While this year’s approach is exceptional, Earth and Mars pass especially close whenever opposition (Mars appears to us directly opposite the Sun) and Martian perihelion occur around the same time. This coincidence happens every 15 or 17 years, and these oppositions are our best viewing opportunities. In fact, very similar oppositions repeat every 79 years, and this year’s approach is actually only a little closer than that of 1924.

As I write, Mars already appears as a disk larger than 20 arc seconds across, with an obvious south polar cap and delicate surface detail visible even in modest amateur telescopes. Take every chance to see it in the eyepiece over the next two months. Since Mars' rotation takes about 30 minutes longer than ours, viewing it at the same Earth-time every night means the features will seem to march gradually around the planet. You will need observations over a more than a month to take in the whole planet.

When you’ve absorbed all the detail you can, step back from the telescope and look with your own eyes at Mars in our southern sky. Alas, that’s the one down side of this extraordinary opposition: its southerly declination means that it never rises more than about 35º above our horizon. Thus we always view Mars through a relatively thick wedge of atmosphere, susceptible to turbulence and poor seeing. The situation is far more favorable from Auckland or Sydney: Mars is spectacular, nearly overhead. At least we get to view it in summer.

Another advantage is that it is easy to visualize Mars’ path through the southern Zodiac. From the beginning of this year Mars has glided eastward from Libra, through Scorpius, Ophiuchus, Sagittarius and Capricornus. At present, it is in Aquarius, just below the Water Jar asterism, above and far outshining the brightest star anywhere nearby, Fomalhault. Just about the time you receive this newsletter, Mars will have stopped and reversed motion--now moving west in a 10 degree-wide opposition loop--and it will continue west through opposition (on August 28, and perihelion, August 30th) for about a month afterward (see graphic, below). By end then of September, Mars will resume its eastward motion into Pisces by year's end.

The reason for this apparent reversal is that the Earth has caught up with and is passing by Mars. An earthbound analogy may help: as you drive toward the Delaware beaches on Route 1, the slower cars appear to move to your rear as you pass them. It is the relative motion that’s important. It’s a bit more complicated when you have two elliptical and inclined orbits, and this accounts for the asymmetrical loop below. The next approach (and opposition) will be in November 2005 in Aries--somewhat higher in our sky, but farther away and smaller in apparent size. Successive oppositions occur in late 2007 and early 2010 in Gemini and Cancer. Curiously, the opposition loop will be progressively wider even as the minimum Earth-Mars distance grows.

Will we see a repeat of 2001’s surface-obscuring dust storm? How much detail can you see in how small a telescope?  Can you draw or image the Red Planet, and what filters help reveal detail? Can you glimpse the Lilliputian moons, Phobos and Deimos? Did you look for Uranus, less than 10º to the northwest? Most importantly, participate in a public Mars-watch. The viewing will never be better. 

Moondark is written by Doug Miller, published on the web, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 27 July 2003. Text and images copyright © 2003 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.