Thursday, 27 August 2009

Mars as large as the Moon?

This popular urban legend is also known as Two Moons.

The brief answer is, no, Mars will not look as big as the Moon.

Every year astronomers, including the one writing this article, get this question; and every year we have to debunk it. People's memories are short while the orbit of Mars is pretty stable.

Apparently this strange hoax first surfaced on the Internet back in 2003. The relevant e-mail went something like this:

The Red Planet is about to be spectacular! 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

Mars will look as large as the full moon to the naked eye. 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 12:30 a.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 month. Share this with your children and grandchildren. NO ONE ALIVE TODAY WILL EVER SEE THIS AGAIN

Or the Greek version (which actually gives an incorrect year - the original doesn't):

Αυτό γίνεται μόνο μια φορά στην ζωή μας
Την 27η Αυγούστου 2009, 30 λεπτά μετά τα μεσάνυκτα, κοιτάξτε στον ουρανό.
Ο πλανήτης Άρης θα είναι πολύ λαμπερός μέσα στον ουρανό
Θα είναι το ίδιο μεγάλος όπως και το φεγγάρι παρόλο που ο πλανήτης Άρης θα είναι 34,65 εκατομμύρια μίλια μακριά από την Γή.
Προσπαθήστε λοιπόν να μην χάσετε αυτό το γεγονός
Θα το βλέπουμε με γυμνό μάτι σαν να και η γη έχει δύο φεγγάρια!
Η επόμενη φορά που θα λάβει χώρα αυτό το γεγονός θα είναι το έτος 2287.
Μοιραστείτε αυτή την πληροφορία με όλους τους φίλους σας γιατί κανένας ζωντανός δεν θα μπορέσει να το δει για δεύτερη φορά...

This is plainly wrong. Mars isn't going to be making a close approach on August 27. The close approach this e-mail is alluding to happened back in 2003. It did indeed get closer than it had in at least 50,000 years, but this was a very small amount. On August 27th, 2003, Mars closed to a distance of only 55,758,006 kilometers (34,646,418 miles). The Moon, by comparison, orbits the Earth at a distance of only 385,000 kilometers (240,000 miles). Mars was close, but it was still 144 times further away than the Moon. The Moon's diameter is 3474 kilometres (2159 miles), a little more than a quarter of that of the Earth while that of Mars is 6,800 km, about half that of the earth.

[Here is a little experiment you can do. Put an orange 114 meters away from you and a golf ball at your feet, lie down and look at them. Do they look about the same size?]

So what happened was this: Instead of appearing like a huge red orb in the sky, Mars looked like a bright red star. Amateur astronomers around the world set up their telescopes, and had a look at this close encounter. But you still needed a telescope and it really didn't look that much different. And everyone was happy; because if Mars did actually come close enough to rival the Moon, its gravity would alter the Earth's orbit and raise terrible tides.

See what NASA has to say about this hoax.


Macko Usko said...

duly noted, thanks for the clarification, interesting