1. Winter Sky – Facing North

2. Celestial Motions
The rotation of the Earth about its spin axis once every 24 hours causes diurnal effects: day and night and the “rising” and “setting” of celestial objects.
The revolution of the Earth about the sun once every … days produces annual effects: the sun appearing to move with respect to the stars along a path in the sky called the ecliptic. The twelve constellations lying along the ecliptic comprise the zodiac.
The apparent motions of celestial objects on the sky are the combined result of diurnal and annual motions and, in the case of the planets their own orbital motions around the sun.

3. The Celestial Sphere north celestial pole Earth’s spin axis north pole
equator
celestial equator
south pole
celestial
sphere
south celestial pole

4. the “ecliptic plane” (plane of the Earth’s orbit around the sun)
Tilt of Earth’s Spin Axis
23.5o tilt
The Earth’s spin axis is tilted by 23.5 degrees off vertical with respect to
the “ecliptic plane” (plane of the Earth’s orbit around the sun)
The spin axis remains essentially parallel to itself during the course of the year

5. Summer Solstice – 21 June tropic of Cancer arctic circle equator
sunlight
equator
antarctic
circle
sunlight

6. Winter Solstice – 21 December
tropic of
Cancer
arctic
circle
sunlight
equator
sunlight
antarctic
circle
tropic of
Capricorn

7. Vernal Equinox – 21 March arctic circle tropic of Cancer equator
sunlight
tropic of
Cancer
equator
tropic of
Capricorn
sunlight
antarctic
circle

8. Autumnal Equinox – 21 September
arctic
circle
sunlight
tropic of
Cancer
equator
tropic of
Capricorn
sunlight
antarctic
circle

10. The Culminating Sun
The sun culminates in the zenith (i.e. straight overhead) at noon for observers located on the tropic of Cancer (latitude = 23.5o N) on the day of the summer solstice.
The sun culminates in the zenith at noon for observers located on the tropic of Capricorn (latitude = 23.5o S) on the day of the winter solstice.
The sun culminates in the zenith (i.e. straight overhead) at noon for observers located on the equator (latitude = 0o) on the days of the equinoxes.

11. Other Seasonal Extremes
The sun never rises for observers north of the arctic circle on the day of the winter solstice
The sun never sets for observers north of the arctic circle on the day of the summer solstice
The above conditions are reversed for the antarctic circle.
The sun moves 360o around the horizon for observers located at the north and south poles on the days of the equinoxes.

12. Local Position

13. Tilt of Earth’s Spin AxisW N S E
winter
solstice
equinox
summer
solstice
The sun rises on the east point and sets on the west point on the days of the equinoxes, giving
equal periods of “day” and “night”.
The sun is in the sky for the longest duration on the summer solstice and illuminates
the northern hemisphere most directly.

14. Temperature Effect
Summer days are longer and the sun is more intense (due to the more direct illumination angle). Thus summer is hotter than winter.
There is a lag of the seasons when comparing the dates of the solstices with the actual extremes in temperature because it takes time to heat up the oceans and atmosphere at the onset of summer and to cool them off at the onset of winter.
If the Earth’s spin axis were not tilted by some angle, we would have no seasons.

15. Motion of Sun, Moon and other Planets
All major bodies in the Solar System move around ecliptic
Slow drift (from W to E) against the background of stars
Skyglobe demo
7 visible “planets” incl. the sun and moon
Planet = “wanderer”
Days of the week named after the planets

16. Why do all planets move in the same plane?
Reason: Formation process of the Solar System
Condenses from a rotating cloud of gas and dust
Conservation of angular momentum flattens it
Dust helps cool the nebula and acts as seeds for the clumping of matter

17. Formation of Planets Orbiting dust – planitesimals
Planitesimals collide
Different elements form in different regions due to temperature
Asteroids
Remaining gas

18. The Zodiac 12 h RA - Libra the Scales 0 h RA - Aries the Ram
The Zodiac is:
The ecliptic is divided into 12 segments
Each segment subtends 30º or 2 h of RA
Your zodiac “sign” is the constellation that the sun was in when you were born
12 h RA - Libra the Scales
30º
0 h RA - Aries the Ram
*Note: Due to precession of Earth’s axis, the position of the sun in the zodiac constellations is actually shifting
Image taken from

19. Precession

20. 2. Moon’s orbital plane is tilted by 5o from our equator
Precession of Earth’s Spin Axis
5. Spin axis now points to Polaris. 13,000 years from now, Vega will be our “pole star”
4. Earth responds to this pull by slowly “precessing” its spin axis around a circle in the sky once every 26,000 years
23.5o
2. Moon’s orbital plane is tilted by 5o from our equator
3. Moon’s gravitational pull on Earth attempts to pull bulge into lunar orbital plane
1. The rotation of the Earth distorts it into an “oblate” spheroid flattened at the poles

21. http://www. upscale. utoronto