About every two years Mars makes a close approach to Earth, but due to the eccentricity of Mars' orbit, the appearance of Mars can vary considerably.

The closest Mars can come to Earth is 56 million kilometers during a perihelic opposition at which time Mars has a diameter of 25.1" (arc seconds). During an apehelic opposition Mars is about 96 million kilometers from Earth. The maximum distance from Earth to Mars during a conjunction is 399 million kilometers. At that distance Mars appears only 3.6" in diameter.

The Moon at full is about half a degree of arc in diameter. There are 60 arcminutes in a degree and 60 arcseconds in a minute, making the full moon 1800" (arcseconds) in diameter. At greatest extent Mars appears but as large as a big crater on the Moon.