The Horizontal System


What does the celestial sphere look like when observed from a position on the Earth's surface?

We ignore any landscape around the observer, so the observer has the impression of being on a flat plane and at the centre of a vast hemisphere across which the celestial bodies move. On all sides, the plane stretches out to meet the base of this celestial hemisphere at the horizon. Directly above the observer's head is the zenith (and directly below his feet is the nadir). Directions around the horizon are labelled with the familiar cardinal points, N, S, E and W. Finally, the line passing through the zenith and the south point on the horizon is known as the observer's meridian.


It is useful to define coordinates which specify the position of the star from the point of view of the observer. These are altitude and azimuth.

Azimuth


Azimuth is measured around the horizon. Azimuth ranges from 0 to 360°. North is at 0° and azimuth increases to the east, so east is 90°, south 180° and west 270°. Stars with the same azimuth lie on a vertical circle running from the zenith down to the horizon. This vertical circle is a great circle. Azimuth is marked in red on the figure to the right.

Altitude

A star's altitude measures how high it appears in the sky. It is measured along the vertical circle defined earlier, and increases from 0° at the horizon to 90° at the zenith. Since a star can be below the horizon, altitude ranges from -90° to 90°.

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The horizon coordinate system


The celestial sphere in the sky

We need to know where the features of the celestial sphere appear from the point of view of the observer.

The diagram to the right shows an observers view of the celestial sphere. Positional astronomy is often easy if you know what diagram to draw; the diagram to the right is the most important one in the whole course; it allows you to draw the celestial sphere from any point on the Earth's Surface. It is called the horizon diagram.

Because the radius of the celestial hemisphere is infinite compared with the radius of the Earth, the directions of the north celestial pole and the celestial equator as viewed by the observer are indistinguishable from their real directions (which are defined relative to the centre of the Earth.)

Now we can see how the celestial sphere appears from any given latitude, Φ. Assuming our observer is in the northern hemisphere, as shown in the figure, the north celestial pole appears in the sky at an altitude equal to Φ, and towards the north. The celestial equator is located at an altitude of 90-Φ, and towards the south. At the north and south poles, the celestial equator lies along the horizon.

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