The celestial sphere

To an observer on Earth, the stars appear to move together across the sky during the night, rising in the east and setting in the west.

In reality, this does not happen. The stars appear to rise and set for the same reason the Sun does. As the Earth rotates on its axis, the spin carries the visible portion of the sky past us, moving from east to west. Depending on your latitude, some stars never appear to set – only to fade out as the rising Sun turns the night sky into day. These stars are generally near the South Pole or North Pole and can be used to figure out your direction (and time) in the dark.

A celestial sphere – an ancient idea

Many ancient peoples thought that a dome enclosed the Earth and that the stars moved from east to west across the inside of the dome or sphere. Only a few centuries ago, astronomers came to realise that the stars are actually very far away, are scattered throughout the Milky Way galaxy or beyond and don’t move – as they appear to – from east to west inside of a vast sphere.

A model for navigating without instruments

This old idea, however, is still useful. The idea of a celestial sphere provides a simple way of thinking about the appearance of the stars from Earth without the complication of a more realistic model of the universe. Observing stars within a celestial sphere offers a convenient way of describing what we see from Earth. When referring to the celestial sphere, we are imagining that everything we see in the sky is set inside a huge spherical dome that surrounds the Earth. Reference points of the celestial sphere are the basis for several co-ordinate systems – including the star compass (kāpehu whetū) – and are used to place celestial locations with respect to one another and to us. Hence, the celestial sphere is crucial for navigating without instruments.

Navigating without instruments uses the celestial sphere to determine location and direction. Therefore, for ease and simplicity, navigators use terms such as ‘rising’ and ‘setting’ although they know that the Sun, Moon and stars don’t actually rise and set for the reasons given above.

Reference points of the celestial sphere

The celestial sphere that encloses the Earth has no defined size and has the observer on the Earth as its centre. As the observer, you would have a particular view of the stars based on your location on the Earth. However, regardless of your location, there are reference points that always stay in the same position, for example, the zenith is always directly above you and the horizon is always level. These are the reference points of the celestial sphere:

• The great circle: This is an imaginary circle drawn on the celestial sphere where the Earth is the centre. It is the largest possible circle that can be drawn on a sphere.
• The horizon where the Earth and sky meet: Navigators on ocean-going vessels have an idealised view of the horizon. This is what it looks like without any obstructions such as buildings or trees.
• A star’s altitude: This is the angle between it and the horizon.
• The cardinal points: These are the points on the celestial sphere that are on the horizon and due north, south, east and west. The north point, for example, is the point due (true and straight to the pole) north on the horizon.
• The zenith: This is the point in the sky directly above you. Any point on the horizon is 90° from the zenith.
• The meridian: This is the great circle that passes through the north point, the south point and the zenith and lies on the celestial sphere.