There are two ways to describe the size of an object. One is to give its actual size, in miles, for instance. Mars' actual size is about 4200 miles in diameter. However, unless you are standing on Mars that doesn't really help you understand how big it looks. So astronomers measure size by stating how many degrees in angle you'd have to turn from one side to the other. This is the angular size of the object.
In 2003, Mars made its closest approach to Earth in many thousands of years. At that time Mars had an angular size of about 24 arc-seconds. How big is that? We're familiar with measuring angles in degrees. Ninety degrees is a right angle. However, even one degree is too big to be useful for describing the size of many astronomical objects. A degree is broken up into 60 arc-minutes. Each arc-minute is further divided into 60 arc-seconds. So 1 arc-second is 1/3600 of one degree.
So where did this "as big as the full moon" come from? In 2003, when Mars was 24 arc-seconds in angular size, if it were viewed through a telescope at 75x, it would appear to the observer through the telescope to be the same size as the moon would look without a telescope (the full moon as seen from Earth is about 30 arc-minutes or 1/2 degree in angular size). Someone misunderstood and it snowballed from there.
Where did August 27th come from? That was the day of the closest approach in 2003. However, Mars was close to that angular size for weeks before and after. The "one day only" was another misunderstanding.
The interesting thing is that Mars has a close approach (called an opposition) every 2 years and 2 months, so not only is the hoax extravagant in its claims of size and duration, but it has been around every year, even in years when Mars wasn't visible at all in August.
Such was the case several years ago when my mother called me about the Mars story to ask if I were going to view it. I was curious, since my mother doesn't "do" email. She had read it in her local paper! I won't name the paper, but there was the story, printed straight from the email. Apparently, they staff of that newspaper (and not a hokey one, but a "real" New York newspaper) didn't think to check the source or ask an astronomer before printing the sensational story.
Did I view Mars during that approach in 2003? You betcha. Right from my backyard. But I have to say it was not as spectacular as you might imagine. Next time you are out during a full moon look up. How much can you actually see on the moon? Now imagine a dull orange ball that size, with very faint fuzzy markings, and you'll get an idea of the reality of looking at Mars through a telescope in August 2003. In fact, I've taken an image of Mars and degraded it to look like I remember Mars looking through my telescope in 2003. A far cry from the highly processed images we typically see on the web!
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