Friday, August 27, 2010

Mars-as-big-as-the-moon hoax

Update Aug 27 2015:  This hoax is still making the rounds,  even though Mars is not very close this year!  Amazing!  

In August 2015,  Mars rises at or just before dawn.  It is not visible for most of the night,  and is dim enough to not be visible in the pre-dawn brightening sky.  

At no time is it ever more than a single point of light to the naked eye.  It takes a 75-power or higher telescope for it to show a disk comparable to the naked eye moon.  Period.  End of discussion.

For details,  read my original hoax-debunk article from 8-27-2010:

For the last several years I keep getting email forwards that are almost identical in their wording, just dressed up differently by different folks. Some have photoshopped illustrations. Most read just about like this one recently sent by a good friend:

27th Aug the Whole World is waiting for.............
Planet Mars will be the brightest in the night sky starting August. It will look as large as the full moon to the naked eye. This will cultivate on Aug. 27 when Mars comes within 34.65M miles off earth. Be sure to watch the sky on Aug. 27 12:30 am. It will look like the earth has 2 moons. The next time Mars may come this close is in 2287. Share this with your friends as NO ONE ALIVE TODAY will ever see it again.

Here is the text of my reply, which is based on cold hard numbers. The source data for this are the average distance of the moon from Earth (240,000 miles), and the average distances of Earth and Mars from the sun: 93,000,000 and 140,000,000 miles, respectively. The diameters of the moon and Mars are 2160 and 4200 miles, respectively.

Oops, this is a reprise of something incorrect that has been circulating since the 35 million mile unusually-close approach opposition of 2003.

The usual average opposition distance is around 60 M miles, and this happens crudely every 2 years.

Figure it yourself: Mars can never look to be comparable in size to the moon, to the naked eye, no matter what opposition distance you use. To first order, the angle subtended by the moon is its diameter divided by its distance, or 2160 miles/240,000 miles = .009 radians = .516 degrees = 31 minutes of arc. This is using the arc length formula, and ignoring arc curvature to use diameter instead of arc length.

Whether you figure that angle from 35 M or 60 M miles, makes little real difference. The actual diameter of Mars is right at 4200 miles. The subtended angle is as large as 4200 miles / 35,000,000 miles = .00012 radians = .0069 degrees = .413 minutes = 25 seconds of arc. It is more often 4200 miles / 60,000,000 miles = .000070 radians = .0040 degrees = .241 minutes = 14 sec of arc.

That's at least 75 times, and more commonly 129 times, smaller apparent diameter in the sky. And those figures are for closest approach at opposition. The most common viewing distance to Mars is closer to 140 million miles, and the worst case is close to 230 million miles. Those angles are .000030 radians = .0017 degrees = .103 minutes = 6.2 seconds, and .000018 radians = .00105 degrees = .0628 minutes = 3.8 seconds of arc, respectively.

That most common distance makes Mars appear 301 times smaller than the moon, and the worst case is 494 times smaller. And that is why the moon looks to us like a disk, while Mars is never more than a single tiny point of light, in the sky.

No comments:

Post a Comment