Every 26 months, the planet Mars fulfills its cycle of closest approach to Earth. Unlike the Moon, which revolves around our planet and keep its distance from us more or less constant, the planet Mars orbits the Sun, as we know from the time of Galileo, so its distance varies considerably.
A peculiarity that it is sometimes difficult to mentally visualize is that the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune are not always aligned all on the same side of our star, the Sun. All of them are constantly moving, at different speeds and taking different times to do the entire lap. The fastest is Mercury, which takes only 88 days, and the slowest one is the last known, Neptune, which takes 165 years to make one revolution around the Sun. The Earth, as we all know, takes 365 days.
Mars is a little further out than Earth and therefore takes 687 days. That means that in the time Earth goes around the Sun twice Mars goes around only once. There are times when Earth and Mars are on the same side, but then our planet quickly overtakes it, goes to the opposite side of the Sun and after 26 months runs by the planet Mars again. This is called synodic period.
These planet encounters occur, for example, in the February-March 2012 season, March-April 2014, May-June 2016, July-August 2018, September-October 2020, Novembre-December 2022, and so on.
In each of those times Mars and the central star of the system, the Sun, are visually at "opposites" sides as they appear to an observer on Earth which is temporarily located between the two; hence the term "opposition".
It is around such times that the spacefaring nations send probes to our neighboring planet, for the distance is at it shortest and therefore the journey easier and quicker.
Mars is a small planet, only two times larger than the Moon, so this distance variation we are talking about makes its appearance as seen from Earth significantly changing. When it is far away, it is very difficult to see details on its surface, even with a good telescope. Its brightness to the naked eye fades very much, being less bright than many stars. But when the Earth encounters it back it can be seen 7 times larger, becoming the brightest star in the night sky after Venus, the "Morning Star".
Oh ... and despite the doomsayers who like to cash in on the subject, during the oppositions Mars does not cause any alteration on Earth, because it is still about one hundred million kilometers away, a couple of hundred times farther than our Moon.
It has a characteristic reddish color, that for ancient people reminded blood, but today we know that is simply sand covering its surface. Mars is a giant desert. We can see with telescopes that it has two small caps in the polar regions, which mostly consist of dry ice (CO2, the main component of its atmosphere). Dark spots are also seen, in the rest of the planet. It was once thought that this could be vegetation, but now we know that simply those are areas of different texture. With a little patience we can detect the planet's rotation, which is remarkably similar to that of Earth, or about 24 hours. In fact, it is the planet most like Earth, but colder and almost airless. Certainly it will be the first planet to which humans will go, perhaps in the near future, hopefully in our lifetimes.
To observe Mars the best thing are telescopes from 10 cm in diameter and wider and at least 300 times magnification. Better still if they are telescopes that do not use mirrors. Binoculars usually offer just about a 10-fold magnification, so unfortunately even though they are useful for other astronomical targets they do not help in this case.
In many countries many people have amateur telescopes, and happily there are also public observatories which customary offer "Martian nights" to the public interested in these events. But either way Mars still gives a beautiful show when seen at naked eye. In times of opposition, the best way to locate it from the Southern Hemisphere is looking North, about halfway up from the horizon. From the Northern Hemisphere usually we have to look rather towards the South. It is further to the East at nightfall, and then at dawn, due to the rotation of the Earth, it is seen pulling over to the West.
Almanacs indicate that there are some dates of interest on which it is mathematically closer to Earth, and others in which it is exactly in line with the Earth and the Sun, a disposition similar to the full moon, so mathematically it is the night of maximum brightness. These differences are very subtle, so they cannot be appreciated at naked eye.
For practical purposes, the show is significant for a couple of months. So when the opportunity comes, I wish clear skies for all of you and good observation.
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Originally published in ABC Color, on 24 December 2007. Diagram: "Thus all the wanderers proved to be associated in common dependence on the Sun, and among the members of the solar family thus recognized Mars was found to hold the position next exterior to the Earth, and the path he followed in his circuit of the Sun to be situated with regard to the Earth's as in the following diagram."
"On consulting the diagram we shall at once perceive why it is that every fifteen years Mars becomes so unusually bright as to seem, to one who has not kept track of him, a new and startling star. His orbit, it will be seen, is an ellipse of some eccentricity, and deviates in consequence considerably from a circle. The point marked Perihelion denotes the point where the planet is nearest the Sun; the point marked Aphelion, the point where the planet is the most remote from the Sun. In like manner the points marked Perihelion and Aphelion on the inner circle show the corresponding points of the Earth's orbit, which is much more nearly circular. Now as the two planets revolve in different periods of time, Mars taking 686,98 of our days to complete his circuit, and the Earth 365,26 days to complete hers, the one planet will overtake the other only once every two years and two months or so. Meanwhile they are at great distances apart. But even when they do meet, they do not always meet equally near. For the one orbital period is not an exact multiple of the other, and as the orbits are both ellipses, it is evident that these meetings of the two planets will occur at different points of their orbits, and, therefore, at different distances. If the meeting occur when Mars is in perihelion the planets approach one another within 35 000 000 miles [~ 56 million km]; if in aphelion, only within 61 000 000 miles [~ 98 million km]."
"But even this difference in distance does not measure the full extent of the variation in brilliancy. As the brightness of an illuminated body varies inversely as the square of its distance from the source of light, and as the total amount of light it reflects to an observer varies inversely as the square of his distance from it, it makes every difference in the apparent brilliancy of a body how the body is situated, both with regard to the source of light and with regard to the observer. Now it so chances that at the meetings of Mars with the Earth these two factors attain their maximum effects nearly together, and similarly with their minimum. For at the times when we are closest to Mars, Mars is nearly at his closest to the Sun, and reversely when we meet him at the opposite part of his orbit. It thus comes about that at some meetings,--oppositions, they are called, because Mars then is in the opposite part of the sky from the Sun,--the planet appears four and one half times as bright as at others. Here, then, we have the explanation of the planet's great changes in appearance, changes so great as to deceive any one who has not followed its wanderings, into the belief that it is some new and portentous apparition."
- Percival Lowell, "Mars", 1895, Chapter 1, II. Orbit, page 3. Diagram credit: Percival Lowell, "Mars", 1895, Figure 1.