The apparent close approach of one celestial body to another. This is a line of sight effect.

Diagram showing an appulse

Diagram showing an appulse

Explanation of the diagram: The image on the right is what an observer sees. The diagram shows what is really going on. The orange circle represents a planet, the blue circle represents the Earth. As the planet travels round its orbit, to an observer on Earth, it will be seen to pass close to some stars. In reality, the two objects are separated by many light years. This is referred to as a ‘line of sight effect’.

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