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.
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.