La Société Guernesiaise Astronomy Section

Mars hoax

 

In 2007 an email was widely circulated which claimed that on 27 August 2007 Mars would be closer to the Earth than it has been for thousands of years, and would appear as big as the Moon.  No doubt, a similar email will be circulated in 2008.

 

This is nonsense.  It is probably a hoax, though what the motivation is for it I cannot guess.  The same hoax circulated in 2005 and 2006. Whoever perpetrates this misinformation cannot even get the year right, as the event to which it refers actually happened several years ago, although Mars certainly did not (and will not, and cannot) appear as large as the Moon.

 

In August 2003 time Mars was indeed (marginally) closer to the Earth than it had been for thousands of years, being about 34.6 million miles from the Earth.  It was given a lot of publicity at the time; we had open evenings at the Guernsey Observatory to view Mars.  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 Mars 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 last opposition of Mars took place on 24 December 2007, when it was about 55 million miles from the Earth.  The next one will occur on 29 January 2010.

 

On 27 August 2008 Mars will be at a distance of about 220 million miles, and its angular size will be less than 4 arc-seconds.  The Moon will therefore appear over 450 times the diameter of Mars.  Mars will set in the west, just after the Sun, in the constellation Virgo.  The sky will probably be too bright for it to be seen.

 

This “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).  It has also been suggested 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 (in 2003) a 75-magnification eyepiece.

 

However, common sense and simple mathematics shows that Mars (diameter 6794, closest distance 55 million km) must appear much smaller than the Moon (diameter 3476 km, average distance 384,400 km)!

 

There is a number of web sites explaining this hoax.  For example, see:

 

The August Mars hoax is back

 

Mars spectacular

 

 

©  David Le Conte, August 2007 (revised January 2008)

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