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 the planet on each such occasion would be simply the difference between their two distances from the sun. But, as we have already seen, the orbits are not circular, and consequently there is very considerable variety in the different oppositions as regards the advantages which they offer to the astronomer. It might, for instance, happen that Mars was at its greatest distance from the sun at the time when the earth crossed between it and the sun. Then the interval between the two bodies would be more than 60,000,000 miles, and the opposition would be as unsuitable as it could possibly be. It thus follows that such a very favourable opposition as that through which Mars passed in 1892 only arises from a particular combination of circumstances which but rarely occur.

It may, however, be of interest to lay down the principles which exhibit the law by which the succession of such oppositions is determined. The opposition of Mars can occur while the earth is at any part of its orbit; that is, the opposition may happen in any month of the year. The part of Mars' path which at present lies nearest the sun runs towards that part of the earth's track through which the earth passes in August. Hence it follows that if an opposition takes places in August it does so at a time when Mars is as near to the sun as is possible. It is true that the earth is not then nearest to the sun, but as the effect contributed by the variation of the earth's distance is of little importance, it follows for all practical purposes that when the opposition takes place in August, it does so under the most desirable circumstances.

On the other hand, if it should happen that the opposition took place about February, then the conditions would be as unfavourable as possible, for though Mars, earth, and sun were in a straight line in the order I have named,