HOAX: Two moons 27th August 2010
The annual “Two Moons on Aug 27th” Mars e-mail is circulating again. Alas, the myth is another internet hoax, a best fit for the “liar liar pants on fire” category. Amateur astronomers already know this. This year, for the benefit of everybody else, we do our best to explain why.
A picture is worth a thousand words. See: http://www.astronomy.org.gg/hoax.htm
The photo above is also true to my own experience, as I’ll narrate below.
Wikipedia on the “Mars Hoax” (emphasis mine):
Although nearly all of the claims made in the e-mail are true, the hoax stemmed from a misinterpretation of the third sentence of the second paragraph which states that “At a modest 75-power magnification Mars will look as large as the full moon to the naked eye”. The message was often quoted with a line break in the middle of this sentence, leading some readers to mistakenly believe that Mars would “look as large as a full moon to the naked eye” when, in reality, this only applies when a telescope with a 75-power magnification is used. This is the most likely source of misinterpretation.
We will never, EVER see a sight even remotely like the faked “two moons” e-mail image from Earth (or from anywhere else in the solar system). And 2010 is not even a particularly good year for telescopic viewing of Mars.
We already had Mars’ 2010 “closest approach” in January . Most non-astronomer citizens never would have noticed it. Phoenix and Bay area residents would probably be unable to see it with the naked eye unless it was an exceptionally clear night.
We actually have mathematical “closest approaches” every other year or so (Mars takes 687 Earth “days” to orbit the sun). Obviously, since both planets orbit the sun, there is always going to be some “closest” distance as the Earth swings round past Mars. That distance is not the same each year because the orbits if the two planets are not quite concentric, but elliptical — not quite perfect circles. Mars’ orbit is quite eccentric for a planet – about 9% longer on the long axis compared to the short dimension.
In 2003, we had the celebrated closest Mars approach “in 60,000 years”. Astronomers would have noticed Mars having an apparent diameter of almost twice its “farthest distance”. This difference doesn’t become readily apparent without a telescope of at least 6″ diameter.
The angular size of the Moon is about 1/2 degree (30 ARC MINUTES). By coincidence our Sun is of the same apparent diameter, which is why we can have perfect lunar eclipses. NASA confirms the angular size of Mars varies from a minimum of 3.4 ARC SECONDS to a maximum of 25.1 ARC SECONDS.
An arc second is 1/60 of an arc minute. Mars never stood a chance of looking close in size to the Moon!
From University of Wisconsin:
Even at its closest approaches Mars seldom appears larger than 26 arc seconds, or about 1/69 the apparent size of the moon.
Some sources compare this to the apparent size of a penny at 500 feet.
We all know from experience the Moon NEVER looks about the size of a penny at 500 feet. The Moon might look more like the size of a basketball at 100 feet. So how could Mars ever look like it was almost the same size? It can’t.
If that isn’t bad enough for backyard astronomers, more math (groan) conspires against us too. Remember that the area of a circle is proportional to the square of the radius. So, a planet of radius 1/2 will only display 1/4 the surface detail of a planet of radius 1, all other things being equal. A planet of radius 1/60 can, at best, display 1/3600 the surface detail of the larger one — not counting the distorting effects of the Earth’s atmosphere!
The 2003 Mars approach was a HUGE disappointment to Bob and me, and we (I) wasted a good deal of money trying to be ready to photograph this highly publicized event.
As actually viewed with the naked eye in 2003, it was hard to tell whether any kind of “disk” of Mars could be made out at all, or if Mars was just a really bright reddish point-source star like Aldebaran. With eye to our telescope eyepiece, we were barely able to see Mars’ polar icecap, but that was all. Bob and I tried to photograph Mars through the 8″ telescope with an expensive SLR camera body, without much success as we were inexperienced in photographing the night sky. Bob did the best job, holding up a “Brownie point and shoot” to the telescope eyepiece.
Our photo looked about like a penny at 500 feet. Copper-red, no surface detail visible at all. Unless you blow up the image (below), Mars looks like a little red dot in a huge black frame. This photo has been published here before. For a better color photo (looks like black-and-white), see also our Mars Elusive post (9-17-2003).
The next “good” year for viewing Mars will be 2014, and it won’t get as good as 2003 again during our lifetimes.