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    Re: Loony Baloney
    From: Frank Reed
    Date: 2010 Aug 3, 20:55 -0700

    John Karl, you wrote:
    "But I didn't recognize it as a hoax."

    Just so we're clear, I don't think it qualifies as an actual "hoax" (see below).

    And you added:
    "I just thought it was written by someone who didn't know a darn thing about astronomy."

    See, now here's the funny thing... it's really not far from being literally accurate. There are a few small errors, but mostly it's just misleading. And of course it was news seven years ago, not this year!

    And you asked:
    "Can you remove the whole thing from the List??"

    If you really want me to I will (your post --your choice), but I don't think it will do much harm to leave the file here. Maybe I will modify it so that it refers back to this thread or AT LEAST says that this all happened in 2003. :)

    This particular version of the email forward appears to be very close to the original and maybe it is the original. The folks at Sky and Telescope and many others refer to it as a hoax, but I think that's the wrong word. It's an annoying email forward. The main problem is that people who receive it and forward it mistakenly believe that it applies to THIS August (each August) when it actually referred to August of 2003. It also is distinctly misleading when presented without the backup of an actual human being talking during the slide show. I suspect that this file was originally used as a multimedia tool to accompany a live lecture or presentation. Some of its apparent hyperbole may have been toned down by the live comments.

    Within the above context, and given that it was intended to be read seven years ago, I think you might be surprised by the fact that it's not actually inaccurate of "cringe-worthy". It starts out with "guess no one will get much sleep in August". I was saying much the same thing back in August of 2003... to amateur astronomers! Every astronomy enthusiast with access to a telescope spent every clear evening for two months or more centered around the actual opposition date staying up late and observing Mars. It was indeed "spectacular" just as the file said, in that sense of "spectacular" that astronomers use ;). The file says it was also the "closest approach in recorded history." That's true! The file also says that astronomers can't be sure when it was last (or will next be) this close because of the way "Jupiter's gravity tugs on Mars and perturbs its orbit". This is a clue as to when the file was created. There was some brief confusion in astronomical circles in late 2002 over the exact numbers here because the difference we're dealing with is quite small. Mars was significantly closer to the Earth in August of 2003 than it had been in the previous 18 years, noticeably closer than in the previous century, and very slightly closer than it had been in 5,000 years and just a tiny bit closer than it had been in over 50,000 years. These last two were "wow factor" statistics, but the important point was that it would be the best view of Mars for nearly two decades (because Mars has a significantly eccentric orbit). The confusion, by the way, was not due to some uncertainty in the perturbations of Jupiter. Jupiter IS the main perturbing influence on the orbits of the Earth and Mars, but that's not really the issue. It was simply the choice of proper astronomical ephemerides for periods going back tens of thousands of years. In fact, it really was over 50,000 years, by a margin of a very few miles (out of millions). Some early accounts of this close approach mentioned 5,000 years. Other claimed 58,000 years. It turns out the latter was correct, and Jean Meeus (a name familiar to many NavList members) was among those who investigated the exact details and confirmed that the JPL ephemeris data showed that it really had been over 50,000 years since Mars had been so close to the Earth... by a very small margin.

    The file next says that Mars will be the brightest object in the sky, second only to the Moon, and gives some other statistics, all of which appear to be close to the facts (though I haven't checked how close). But here's where things start to become seriously misleading without a human lecturer accompanying the file. It says "At a modest 75-power magnification..." [then continued on the next slide for dramatic effect] "Mars will look as large as the full moon to the naked eye." Now here's the trick part: that is true... In August 2003, if you looked at Mars THROUGH A TELESCOPE with a modest magnification of 75x, the image of Mars had the same apparent angular size as the Moon when seen in the sky with the naked eye (about 30 minutes of arc, of course). So if you can see thirty to forty "pixels" across the face of the Full Moon when you look at it without a telescope (corrected with eyeglasses), then you would see the same amount of "pixel" detail across the face of Mars seen through a small telescope with very modest magnification. That's much better than normal with Mars, and it's why Mars observers salivate over these once every two decades perihelion conjunctions and especially this very favorable one in 2003. A BIG problem with this particular email forward is that the comparison between the view through a telescope of Mars and the naked eye view of the Moon got lost in translation, and many people unversed in astronomy reading the email forward interpret this to mean that Mars would appear the same size as the Moon in the sky. On the next page, giving instructions for observing times, the author makes his first actual error and uses the word "azimuth" incorrectly. Other than that, the file is well-intentioned and really not inaccurate at all (for the year 2003!), though again, it is easily misunderstood regarding the issue of apparent angular diameters. It ends with advice to "share this with your children and grandchildren" which may offer a clue regarding the original audience for this presentation (retired amateur astronomy group?).


    Finally, while we're at it, suppose Mars really was as big as the Moon in the sky (same apparent angular diameter). What would be the effect on the tides? The tidal acceleration is proportional to M*x/R^3 where M is the mass of the tide-generating object (Sun, Moon, Mars, or other body), x is the distance over which the tides acts (width of the oceans, e.g.) and R is the distance to the object. Now we can replace the mass by the mean density multiplied by the volume. Assuming a nearly spherical object, that means M=(4/3)*pi*rho*r^3 where rho is the mean density of the object and r is the radius of the object. The tidal acceleration, dropping x since we're comparing similar circumstances, is then proportional to rho*(r/R)^3 and for cases where r/R is less than about 0.1 (in other words for any object farther away than about ten times its own radius), this ratio r/R is simply the angular diameter of the object so the tidal acceleration, finally, is
    rho*theta^3
    where theta is the angular diameter. So picture two objects in the sky with the same angular diameter. Their relative tidal accelerations are just rho1/rho2, the ratio of their mean densities (the theta's cancel out). That's why the tides from the Sun in the oceans are only somewhat smaller than the tides from the Moon. So if Mars REALLY WERE visible in our sky with the same angular diameter as the Moon it would not tear the oceans from their beds or cause earthquakes. At most, it would increase the height of the ocean tides by a little more than a factor of two (adding its effect onto the Moon's tides) or it might even cancel them out if the Moon was 90 degrees from this super-Mars. In the real world, Mars never gets bigger than ONE minute of arc in the sky (unmagnified) so the tides generated in the Earth's oceans by Mars are smaller than the common lunar tides by a factor larger than 30^3 or 27,000 --completely unmeasurable. This also implies that the largest planetary tidal influence, still extremely small, comes from Venus (not Jupiter which one might expect).

    -FER

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