Published: 2009-06-08 10:12:00
Updated: 2009-06-09 06:44:45
Posted June 8, 2009 10:12 a.m. EDT
Updated June 9, 2009 6:44 a.m. EDT
Maybe you've received it by now, maybe not, but we've gotten several inquiries in the past month or two from viewers who received an e-mailed PowerPoint file all about Mars making a close approach to earth in August, and implying that the red planet will appear as big as the full moon when it does.
If you receive the message, don't be taken in. There is some truth in the presentation, but it applied to an event that occurred in late August 2003, not in any of the Augusts since then. Nonetheless, the e-mail seems to get passed around anew each spring and early summer.
The basic subject of the message is the occurrence of a Mars "opposition," meaning that Mars and Earth reach points in their respective orbits that place the earth more or less directly in between Mars and the Sun, resulting in a minimum distance between the two planets. Of course, at some later time, the two planets reach a point where the sun is on a line directly between Earth and Mars, at which time they are at a maximum distance apart. Since both planets follow orbits that are slightly elliptical rather than circular, the minimum distance each time an opposition occurs (roughly every 2 years and 7 weeks) can vary significantly.
In 2003, that distance fell to about 34.7 million miles during the late August opposition, resulting in Mars appearing as an especially bright point of reddish light in the sky, due to the combination of being relatively close and also being "full" so that the entire side of Mars facing our planet was lit by the sun. It was a beautiful sight, and I remember well going out to see it on a pier over at Pamlico Beach.
So far, so good. However, the e-mail that keeps making the rounds slyly states on one slide that "At a modest magnification of 75 times..." and then the next slide shows side by side images of Mars and the Moon, along with a label saying "Mars will appear as big as the full moon to the naked eye." Understandably, a lot of people seem to flip through the presentation and miss the part about magnification, and are then disappointed to find that Mars doesn't remotely appear as large as the moon to the unaided eye.
In late August 2003, the angular diameter of Mars as seen from here grew as large as 25.1 arcseconds. By comparison, the sun and moon are both around one-half degree of arc on average, or 1800 arcseconds. At the time of the August opposition, the moon was about 1890 arcseconds, so indeed if you looked at Mars through a 75X telescope and then looked at the Moon with the naked eye, they would appear about the same size, but that is kind of a meaningless comparison, a bit like saying "if you look at a dime through a magnifying glass, it will appear as big as a silver dollar to the naked eye!" Well, sure, but...
I used a very nice astronomical calculations site called HORIZONS provided by NASA's Jet Propulsion Laboratory to dig up some additional numbers for Mars and the Moon that apply to late August of this year (2009) and found that during that time, Mars will be about 151.8 million miles away from us, with an angular diameter of 5.7 arcseconds, while the moon will be around 1830 arcseconds. So, if you want Mars to appear as large in a telescope as the full moon to the naked eye, a 320X scope should do the trick!
One final note about the Mars e-mail is that it notes that it could be 60,000 years before we are as close to Mars again and before Mars looks as large and bright again. This may be true, but it is worth noting that while there will be an opposition in late January 2010 in which Mars' angular diameter will only grow as large as 14.1 arcseconds, there will be one in late July 2018 in which it reaches 24.3 arcseconds and should appear very nearly as bright and large as it did back in 2003.