General

Having now looked at the Solar System from the outside, what does it look like to us standing on the Earth's surface? To us, the sky appears like a great inverted hemispherical bowl over our heads – or so it says in most books but to myself and many people it appears subjectively to be shallower or flatter than a true hemisphere would be. At night the stars, planets, moon and other heavenly bodies appear to be fixed to the inner surface of this bowl. We ourselves seem to stand at the centre of a horizontal circular more or less plane surface extending all the way to the horizon. Four points spaced at right angles on the circumference of the horizon mark the well-known north, south, east and west cardinal points. The point directly overhead on the dome of the sky is called the zenith. The other half of the bowl is out of sight beneath us and is continuous with the hemisphere above. The point on the bowl directly beneath us and therefore exactly opposite the zenith is called the nadir. This great globe of the heavens is called the celestial sphere.

Watching the clear night sky as the hours pass reveals that the stars retain the same relative positions to one another, forming patterns well known to us as the constellations but collectively they appear to drift slowly in an east to west direction across the sky if we face south.