The orbit of Mars is somewhat more elliptical than the Earth's and every 15 to 17 years the orbits of our two planets come close together, and at closest opposition can be 54 to 103 million kilometers away. These distances equate to the size of the disk of Mars to vary rather drastically, so it is nest observed near the 2.1/2.25 year intervals with the closest approach during the 15/17 year interval.

I missed the 27 July 2018 opposition where Mars was 58 million kilometers away, so I attempted to observe it for the 13 October 2020 opposition (the distance was 68 million kilometers away).