Chapter2_Geographic_Astronomical_Systems.pptx

Chapter 2: Geographic & Astronomical Coordinate Systems and Time Systems

Geographic coordinates, latitude/longitude, parallels and meridians

Historical context (Eratosthenes, Ptolemy, etc.)

Celestial reference systems, Right Ascension, Declination

Time systems: Solar, Sidereal, Atomic, UTC, Julian Dates

Summary and practice questions

1. Chapter 2: Geographic & Astronomical Coordinate Systems and Time Systems This chapter explores the latitude-longitude system, its astronomical basis, and time measurement systems connected to Earth's rotation and position. It also provides historical insights into early geodesy.

2. Geographic Coordinates Latitude (Φ): Angle between the equatorial plane and a line from the point to Earth’s center. Positive northward. Longitude (λ): Angle between the meridian plane of the point and the Prime Meridian, positive eastward. These form a polar system for spherical Earth positions.

3. Historical Background Eratosthenes (276–195 BC): First measured Earth’s circumference using sun angles between Syene and Alexandria. Ptolemy: Developed coordinate tables and defined an early prime meridian. Newton: Showed Earth is slightly flattened at poles (ellipsoidal).

4. Earth’s Size and Nautical Mile 1 nautical mile = 1852 m = 1 arc minute of latitude. R = 1852 / ((π/180)*(1/60)) ≈ 6,367 km. Useful approximation for geodesy and corrections in surveying such as curvature and sea level adjustments.

5. Celestial Reference System Stars are imagined on a celestial sphere surrounding Earth. Declination (δ): Angular distance north/south of celestial equator. Right Ascension (RA): Angle eastward along celestial equator from the First Point of Aries (γ).

6. Precession and Epochs Due to axial precession, Earth’s rotational axis traces a circle in ~26,000 years. This causes RA origin (First Point in Aries) to shift ~1° every 72 years. Star catalogues use epochs to define coordinates valid at specific times.

7. Time Systems Overview Solar Time: Based on Sun’s apparent motion. Sidereal Time: Based on Earth’s rotation relative to stars (4 minutes shorter than solar day). Mean Solar Time (UT1): Based on average Sun motion. Atomic Time (TAI): Based on cesium atom oscillations. Coordinated Universal Time (UTC): Combines TAI and UT1, adds leap seconds.

8. Julian Dates and Modern Relevance Julian Date counts days from January 1, 4713 BCE, used in astronomy for continuous time calculation. Used in satellite orbits and astronomical predictions. Understanding coordinate epochs and time systems is crucial in geodesy and GIS.

9. Summary Geographic coordinates measure positions on Earth’s surface using latitude and longitude. Astronomical coordinates (RA, Dec) describe star positions. Time systems ensure synchronization between Earth rotation, atomic standards, and celestial observations.