Season of Mars

Observing Mars is almost always about the timing. During the twenty-six months between Martian conjunctions with the Sun, there are maybe only about four or five months where it is nearly impossible to see. During all the rest of the time, the red planet is visible and reasonably easy to observe, but for most of that time, Mars offers very little to see. An apparition of Mars begins at the time it passes behind the Sun into conjunction. We don't use the terms "superior" or "inferior" conjunction with Mars because it can only pass behind the Sun, never in front of it as Mercury and Venus do. After passing behind the Sun, Mars will slowly emerge into morning twilight. Because it orbits farther out than Earth does, Mars travels more slowly around the Sun that Earth does so Mars will appear to fall behind the Sun on the sky rather than pull out ahead of it like the inferior planets. Mars will appear to travel somewhere between 18 and 25 degrees eastward per month depending upon whether the planet is near perihelion or aphelion. The distance to Mars will also affect the view through the telescope. If Mars is emerging from conjunction in late August and is at or near aphelion, then the planet will be at its smallest and faintest. Mars will be barely three seconds across and fainter than magnitude +2. Even though the angle of the ecliptic with the morning horizon is favorable in late summer and early fall, the planet is so faint that it will be difficult to pick out of the predawn twilight for at least two to three months. For this first eight to nine months after solar conjunction, Mars does not typically offer you much to see in the telescope. The planet gains size only very slowly during this time and observers who are not prepared for what they are going to see tend to be very disappointed. During this early period in the apparition there is just nothing to see because Mars is so tiny in telescopes. Know what to expect because if you are waiting around until three in the morning to see Mars, you should not expect the planet to show you anything more than a tiny, featureless reddish dot. What you can expect to see as the planet slowly gains size is that the planet begins to show a slight phase in your telescope. Because Mars' orbit is not that far outside of Earth's, the viewing geometry allows us a peak behind Mars' terminator at its night side. As Mars' approaches a position about ninety degrees from the Sun, it only appears about 87% to 91% illuminated depending upon its distance from us. This will occur about ten months after solar conjunction and the exact positioning of Mars 90 degrees from the Sun is called quadrature. By now, Mars is also about 10 arc seconds across and nearing zero magnitude. This is considered the minimum size for the planet to begin showing some of its surface detail. Depending upon when we encounter Mars during our trip around the Sun, Mars may show us either its north pole or its south pole. The oppositions of 2001, 2003, and 2005 occurred in late southern spring or southern summer. Subsequent oppositions will more favor the northern hemisphere. The first prominent feature you will have a chance to see is the polar cap of the exposed hemisphere. Because it is somewhere between early spring and late summer at whatever pole you are looking at, it will be shrinking, if not already gone, you want to get a look at it as soon as you can see it. As the season progresses, the cap will rapidly shrink away. Use high power and watch that cap very carefully.

Watch Mars's path against the stars as it passes quadrature. You will notice that the planet's steady eastward march against the zodiac will begin to slow and will eventually stop altogether. The planet itself has not stopped moving but the perspective caused by our beginning to overtake it in space causes the planet to appear to stop and then begin to move backwards against the stars or towards the west. Like with the inner planets, this is also a form of retrograde motion. In this case it is an illusion caused by the fact that Earth's speed overtaking Mars causes the planet to appear to move backwards against the zodiac.

Mars will be at its best when it is directly opposite the Sun in the sky and this point is oddly enough called opposition. If two planets are in a circular orbit, then this point where the inner planet overtakes the outer planet is the point where they are closest to each other. Neither Earth nor Mars has a circular orbit and the orbit of Mars is in fact more elliptical than any other in the solar system other than Mercury or Pluto. Because of this, the time of opposition and closest approach for

Figure 8.2. The illusion of retrograde motion. Power Point illustration by author.

Figure 8.2. The illusion of retrograde motion. Power Point illustration by author.

Mars may differ by a few days. Viewing is still at its best right around opposition because Mars will rise at the same time as the Sun sets and sets as the Sun rises in the morning, thus it is in the sky all night long. During the weeks leading up to opposition, Mars will brighten and grow explosively in size. How much it does so will depend on whether the planet is near perihelion or aphelion in its orbit. Within the past twenty-five years, we've viewed oppositions of Mars at both extremes. On February 25,1980 Mars came to opposition within five hours of reaching aphelion. That year Mars reached only 13.8 arc seconds in diameter, about the smallest it can ever be at opposition and brightened only to magnitude -1.0, also a minimum value for opposition23. The period of time where Mars was larger than 10 arc seconds, that prime viewing time, lasted for less than sixty days. Conversely on August 25, 2003 Mars reached opposition within 40 hours of reaching perihelion. On that night, Mars peaked at 25.1 arc seconds in size and blazed in the late summer sky at magnitude -2.8. Mars was larger than 20 arc seconds for a longer period of time in 2003 than it was larger than 10 arc seconds in 1980. Mars was larger than 10 arc seconds for nearly six months. This was the closest that Mars has come to Earth in over 50,000 years. Favorable oppositions of Mars do occur more frequently than that though. In 1986 and 1988 the planet came to opposition within a month of perihelion and displayed disks of 22.8" and 23.2", respectively and was brighter than magnitude -2.5. The oppositions occurring two years before and after the 2003 event both had Mars sporting a 20.2" disk and brightening to magnitude -2.2. With Mars, though, the period of time where the planet shows a disk large enough to show you large surface features easily in a telescope is very short and so you have to dedicate yourself to getting out there and spend the time observing Mars when the planet is favorably placed in the sky and also close to Earth. The window of opportunity with Mars is always short. For the outer planets for example, they will spend about 45% of their time more than 90 degrees from the Sun. Because Mars is closer to us, it spends as little as 20% of its time more than 90 degrees from the Sun and even less time larger than that precious 10 arc second threshold.

After opposition, the planet begins to fall behind Earth in its orbit and will rapidly begin to shrink and fade. Within about six weeks, another sign that the best part of Mars biennial show is over will occur when the planet's westward motion against the stars slows, stops and then reverses itself again and the planet resumes direct or eastward motion against the stars. Mars' phase will shift back towards a pronounced gibbous by the time the planet reaches quadrature in the evening sky. By this time the planet will have shrunk back below 10 arc seconds and typically have faded back to below zero magnitude. During the final eight to ten months of the apparition, Mars will slip lower and lower in the western evening sky, fading to first and then second magnitude before disappearing in the twilight glow for good about two years after you first saw it emerge into the morning sky. Finally Mars will pass behind the Sun into conjunction and then begin the cycle anew. Because the average speeds of Earth and Mars are relatively close to each other,

23 As a historical footnote, this opposition was one of the most watched in history because Mars was also in a close conjunction with Jupiter, which came to opposition that same night.

the apparitions of Mars, or the planet's synodic period (the time from one conjunction to the next), is the longest of any of the planets in the solar system. So opportunities to view Mars at its best occur with less frequency than that for any other planet and they last a shorter time than any other planet except for Mercury. So when the red planet burns bright at opposition, make it a priority for viewing because a good chance to get a good look at Mars does not last for very long. What will last for long is your wait for another chance if you don't spend time with it.

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