A User s Guide to the Sky

Transcription

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2 A User s Guide to the Sky

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7 Constellations Betelgeuse Rigel Stars are named by a Greek letter ( ) according to their relative brightness within a given constellation plus the possessive form of the name of the constellation: Betelgeuse = Orionis, Rigel = Orionis

8 What is the Zodiac?

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11 The Celestial Sphere Zenith = point on the celestial sphere directly overhead Nadir = point on the c. s. directly underneath (not visible!) Celestial equator = projection of the Earth s equator onto the c. s. North celestial pole = projection of the Earth s north pole onto the c.s.

12 The Celestial Sphere On the sky, we measure distances between objects as angles: The full circle has 360 o (degrees), 1 o has 60 (arc minutes), 1 has 60 (arc seconds).

13 Degrees & Arcseconds Ɵ There are 360 in a full circle. Arcminutes: 1 = 60 Arcseconds: 1 = 60 Extended fist = 10 Ɵ = 31 arcminutes

14 Brainstorm! 1) How many arcseconds? a. 1 b. 1/2 c. 2 d. 2 2) Through how many degrees does the Earth rotate in 1 hour?

15 The Celestial Sphere (II) 90 o - l l From geographic latitude l (northern hemisphere), you see the celestial north pole l degrees above the horizon; From geographic latitude l (southern hemisphere), you see the celestial south pole l degrees above the horizon. The celestial equator culminates 90 o l above the horizon.

16 Example: New York City: l Celestial North Pole 40.7 Horizon North 49.3 Celestial Equator Horizon South The Celestial South Pole is not visible from the northern hemisphere.

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19 Apparent Motion of the Celestial Sphere Looking north, you see stars circling counterclockwise around the celestial north pole.

20 Precession (I) Gravity is pulling on a slanted top => wobbling around the vertical. The Sun s gravity is doing the same to the Earth. The resulting wobbling of the Earth s axis of rotation around the vertical with respect to the Ecliptic takes about 26,000 years and is called precession.

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22 Precession (II) As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. It will be closest to Polaris ~A.D ~12,000 years from now, it will be close to Vega in the constellation Lyra. There is nothing peculiar about Polaris at all (it is neither particularly bright nor nearby, etc.).

23 Brainstorm! 1)Why do stars appear to move east-west? 2)Does precession have any effect on the celestial poles or celestial equator? 3) How many rotations does Earth complete in 1 revolution? 4) What are the Sun s motions?

24 The Magnitude Scale First introduced by Hipparchus ( B.C.): Brightest stars: ~1st magnitude Faintest stars (unaided eye): 6th magnitude More quantitative: 1st magnitude stars appear 100 times brighter than 6th magnitude stars 1 magnitude difference gives a factor of in apparent brightness (larger magnitude => fainter object!)

25 Example: Magnitude Flux Ratio Difference = (2.512) 2 = (2.512) 5 = 100 For a magnitude difference of = 0.27, we find an flux ratio of (2.512) 0.27 = 1.28 Betelgeuse Magnitude = 0.41 mag Rigel Magnitude = 0.14 mag

26 The Magnitude Scale The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Sirius (brightest star in the sky): m v = 1.42 Full Moon: m v = 12.5 Sun: m v = 26.5

27 Brainstorm! 1) Which would appear fainter to us when viewed from Earth with our eyes? a. 4 magnitude star b. +4 magnitude star c. 0 magnitude star 2) If two stars have the same energy output, what would make one star appear fainter than the other, when viewed from Earth?

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29 Cycles of the Sun and Moon

30 The Annual Motion of the Sun Due to Earth s revolution around the Sun, the Sun appears to move through the zodiacal constellations. The Sun s apparent path on the sky is called the ecliptic. Equivalent: The ecliptic is the projection of Earth s orbit onto the celestial sphere.

31 The Seasons (I)

32 The Seasons (I) The Earth s equator is inclined against the ecliptic by The different incidence angle of the Sun s rays causes the seasons on Earth.

33 The Seasons (II)

34 The Seasons (III) Northern summer = Southern winter Northern winter = Southern summer

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37 The Seasons (IV) Earth in January Earth s orbit (eccentricity greatly exaggerated) Sun Earth in July The Earth s distance from the Sun has only a very minor influence on seasonal temperature variations.

38 Astronomical Influences on Earth s Climate (I) Factors affecting Earth s climate: Eccentricity of Earth s orbit around the Sun (varies over period of ~100,000 years) Precession (period of ~26,000 years) Inclination of Earth s axis versus orbital plane Milankovitch Hypothesis: Changes in all three of these aspects are responsible for long-term global climate changes (ice ages).

39 Astronomical Influences on Earth s Climate (II) Last glaciation End of last glaciation Polar regions receiving less than average energy from the Sun Polar regions receiving more than average energy from the Sun

40 Brainstorm! 1) Refer to your celestial sphere map. Locate the position of the Sun for each season. 2) Will the seasons on Earth ever change? Explain. 3) At what latitude is the Sun directly overhead, at noon, on the first day of our summer? What is this latitude called? 4) At what latitude is the Sun directly overhead, at noon, on the first day of our winter? What is this latitude called?

41 Motions of the Planets Mercury Venus Earth All planets in almost circular (elliptical) orbits around the Sun, in approx. the same plane (ecliptic) Sense of revolution: counter-clockwise The Moon is orbiting Earth in almost the same plane (ecliptic) (distances and times reproduced to scale)

42 Apparent Motion of the Inner Planets Mercury appears, at most, ~28 from the Sun. It can occasionally be seen shortly after sunset in the west or before sunrise in the east. Venus appears, at most, ~46 from the Sun. It can occasionally be seen for a few hours (at most) after sunset in the west or before sunrise in the east.

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44 The Phases of the Moon (I) As the Moon orbits around Earth, we see different portions of the Moon s surface lit by the Sun, causing the phases of the Moon.

45 The Tidally-Locked Orbit of the Moon The Moon is rotating with the same period around its axis as it is orbiting Earth (tidally locked). We always see the same side of the Moon facing Earth.

46 The Phases of the Moon (II) New Moon First Quarter Full Moon Evening Sky

47 The Phases of the Moon (III) Full Moon Third Quarter New Moon Morning Sky

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50 Synodic vs Sidereal Synodic motion relates to an Earth rotation/revolution which results in the same view of the Sun or Moon Sidereal motion is the process of returning to the same position with respect to the background stars ml

51 The Month is based on the motion of the Moon. One synodic month = days (complete cycle moon phases) One sidereal month = 27.3 days (one moon orbit)

52 A Day The day is based on the rotation of Earth. A sidereal day = 23 h, 56m, 4.09 s A solar day = 24 hours 1 2 Earth Observer repoints to distant star (sidereal) Earth rotates Earth Observer repoints to sun (synodic)

53 Brainstorm! 1) In general, what does sidereal time measure? 2) Why do we have leap year? 3) What is the change in position of the moon in the sky, in 24 hours? (Number of degrees)

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55 Lunar Eclipses Earth s shadow consists of a zone of full shadow, the umbra, and a zone of partial shadow, the penumbra. If the Moon passes through Earth s full shadow (umbra), we see a lunar eclipse. If the entire surface of the Moon enters the umbra, the lunar eclipse is total.

56 A Total Lunar Eclipse (I)

57 A Total Lunar Eclipse (II) A total lunar eclipse can last up to 1 hour and 40 minutes. During a total eclipse, the Moon has a faint, red glow, reflecting sunlight scattered in the Earth s atmosphere.

58 Typically, there are 1 or 2 lunar eclipses per year.

59 Brainstorm! 1) What is the phase of the Moon during a total lunar eclipse? 2) Why is the shadow dark and then red, during a total lunar eclipse? 3) How often can an eclipse occur? 4) Which are more commonly seen, solar or lunar eclipses? Why?

60 Solar Eclipses (I) The angular diameter of the Moon (~0.5 o ) is almost exactly the same as that of the Sun. This is a pure coincidence. The Moon s linear diameter is much smaller than that of the Sun.

61 Solar Eclipses (II) Due to the equal angular diameters, the Moon can cover the Sun completely when it passes in front of the Sun, causing a total solar eclipse.

62 Total Solar Eclipse The Moon s shadow sweeps across the Earth, over points from where we can see a solar eclipse.

63 Total Solar Eclipse During a total solar eclipse, the solar chromosphere, corona, and prominences can be seen.

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65 The Diamond Ring Effect

66 Earth s and Moon s orbits are slightly elliptical: Perihelion = position closest to the sun Earth Apogee = position furthest away from Earth Sun Perigee = position closest to Earth Moon (Eccentricities greatly exaggerated!) Aphelion = position furthest away from the Sun

67 Annular Solar Eclipses Perigee Apogee Perihelion Aphelion The angular sizes of the Moon and the Sun vary, depending on their distance from Earth. When the Earth is near perihelion, and the Moon is near apogee, we see an annular solar eclipse.

68 An almost total, annular eclipse of May 30, 1984.

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70 Approximately 1 total solar eclipse per year

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72 Very Important Warning: Never observe the Sun directly with your bare eyes, not even during a partial solar eclipse! Use specially designed solar viewing shades, solar filters, or a projection technique.

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74 Brainstorm! 1) What is the phase of the Moon during a total solar eclipse? 2) The Moon is much smaller than the Sun. Why then is it possible for a solar eclipse to occur?