Geodesy

• ZENITH - POINT ON THE CELESTIAL SPHERE DIRECTLY ABOVE THE OBSERVER
• NADIR – POINT ON THE CELESTIAL SPHERE DIRECTLY BELOW THE OBSERVER
• VERTICAL CIRCLE – GREAT CIRCLE ON THE SPHERE PASSING THROUGH THE NADIR AND ZENITH.
• CELESTIAL POLES – INTERSECTION OF THE EARTH’S ROTATIONAL AXIS AND THE CELESTIAL SPHERE (NORTH AND SOUTH)
• CELESTIAL MERIDIAN – THE VERTICAL CIRCLE PASSING THROUGH THE CELESTIAL POLES
• PRIME VERTICAL – THE VERTICAL CIRCLE PERPENDICULAR TO THE PLANE OF THE CELESTIAL MERIDIAN.
• CELESTIAL HORIZON – GREAT CIRCLE PERPENDICULAR TO THE PLANE OF THE CELESTIAL MERIDIAN AND THE PRIME VERTICAL.
• NORTH AND SOUTH POINTS – INTERSECTION OF THE CELESTIAL MERIDIAN AND THE HORIZON
• EAST AND WEST POINTS – INTERSECTION OF THE PRIME VERTICAL AND THE HORIZON.
• HOUR CIRCLE – A GREAT CIRCLE PASSING THROUGH THE CELESTIAL POLES AND PERPENDICULAR TO THE PLANE OF THE CELESTIAL EQUATOR.