Thursday, January 19, 2006

June 7, 2001

Mars dominates the evening sky this spring and summer. At its brightest since 1988, the ruddy planet cannot be missed as it ascends after sunset in the southeast. Not even the star Antares, of similar hue and close on the right, can presently live up to its name of "rival of Mars."

The cause of this phenomenon is the coincidence of Mars' near approach to both the Earth and the Sun. Because Mars' orbit is relatively eccentric, its distance from the Sun varies over tens of millions of miles. Meantime, Mars' distance from the Earth varies over a much greater range, as the latter, being closer to the Sun, speeds ahead to the other side of our star and then comes round again from behind. As a result, every 16 years on average these two values reach a minimum at the same time. Then Mars presents a bigger disk in our sky, hence reflecting more sunlight into our eyes.

Another coincidence is that Mars is changing seasons just as we are, although our northern hemisphere is heading into summer (on June 21) while our neighbor's is leaving it (on June 17). The occasion is particularly serendipitous for telescopic observers during this close apparition, since, on Mars as on Earth, the beginning of fall is a point in a planet's orbit when its entire surface, including both poles, is illuminated for half the day.

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