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Chapter 2: The Sky

0. Chapter 2: The Sky. :30. 0 of 102. If you look up into the sky, towards the south, then …. east is to your right, north is behind you, west is to your left. east is to your left, north is behind you, west is to your right. east is to your left, west is behind you, north is to your right.

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Chapter 2: The Sky

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  1. 0 Chapter 2:The Sky

  2. :30 0 of 102 If you look up into the sky, towards the south, then … • east is to your right, north is behind you, west is to your left. • east is to your left, north is behind you, west is to your right. • east is to your left, west is behind you, north is to your right. • east is to your right, west is behind you, and north is to your right. • east is behind you, west is to your left, north is to your right.

  3. 0 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.

  4. 0 The Celestial Sphere (II) • Fromgeographic latitude l(northern hemisphere), you see thecelestial north pole l degrees above thenorthernhorizon; • From geographic latitude –l (southern hemisphere),you see the celestial south pole l degrees above thesouthernhorizon. 90o - l l • Celestial equator culminates 90o – l above the horizon.

  5. 0 Example: New York City: l ≈ 40.70 Celestial North Pole Celestial Equator 49.30 40.70 Horizon Horizon North South The Celestial South Pole is notvisible from the northern hemisphere.

  6. :30 0 of 102 Athens, OH, is located atl≈ +39o.Where in the sky would you see the highest point of the celestial equator? • North, 39o above the horizon. • South, 39o above the horizon. • North, 51o above the horizon. • South, 51o above the horizon. • South, 45o above the horizon.

  7. 0 Athens, OH: l ≈ 390 Celestial North Pole Celestial Equator 510 390 Horizon Horizon North South

  8. 0 The Celestial Sphere (III)

  9. 0 Apparent Motion of the Celestial Sphere

  10. 0 Apparent Motion of the Celestial Sphere II

  11. A View of the Sugarloaf Mountain (Rio de Janeiro, Brazil) :30 0 of 102 Sun’s position in the morning Where will the sun be in the evening before sunset? • Far off to the right • Far off to the left • Near the Zenith • Near the Nadir • Close to where it is now.

  12. 0 The Magnitude Scale First introduced by Hipparchus (160 - 127 B.C.): • Brightest stars: ~1st magnitude (mv = 1) • Faintest stars (unaided eye): 6th magnitude (mv = 6) More quantitative: • 1st mag. stars apear 100 times brighter than 6th mag. stars • 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!)

  13. 0 The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Sirius (brightest star in the sky): mv = -1.42 Full moon: mv = -12.5 Sun: mv = -26.5

  14. 0 The Sun and its Motions (I) Earth’s rotation is causing the day/night cycle.

  15. 0 The Sun and its Motions (II) 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.

  16. :30 0 of 102 What is causing the seasons? • Brightness variations of the sun. • The Earth being closer to the sun in the summer and further away in the winter. • A steeper angle of incidence of the sun’s rays in the summer than in the winter. • A denser cloud cover in the winter than in the summer. • Stronger heat output from within the Earth in the summer than in the winter.

  17. 0 The Seasons (I) The Earth’s axis of rotation isinclinedvs. the normal to its orbital plane by23.50, which is causing theseasons.

  18. 0 The Seasons (II) • The Seasons are only caused by a varyingangle of incidenceof the sun’s rays. • They arenotrelated to theEarth’s distance from the sun. • In fact, the Earth is slightlycloser to the sun in (northern-hemisphere)winterthan in summer. Steep incidence → Summer Light from the sun Shallow incidence → Winter

  19. 0 The Seasons (III) Earth’s orbit (eccentricity greatly exaggerated) Earth in January Earth in July Sun The Earth’s distancefrom the sun has only a very minor influence on seasonal temperature variations.

  20. :30 0 of 102 When it’s summer in the U.S., it is … in Argentina • spring • summer • fall • winter • midnight

  21. 0 The Seasons (IV) Northern summer = southern winter Northern winter = southern summer

  22. :30 0 of 102 What happens to a spinning top that you set on a table top slanted? • It will immediately fall over • It will quickly adjust itself to a perfectly upright position. • It will wobble around in a circular motion around the vertical. • It will wobble around back and forth through the vertical position. • It will take off and become airborne.

  23. 0 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 w.r.t. the Ecliptic takes about 26,000 years and is called precession.

  24. 0 Precession (II) As a result of precession, thecelestial north pole follows a circular patternon the sky,once every 26,000 years. It will beclosest to Polaris ~ A.D. 2100. ~ 12,000 years from now, it will be close toVegain the constellation Lyra. There is nothing peculiar about Polaris at all(neither particularly bright nor nearby etc.)

  25. :30 0 of 102 What is the radius of the circle that the celestial north pole traces out on the celestial sphere in the course of a precession cycle? • 90o • 45o • 39o • 23.5o • 66.5o Radius

  26. 0 The Motion of the Planets (I) The planets are orbiting the sun almost exactly in the plane of the Ecliptic. Jupiter Venus Mars Earth Mercury The Moonis orbitingEarth in almost the same plane (Ecliptic). Saturn

  27. 0 The Motion of the Planets (II) • Allouter planets(Mars, Jupiter, Saturn, Uranus, Neptune and Pluto) generally appear tomove eastwardalong the Ecliptic. • Theinner planetsMercury and Venus can never be seen at large angular distance from the sun and appear only asmorning or evening stars.

  28. :30 0 of 102 Which planet is the most difficult one to observe? • Mercury • Venus • Mars • Jupiter • Saturn

  29. Mercuryappears at most~280from the sun. It can occasionally be seenshortly after sunset in the westorbefore sunrise in the east. Venusappears at most~ 460from the sun. It can occasionally be seen forat most a few hours after sunset in the westorbefore sunrise in the east. 0

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