© David Le Conte
For several years emails have been circulating falsely claiming that on 27 August of each year Mars would be closer to the Earth than it has been for thousands of years, and would appear as big as the Moon. Another will probably do its rounds this year. There is even a totally misleading PowerPoint presentation.
It is gross misinformation – probably a hoax, though what the motivation is for it I cannot guess. Whoever perpetrates this nonsense cannot even get the year right, as the event to which it refers actually happened in 2003, although Mars certainly did not (and will not, and cannot) appear as large as the Moon.
In August 2003 Mars was indeed (marginally) closer to the Earth than it had been for thousands of years, being about 56 million kilometres (34.6 million miles) from the Earth. It was given a lot of publicity at the time, and, in common with many astronomical societies, we had open evenings at the Guernsey Observatory to view the planet. Its angular size then was 25 arc-seconds, compared with the Moon’s 30 arc-minutes. In other words, Mars appeared one-75th of the angular diameter of the Moon. That is about as large as we ever see Mars, and occurs when it is at ‘opposition’, ie on the same side of the Sun as the Earth is. The Earth moves faster than Mars in its orbit around the Sun, and so regularly overtakes Mars, with over two years between each of these oppositions.
The table shows the dates and distances of Mars at oppositions for the first thirty years of the 21st century. The next best opposition will be in 2018.
|Date||Distance (km x 106)||Distance (miles x 106)||Angular diameter (arc-seconds)|
|2001 June 13||68||42||21|
|2003 August 28||56||35||25|
|2005 November 07||70||43||20|
|2007 December 24||89||55||16|
|2010 January 29||99||62||14|
|2012 March 03||101||63||14|
|2014 April 08||93||58||15|
|2016 May 22||76||47||18|
|2018 July 27||58||36||24|
|2020 October 13||63||39||22|
|2022 December 08||82||51||17|
|2025 January 16||96||60||15|
|2027 February 19||101||63||14|
|2029 March 25||97||60||14|
The reason why the distance of Mars changes with each opposition is because of the relationship between its orbit and that of the Earth. Mars’s orbit has an eccentricity twice Earth’s. The diagram shows the 2005 opposition (source: StarryNight). You can see that as the two planets follow their paths around the Sun they periodically meet up, at separations which depend on their relative positions in their orbits.
The “hoax” may, of course be someone's totally mistaken interpretation of angular diameter, although the fact that it is perpetrated year after year implies a mischievous intent. It is possible that someone originally has confused arc-seconds (1/3600 degree) with arc-minutes (1/60 degree). The most common explanation of the hoax, however, is that it arises because of the omission of the fact that to magnify the angular diameter of Mars to that of the Moon in August 2003 would have required a telescope and a 75-magnification eyepiece.
However, common sense and simple mathematics shows that Mars (diameter 6794, closest distance 55 million km) must always appear much smaller than the Moon (diameter 3476 km, average distance 384,400 km)!
There are several web sites explaining this hoax. For example, see:
© David Le Conte, Fellow of the Royal Astronomical Society August 2007 (revised January 2008, October 2008, April 2009, June 2009, July 2010, October 2011, October 2013) Acknowledgements to SkyMap Pro and StarryNight