View Full Version : Has Mars gone on a Diet?

06-16-2010, 01:31 AM

I just wrote my first wiki article for the site... and wouldn't you know it I have to contradict Edgar Rice Burroughs himself!

Do any of you know if the perceptions of Mars' size has changed in the last 100 years? Burroughs used as his base measurement of distance, 1 degree of longitude for Mars... However he based it on a planet that was 15,947 miles in circumference... and Mars is about 13,300.

Is it possible that in the early 1900s we thought the planet was almost 2000 miles wider than it actually is? I was unable to google-fu the answer to this.

(curious fans can see my site, and the page in question at:

06-16-2010, 01:48 AM
It sounds possible, though i have no idea if it's more likely than Burroughs simply being mistaken.
Hm, and the only idea i can come up with to confirm this is to find some astronomy textbook from 1900.

06-16-2010, 08:49 AM
The 1911 Britannica (http://www.1911encyclopedia.org/Planet) gives the diameter of Mars as 4,316 miles, which would make the circumference 13,559 miles, only about 300 miles too large.

06-16-2010, 05:36 PM
And that makes sense -- because back then they wouldn't have known about that whole "angular size" thing (planets may appear a little smaller or larger depending on how it's rotated toward us at the time of observation) -- I don't think the exact circumference was known until the first satellite got there.

Ok. Mystery solved maybe.

07-05-2010, 11:53 AM
That looks like a case of measurement imprecision.

First, Mars's radius in kilometers (1 mi = 1.609344 km)

Present-day values, from Mars Fact Sheet (http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html) (NASA)

Equatorial: 3396.2
Polar: 3376.2
Average: 3389.5

Now the other values and their errors:
ERB: 4290 +27%
Brit: 3473 +2%
OP: 3578 +6%
dclary, the number you used is a bit off, though that doesn't explain ERB's excessively high figure.

The Britannica value is very close to the present-day value, meaning that I don't have a clue why ERB's value is off. Did he use some other estimate?


Mars's radius was not measured directly, but as a combination of two other quantities, the distance from the Earth to the Sun (Astronomical unit - Wikipedia (http://en.wikipedia.org/wiki/Astronomical_unit)), and Mars's angular size.

About that distance, astronomer Simon Newcomb in 1895 came up with a figure that was too small by 0.1%.

So the main error would come in measurements of Mars's angular size. Its maximum observed angular diameter is about 25.1 seconds of arc, so it's likely that that value was used to determine Mars's linear dimensions. The Britannica value is thus 0.5" too large, which seems plausible - it's hard to get much better than 1" with a ground-based telescope.