# Appearance of the Sky Orientation Motion of sky Seasons Precession (?)

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1 Today Appearance of the Sky Orientation Motion of sky Seasons Precession (?)

2 The Celestial Sphere Stars at different distances all appear to lie on the celestial sphere. The ecliptic is the Sun s apparent path through the celestial sphere.

3 The Celestial Sphere The 88 official constellations cover the celestial sphere. The celestial sphere is like a globe of the earth - the 2D surface of a sphere that maps where things are. BUT we look up at it from the inside rather than down on it from above. East & West get flipped like left and right in a mirror.

4 The Milky Way A band of light that makes a circle around the celestial sphere: our view into the plane of our galaxy.

5 The Local Sky An object s altitude (above horizon) and direction (along horizon) specify its location in your local sky. Everything is measured in angles!

6 The Local Sky Zenith: The point directly overhead Horizon: All points 90 away from zenith Meridian: Line passing through zenith and connecting N and S points on the horizon

7 We measure the sky using angles

8 Angular Measurements Full circle = 360º 1º = 60ʹ (arcminutes) 1ʹ = 60ʺ (arcseconds)

9 Angular Size An object s angular size appears smaller if it is farther away. = L D angular size (in radians) = physical size distance

10 Why do stars rise and set? Earth rotates west to east, so stars appear to circle from east to west. Stellarium

11 Our view from Earth: Stars near the north celestial pole are circumpolar and never set. We cannot see stars near the south celestial pole. All other stars (and Sun, Moon, planets) rise in east and set in west. A circumpolar star never sets Celestial equator This star never rises Your horizon

12 Thought Question What is the arrow pointing to?

13 Thought Question What is the arrow pointing to? The North Star

14 Why do the constellations we see depend on latitude and time of year? They depend on latitude because your position on Earth determines which constellations remain below the horizon. They depend on time of year because Earth s orbit changes the apparent location of the Sun among the stars.

15 Review: Coordinates on the Earth Latitude: position north or south of equator Longitude: position east or west of prime meridian (runs through Greenwich, England)

16 The sky varies with latitude but not longitude.

17 Altitude of the celestial pole = your latitude

18 The sky varies as Earth orbits the Sun As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. At midnight, the stars on our meridian are opposite the Sun in the sky.

19 Solar & Sidereal Day Solar day = 24:00 hours (noon to noon) combination of Earth s spin plus Earth s orbital motion Sidereal day (Earth s spin period) = 23:56 time between meridian crossings of one star

20 What causes the seasons? Seasons depend on how Earth s axis affects the directness of sunlight.

21 Sun s altitude also changes with seasons Sun s position at noon in summer: higher altitude means more direct sunlight. Sun s position at noon in winter: lower altitude means less direct sunlight.

22 Summary: The Reason for Seasons Earth s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun. Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct. AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth. DISTANCE from the sun matters relatively little because the Earth s orbit is nearly circular. The variation of the Earth-Sun distance is only about 3%. Distance variation could matter (e.g., for comets); it just isn t an important factor for the Earth. Hypothesis check: how would seasons in the northern and southern hemisphere relate if distance from the sun caused the seasons?

23 How do we mark the progression of the seasons? We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox Fall equinox 2 weeks from Friday

24 We can recognize solstices and equinoxes by Sun s path across the sky. Summer solstice: Highest path, rise and set at most extreme north of due east Winter solstice: Lowest path, rise and set at most extreme south of due east Equinoxes: Sun rises precisely due east and sets precisely due west.

25 Seasonal changes are more extreme at high latitudes. Path of the Sun on the summer solstice at the Arctic Circle

26 How does the orientation of Earth s Precession: axis change with time? Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years. Polaris won t always be the North Star. Positions of equinoxes shift around orbit; for example, the spring equinox, once in Aries, is now in Pisces! Earth s axis precesses like the axis of a spinning top.

27 What have we learned? How do we mark the progression of the seasons? The summer and winter solstices are when the Northern Hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight. How does the orientation of Earth s axis change with time? The tilt remains about 23.5 degrees (so the season pattern is not affected), but Earth has a 26,000 year precession cycle that slowly and subtly changes the orientation of the Earth s axis.

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