Monday 14 April not only includes the start of a Lunar Eclipse (depending on the time zone), two asteroids at or near opposition, Mars just past its opposition, Comet PanSTAARS 2012 K1 at opposition, Dwarf Planet Pluto begins retrograde motion, and at 8 am CDT (13 UT) Mars will at its closest to the Earth for this particular orbit. At that time the separation between Earth and Mars is 0.618 AU (57,446,689 miles – 92,451,484 km).
A Martian Urban Legend
“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 ever be to the Earth (55,755,723 km – 34,645,000 miles) 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.
What is opposition?
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.
Mars at the April 2014 and May 2016 Oppositions.
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. Since opposition for Mars is during April this year rather than in August as it was for the 2003 opposition, this should dispel the urban myth that appears every July or August since the 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 this year’s closest approach on 14 April and some basic arithmetic. 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 14 April 2014, Mars will be 92,385,661 km (57,405,788 miles) 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 the distance (km) values from the table below and the following 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
Seven oppositions of Mars showing the distance when closest to Earth
Date of opposition Date closest to Earth Distance (AU) Distance (km)
August 28, 2003 August 27, 2003 0.37272 55,758,118
November 7, 2005 October 30, 2005 0.46406 69,422,387
December 24, 2007 December 28, 2007 0.58935 88,165,505
January 29, 2010 January 27, 2010 0.66398 99,329,994
March 3, 2012 March 5, 2012 0.67368 100,781,093
April 8, 2014 April 14, 2014 0.61756 92,385,661
May 22, 2016 May 30, 2016 0.50321 75,279,144
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.