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You are not the center of the world!

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Parallax helps astronomers take into account the fact that when observing from the earth's surface, you are not at the center of the world. In this diagram, the first circle represents the earth ‘terra’. From its center different straight lines are drawn to each of the planets and luminaries which are on different circles and distances to the earth. On the earth surface, just above, a second series of straight lines are drawn to the same planetary positions. The angles between these two series of straight lines is parallax.

On the right side of the diagram, solar and lunar parallax are compared when the luminaries are in conjunction at a Solar eclipse. This illustrates that parallax needs to be computed to correctly predict the time of a solar eclipse. The left side of the diagram compares the parallax angles of all the planets. It shows, schematically, that they grow smaller and smaller.

The diagram also clearly displays how parallax decreases towards the zenith where it is zero.

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