Lecture Notes: Celestial Sphere and Earth Motions

Celestial Sphere Concepts

• Selectional Sphere: Modeled as a large sphere surrounding the Earth with fixed points (North and South Selectional Poles, and Selectional Equator).
• Latitude (L):
  • Determined by the angle between the northern horizon and the North Selectional Pole.
  • It influences what part of the sky is visible: 90-L degrees from the southern sky and the rest from the northern sky.
  • Example: Al Ain has a latitude of 24°, allowing 66° view of the southern sky.
  • In the southern hemisphere, focus is on the South Selectional Pole.

Special Latitudes

• Latitude 90°: North Pole.
  • North Selectional Pole at zenith, only northern sky visible.
• Latitude 0°: Equator.
  • Selectional Equator at zenith, equal view of northern and southern skies.

Observing the Sky

• Star and Sun Movements:
  • At different latitudes, the sun and stars have distinct trajectories due to Earth's rotation.
  • At the equator, stars rise vertically, and at the poles, they circle horizontally.
  • Time Lapse: Shows movement due to Earth's rotation.

Polaris and Navigation

• Polaris (North Star): Close to the North Selectional Pole, used for navigation.
• Circumpolar Constellations:
  • Rotate counterclockwise around the North Pole.
  • Help in navigation and time-telling.

Earth's Motions

• Revolution: Earth’s orbit around the Sun (~365 days).
• Rotation: Earth’s spin on its axis (~24 hours).
• Precession: Slow wobble of Earth's axis (26,000 years cycle).
  • Caused by gravitational forces from the Sun and Moon.

Impact of Earth's Tilt

• Seasons: Result from Earth's axial tilt (23.5°).
  • Summer Solstice: Sun at its highest point.
  • Winter Solstice: Sun at its lowest point.
  • Equinox: Equal day and night, Sun crosses the celestial equator.
• Day/Night Length:
  • Varies with latitude, affecting phenomena like "midnight sun" in polar regions.

Ecliptic and Astrology

• Ecliptic: Earth's orbital path around the Sun influences sun's apparent path.
  • Determined using zodiac constellations visible at night.
  • Horoscopes: Based on constellations' positions over time.

Effects and Observations

• Tilt Effects:
  • Responsible for different lengths of day and night.
  • Creates phenomena such as 24-hour daylight or night in polar circles.
• Latitude and Viewing:
  • Affects visible constellations and sun’s trajectory.
  • Important for understanding local climate and seasons.

Summary

• Latitude impacts visible sky and celestial navigation.
• Earth's movements (revolution, rotation, precession) lead to observable phenomena.
• Axial tilt is central to understanding seasons and solar paths.

Key Concepts to Remember:

• Understanding your latitude is crucial for celestial orientation.
• Earth's axial tilt and movement are primary reasons for seasonal changes and sky observations.