“This year, during August, Mars will be at its closest to the Earth; so close, in fact, that it will appear as large as the full Moon.” Have you heard this urban legend before? Since 2003, emails describing this myth have circulated the internet usually during July or August. It all started during the opposition of August 2003 when Mars was about as close as it can be to the Earth: 0.3727 AU; 55,755,723 km; 34,645,000 mi. in an approximately 20-year cycle of varying distances at opposition. Since then, usually around August, misinformation about the appearance of Mars circulates around the internet.
The outer planets reach opposition when the Earth has moved into a position with the Sun on one side and the outer planet on the other side. Because all planets orbit in the same direction (toward the east), and all follow orbits that are slightly more elliptical than circular, oppositions occur at regular intervals of about 12 months (except for Mars). Mars is considerably closer to Earth and is moving faster than the other outer planets, so it takes approximately 26 months for Earth to catch up with Mars for an opposition. In the time that it takes the Earth to catch up with and move into an opposition position, the outer planets have also been moving eastward relative to the stars in the background. Over time, the constellation where the outer planet is located during opposition and its retrograde loop gradually shift to the east. An opposition is also near the time when the distance between the Earth and an outer planet is around the minimum distance. Keep in mind that because the shape of the orbit is elliptical, this minimum distance will be different each time.
The opposition of July 27th 2018 at a distance of 0.38496 AU; 57,589,196 km; 35,784,268 miles will be the closest for quite some time to come after the 2003 opposition (0.37272 AU). While this is not as close as the 2003 very close opposition, there is a conclusive way to show that Mars could never be so close that it would appear as large as the Moon. To see how use the equations below and the distance for the 2018 opposition, or any opposition for that matter. You will see that Mars could not appear to be as large as our Moon unless some force somehow caused Mars to change its orbital position and literally move closer to Earth.
On July 27th 2018 Mars will be 0.38496 AU; 57,589,196 km.; 35,784,268 mi. from Earth. Mars is 6,792 km (4,220 mi.) in diameter. The Moon is 3,475 km (2,159 mi.) in diameter and is an average 384,400 km (238,900 mi.) from Earth.
To calculate how large Mars will appear as compared to the Moon:
(Mars’s diameter ÷ Mars’s distance)
(Moon’s diameter ÷ Moon’s distance)
To calculate how close Mars would have to be to appear as large as the full Moon:
(Mars’s diameter × Moon’s diameter) × Moon’s distance =