later => add 4 hr => 10 hr overhead at midnight 3:00 am => 3 hr later => 13 hr overhead Lec. 3: The Night Sky II 55 28