Why is Earth missing from Gustav Holst's symphony "The Planets" and why did he set the planets in the order that he did, with Mars first rather than Mercury?

The result of this encounter was a "series of mood pictures" begun in 1914, which became known as The Planets Suite. The seven movements each portray the astrological traits of the seven known planets at the time, in order of distance from the Earth. Pluto was discovered in 1930, well after The Planets was written, and so does not appear in Holst's work.
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