Mars at opposition this week

As we approach the red planet’s January 16 opposition this week, interest in observing and imaging Mars understandably increases.

At opposition, Mars will appear as a magnitude -1.4, a 15-arcsecond red disc which, under good seeing conditions and with a moderate telescope, will reveal its now familiar surface details, which include polar caps and lighter and darker regions.

This perspective of Mars' Valles Marineris hemisphere, from July 9, 2013, is actually a mosaic comprising 102 Viking Orbiter images. At the center is the Valles Marineris canyon system, over 2,000 kilometers long and up to 8 kilometers deep (JPL-Caltech/NASA).

Opposition day is the best time to view Mars, occurring on average every 780 days or just over 2 years.

However, not all Martian oppositions are equal. Its brightness and apparent size will vary depending on its position with respect to the Earth and the Sun.

Mars has a relatively higher eccentric orbit compared to Earth’s. This means that its distance at opposition from Earth can vary between 56 million km when it is closest to the Sun (perihelic opposition) and 101 million km when it is furthest (aphelic opposition).

Between perihelic and aphelic oppositions, the red planet’s brightness can vary between magnitude -2.8 down to -1.2, while its apparent size will range from 25 to 12 arcseconds. 

We are approaching an aphelic opposition, due February 2027, so this year’s opposition isn’t rated that highly.

Our next perihelic Martian opposition, which occurs every 15 to 17 years, will occur in September 2035. At that time, Mars will appear as a 25-arcsecond, magnitude -2.8 disc, revealing a lot more detail.

Mars' oppotsitons

And why does Mars appear to change its apparent direction of motion in the sky during its orbit?

This is known as apparent retrograde motion due to the Earth’s faster orbital speed and the relative positions of Earth, Mars, and the Sun.

As Earth overtakes Mars in its inner orbit, it appears from Earth as if Mars momentarily stops and then moves in the opposite direction. This is analogous to a faster car on a racetrack overtaking a slower car, giving the faster driver the impression that the slower car starts to move backwards.

As Earth passes Mars in its inner orbit, it briefly stops in the sky and then moves in the opposite direction.

Mars began its retrograde motion on 8 December 2024 and will continue until 24 February 2025, when it will change back to its prograde motion.

Mars' motion between October 2024 and May 2025 as seen from Melbourne.