The angular diameter of Mars in radians and in arc minutes and the evaluation of the capability of human eyes to resolve Mars of diameter 6.8 × 10³ km, located 5.5 × 10⁷ km away are as follows;
(d) The angular diameter of Mars is 1.23 63 × 10⁻⁴ rad
(e) The angular diameter in arc minutes is about 0.425 arc minute.
What is an angular diameter?
The angular diameter is an apparent dimension used to describe the size of a circular object, observed from a specified point of view or location.
The diameter of Mars = 6,800 km
The closest distance between Mars and Earth when Mars gets closest to Earth = 55,000,000 km
(d) The angular diameter of Mars can be obtained by finding the vertex angle formed by the diameter of Mars and the ray from the observer on Earth to the left and right extremities of Mars using the trigonometric ratio of tangent as follows;
Angular diameter = 2 × arctan((6,800/2)/55,000,000) = 0.000123636363 radians
The angular diameter of Mars as observed from Earth is 1.23 × 10⁻⁴ radians
(e) 1 radians = 3437.75 arc minute
0.000123636363 radians = 0.000123636363 × 3437.75 arc minute
0.000123636363 × 3437.75 ≈ 0.425
Therefore;
0.000123636363 radians ≈ 0.425 arc minute
The specified level of detail that the human eye can resolve is one arc minute (1')
The apparent angular diameter of Mars, which is about 0.425 arc minute is lesser than the angular limit of detail of one arc minute that the eye can resolve, therefore, humans can not resolve Mars without optical aid.