Mars is in opposition, so what does it mean? – UWMadScience

By Jim Lattis, Director of UW Space Place at the University of Wisconsin-Madison

You may have heard that March came to the opposition on July 27, 2018. But what about Does it mean? This means that Mars is clear and easy to find in the night sky. This is what is called opposition because it is at this point that Mars is at 180 degrees of – if directly in line with – the sun. When the sun sets, Mars gets up and crosses the sky all night long, going to bed at sunrise.

The opposition is also when the distance from the planet to the Earth reaches a relative minimum. Because it's closer, it appears bigger and brighter in our sky. Already since spring, we saw oppositions of Jupiter (May 9), then Saturn (June 27), so it was a good summer for the viewers of the planet. (Uranus, Neptune and Pluto have also reached the opposition this year, but they are all weak enough that most astronomers do not see them at all.)

Planetary orbits lead the opposition and the opposites of Mars are a little more complicated than those of the others because the Martian orbit is much more elliptical than the orbits of the planets such as Jupiter and Saturn. As the astronomer Johannes Kepler described in the early 1600s, the planets follow elongated circular trajectories – ellipses – rather than perfectly circular trajectories around the sun

When opposition occurs near the perihelion of Mars (when Mars is at the point closest to its orbit to the sun), Mars is also quite close to the Earth.But when the opposition occurs near the Martian aphelion (when it is at its point the farthest from the sun), it is probably quite far from the Earth. These differences in distance can be dramatic.

For example, Mars is close to perihelion for this year's opposition – a so-called perihelic opposition – and at its closest point it has come within 58 million kilometers of the Earth. Compare this to the March 2012 opposition, an aphelic opposition, while Mars was never within 100 million kilometers of the Earth. Because of this difference in distance, March at the opposition of this year is nearly three and a half times brighter for the terrestrial observer than for the opposition in 2012.

However, even Perihelic oppositions never occur precisely at perihelion, so there are small differences from one event to the other. For example, at the opposition of August 27, 2003, the minimum distance Earth-Mars was just under 56 million kilometers. This means that Mars has appeared up to 10% brighter in 2003 than in 2018.

This is nothing exciting, really, but it still excited people . It turns out that the opposition of 2003 brought it closer to Earth than in any previous March oppositions over the last 60,000 years. This slight advantage in the "favorability" of this opposition was responsible for a frenzy of interest among astronomy enthusiasts around the world. But the dramatic nature of this event was due more to its historical context than to its intrinsic virtues.

The headlines claiming that the closest opposition in 15 years – which would be this year's – would not get the same attention.

March artist's illustration to the opposition. Note that the sun, Earth and Mars align, with the Earth in the center. Not on the scale. Credit: NASA / JPL-Caltech

Despite the favorability of the two oppositions of 2003 and 2018, neither was particularly easy to observe for potential viewers in the temperate regions of the north of the Earth. The perihelion of the orbit of Mars lies in the direction of the southern parts of the ecliptic (the plane of the Earth's orbit), roughly towards constellations such as Capricornus and Aquarius. As a result, perihelic oppositions favor south observers and always occur low in the sky, near the horizon, compared to the Nordics

In 2003, March at the opposition was nearly 16 degrees south of the celestial equator. , and in 2018, March at the opposition is over 25 degrees south, which is worse than it was in 2003. Concretely, this means that even the best telescopes (from the North) are watching Mars through our dense and unstable atmosphere, making the reddish disk blurry and unstable even a good night's sleep. The very good Martian oppositions favor the southern hemisphere, while the northerners have the best views of much less favorable aphelion oppositions, which occur in the northern parts of the ecliptic, towards the constellation Leo, for example.

That does not mean you should not look for an opportunity to see Mars this summer, of course, but if watching on Mars is a priority, pack your telescope and make parts closer to the equator ! But also make a note of October 2020, the next opposition of Mars. Although it is not a record in terms of distance, it will occur considerably further north, about 5 degrees north of the celestial equator. Even if it will be a little further, in the clear skies of October, the views of Mars should be breathtaking.

NASA's Hubble Space Telescope photographed Mars on July 18, 2018 near its closest approach to the Earth since 2003. The planet has been observed the Sun, the Earth and Mars are aligned, with the Earth sitting between the Sun and Mars. This proximity gives the red planet its most beautiful appearance in the night sky since the opposition of 2003. Credits: NASA