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? According to some tomorrow, the 27th, it will happen again. For the record the full Moon was the 21st, not that the Moon needs to be visible for this non-event. Consider that since 2003, emails describing this urban 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 (55,755,723 km [34,645,000 mi.]) in a 20-year cycle of varying distances at opposition. Since then, usually around August, misinformation about the appearance of Mars circulates around the internet.
Mars and Earth 3 March 2012 and 27 august 2013
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.
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Since opposition for Mars was March last year (2012) and not during August as it was during August 2003, this should help dispel this particular urban myth that has appeared every July or August since that August 2003 opposition. However, there is an even more 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 information in the data table and the math formulas below. 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 March 5, 2012, Mars was 100,781,093 km (62,622,472 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 other dates, use other distance (km) values from the data table and the math formulas:
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
Click here to go to the Qué tal in the Current Skies web site for more observing information.