Unequal oppositions
The Red Planet reaches opposition every few years, but there are also cycles that play out over longer periods of time
Not all close approaches of Mars are the same: some are distinctly better than others. The maximum apparent size of Mars varies in a cyclical fashion through subsequent oppositions, because the orbits of Earth and Mars are not exactly circular; they’re elliptical. When Mars is very far from opposition, its disc can shrink as small as 3.5 arcseconds – similar to the apparent size of Uranus. In contrast, at a really favourable opposition the largest size the Red Planet can reach is 25.1 arcseconds, and at the 2018 opposition we saw the apparent diameter of Mars get very close to that, at 24.2 arcseconds.
For the UK at least, the 2018 opposition took place when Mars was low in the sky. This was a ‘perihelic opposition’, one taking place when Mars was close to (technically within 90° of) perihelion, the position where the planet has its smallest orbital distance from the Sun.
The maximum size of Mars in 2020, also technically a perihelic opposition, will be 22.6 arcseconds, smaller than the 24.2 arcseconds presented during 2018, but still a reasonable value. Opposition diameters will continue to shrink for a number of future oppositions. At the 2022 opposition, the biggest the Red Planet’s disc gets is
17.0 arcseconds, while the 2025 opposition presents a 14.5 arcsecond disc. It shrinks further still for the 2027 opposition, when it will appear 13.8 arcseconds across.
With these forecasts in mind, we can see that the 2020 opposition of Mars will be the most favourable for UK viewing for some time – it won’t be this big again until 2035.
Distance at closest approach
S
S
Southern polar cap
S
S