Oppositions of Mars occur at an average interval of 780 days; because of the eccentricity of the orbits, opposition's date and least distance don't coincide, but can be as large as 9 days.
In the favorable periheliac oppositions, Mars approaches the Earth within 0.4 astronomical unit; these oppositions repeat at intervals of 15 years or 17 years. Each opposition is followed by a very similar one 79 years later.
Mars has a risonance with the Earth of 79 years (15+15+15+17+17) or 284 years or 363 years.
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www.pierpaoloricci.it/dati/opposizionimarte_eng.htm
