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Chapter 2:The Sky: 

Chapter 2: The Sky 0

The Celestial Sphere: 

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. 0

The Celestial Sphere (II): 

The Celestial Sphere (II) 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. Celestial equator culminates 90o – l above the horizon. l 90o - l 0

Example:: 

Example: New York City: l ≈ 40.70 Horizon North Celestial North Pole 40.70 South 49.30 Celestial Equator The Celestial South Pole is not visible from the northern hemisphere. Horizon 0

Athens, OH, is located at l ≈ +39o. Where in the sky would you see the highest point of the celestial equator? : 

Athens, OH, is located at l ≈ +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. 0

Slide6: 

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

The Celestial Sphere (III): 

The Celestial Sphere (III) 0

Apparent Motion of the Celestial Sphere: 

Apparent Motion of the Celestial Sphere 0

Apparent Motion of the Celestial Sphere II: 

Apparent Motion of the Celestial Sphere II 0

A View of the Sugarloaf Mountain (Rio de Janeiro, Brazil): 

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

Constellations: 

Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups, representing mythological figures. 0

Slide12: 

Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions. 0

Are the stars of a constellation really relatively close to each other?: 

Are the stars of a constellation really relatively close to each other? Yes, because we see them close to each other. Yes, because they all seem to move in the same direction. No, because they are very distant from us. No, because we don’t know if they are at comparable distances from us. No, because they tend to move in different directions. 0

Slide14: 

The stars of a constellation only appear to be close to one another Usually, this is only a projection effect. The stars of a constellation may be located at very different distances from us. 0

Slide15: 

Stars are named by a greek letter (a, b, g) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Betelgeuze = a Orionis, Rigel = b Orionis Betelgeuze Rigel Orion 0

The Magnitude Scale: 

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!) 0

Example:: 

Example: Betelgeuze Rigel Magnitude = 0.41 mag Magnitude = 0.14 mag For a magnitude difference of 0.41 – 0.14 = 0.27, we find an intensity ratio of (2.512)0.27 = 1.28 0

Alpha Centauri B has an apparent magnitude of mv = 1.5; the star t Ceti has mv = 3.5. This means that: 

Alpha Centauri B has an apparent magnitude of mv = 1.5; the star t Ceti has mv = 3.5. This means that Alpha Centauri B is 2.5 times brighter than t Ceti Alpha Centauri B is 2.5 times fainter than t Ceti Alpha Centauri B is 6.3 times brighter than t Ceti Alpha Centauri B is 6.3 times fainter than t Ceti Alpha Centauri B is 5 times brighter than t Ceti 0

Slide19: 

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 0

The Sun and its Motions (I): 

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

The Sun and its Motions (II): 

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. 0

What is causing the seasons?: 

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. The longer daytime period in the summer than in the winter. 0

The Seasons (I): 

The Seasons (I) The Earth’s axis of rotation is inclined vs. the normal to its orbital plane by 23.50, which is causing the seasons. 0

The Seasons (II): 

The Seasons (II) The Seasons are only caused by a varying angle of incidence of the sun’s rays. They are not related to the Earth’s distance from the sun. In fact, the Earth is slightly closer to the sun in (northern-hemisphere) winter than in summer. Light from the sun Steep incidence → Summer Shallow incidence → Winter 0

The Seasons (III): 

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

When it’s summer in the U.S., it’s … in Argentina: 

When it’s summer in the U.S., it’s … in Argentina spring summer fall winter midnight 0

The Seasons (IV): 

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

Precession (I): 

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. 0

Precession (II): 

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. 2100. ~ 12,000 years from now, it will be close to Vega in the constellation Lyra. There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) 0

The Motion of the Planets (I): 

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

The Motion of the Planets (II): 

The Motion of the Planets (II) All outer planets (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto) generally appear to move eastward along the Ecliptic. The inner planets Mercury and Venus can never be seen at large angular distance from the sun and appear only as morning or evening stars. 0

Which planet is the most difficult one to observe?: 

Which planet is the most difficult one to observe? Mercury Venus Mars Jupiter Saturn 0

Slide33: 

Mercury appears at most ~280 from the sun. It can occasionally be seen shortly after sunset in the west or before sunrise in the east. Venus appears at most ~ 460 from the sun. It can occasionally be seen for at most a few hours after sunset in the west or before sunrise in the east. 0