Martian Bust

The Bad Astronomer once again says, “Mars. Moon. No.”  Once again, the junk chain email traffic going ’round the internets includes the claim that Mars will be as big as the full moon in August.  Although the BA and numerous others have debunked this claim time and time again, I decided to go ahead and work through the debunking myself when asked about it, rather than just forward on a link.  The email in question made its way to a relative of Tim. She then sent it to him and asked him to forward it to me for an explanation, since he’s now dating a skeptical astronomer and all.

The email comes in the form of a powerpoint presentation which I’ll try and recreate here.

Slide 1:

Check it out, guess no one will get much sleep in August.

  • The Red Planet is about to be spectacular!

Slide 2:

  • This month and next, Earth is catching up with Mars in an  encounter that will culminate in the closest  approach between the two planets in recorded  history.
  • The next time Mars may come this close is in 2287.
  • Due to the way Jupiter’s gravity tugs on
    Mars and perturbs its orbit, astronomers can only be
    certain that Mars has not come this close to Earth
    in the Last 5,000 years, but it may be as long as
    60,000 years before it happens again.

    The encounter will culminate on August 27th when
    Mars comes to within 34,649,589 miles of Earth and

    will be (next to the moon) the brightest object in
    the night sky. It will attain a magnitude of -2.9
    and will appear 25.11 arc seconds wide.
  • At a modest 75-power magnification

Slide 3:

Mars will look as large as the full moon to the naked eye.

Slide 4:

Mars will be easy to spot. At the
beginning of August it will rise in the east at 10p.m.
and reach its azimuth at about 3 a.m.

By the end of August when the two planets are
closest, Mars will rise at nightfall and reach its
highest point in the sky at 12:30a.m. That’s pretty
convenient to see something that no human being has
seen in recorded history. So, mark your calendar at
the beginning of August to see Mars grow
progressively brighter and brighter throughout the

Slide 5:

Share this with your children and grandchildren.


Phew ok, sorry for the colors… moving on to my response!

“No, Mars will never be as big or bright as the full moon, sadly, to the naked eye.  Unless someone moved it!  I’ve seen this before, and there is some truth there to go through…

Mars went through a very close approach to Earth in August of 2003, and I believe this email started that year.  I was at Lyco* at the time and it was a WONDERFUL time to take out the telescopes and see Mars.  It looked just like a bright red star in the sky, but through a modest telescope, you could start to see surface features, like the ice caps.  I remember one night, we had 300 people outside the academic center looking through our telescopes!  (That may have been my first outreach activity.)  This closest approach in 50,000 years was quite a bit closer than most close approaches, which are all pretty spectacular.  But we use the news hype to get people out under the stars if we have to!  You can read more about that event here:

So as you can see, the first parts of this slideshow are true!  Well, for August 2003, anyway. At least up to the point where Mars is 25 arcseconds wide in the sky.  (You can calculate that knowing the size of and distance to Mars.)  An arcsecond is a measure of angle.  Since we can’t take a ruler to the sky to measure the distance between objects, we use angles.  If you hold up your index finger at arms length, the width of your finger is about one degree.  One arcsecond is 1/3600th of that! So 25 arcseconds is really small, certainly not resolvable by the naked eye.**  However, a small telescope and a still atmosphere can get a few arcseconds of resolution, meaning you can start to see features on the disk!  So, Mars cannot be as big as the full Moon in the sky, which is half a degree in width (half your finger width at arms length.  No really, try it!)

Here is where the email runs into a problem.  It implies on the 3rd slide that Mars will be as big as the Moon! We just calculated it and see that’s not the case.  The pictures that are side by side are misleading.  But, I almost overlooked the last line of the second slide… “At 75 power magnification.”  So with a telescope of those specs, that 25 arcsecond disk gets blown up to… half a degree, the size of the full Moon. So what they really want to say is that the size of Mars through a telescope is as big as the full Moon is to the naked eye.  However, the atmosphere is turbulent, and you won’t see a picture that is clear like that nice Hubble shot in the slideshow. What I saw, through our department telescopes, was something like what you see on the right in this image.***

The next slide which talks about the times is fairly correct, based on my memory of the event.  And it is true that it was a spectacular sight!  But the night sky always has something cool.  When I took you up to McCormick****, you got to see the rings of Saturn almost edge on, and later this year they will seem to disappear as they go completely edge on.  Mars oppositions occur every 26 months, and the next one is Jan 2010, but we have to wait until 2050 to see a really close approach like the last one.  That email has yet to die because it makes quite an wonderful claim, although it is misleading, and doesn’t have the year of the the event, so it shows up in force every spring.  But a good observatory, star party, or astronomy outreach program will always have something good to show you.”

As far as I can tell, the originator of this email was trying to do the right thing and spread information about a wonderful event.  And although I show that it is technically making a correct claim, it is extremely, terribly, misleading.  So always check with your friendly local astronomer if you see something like this.  I got the best compliment in her response, “We don’t need Snopes or, we have gugnico.”  *Flattered*

Aaaaaaand… Carnival of Space #106 is up at Next Big Future!

* Lyco = Lycoming College, where Tim and I went to school.
** Actually, it’s not that far off. The angular resolution of the human eye is ~1-2 arcminutes.
*** Not the same “astronoise” as me!
**** McCormick Observatory at the University of Virginia. I had taken Tim up there for a tour with Richard Drumm just prior to this exchange.

3 thoughts on “Martian Bust

Comments are closed.