My Dark Sky

I bet most, if not all, of you must have got this email before…

Two moons on 27 August
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.

So, do you really believe that you will be able to see Mars as large as the Moon on 27 August?

NO! Don’t believe it. It’s wrong. Mars can never be as large as the Moon in our sky!

Since 2003, every year in August, I’m sure to receive this email, and this year is no exception. But I’m happy that those who sent me the email are sceptics. They don’t just accept what the article said; they sent me the email for clarification, to confirm the facts, because they don’t think it’s possible. This is a good sign, when you get some emails that you are doubtful, you must check the facts, ask someone, googled it and not just forward the “sometime can be so wrong” information out to everyone in your address book and “help” to spread the wrong information.

OK, now to the facts.

First, how did all this started?

The story began in August 2003, when Mars came historically very close to Earth, which is the closest approach for the past 60,000 years. How close? 55.76 million km (34.65 million miles) away from Earth. If you think that was not close, consider this: when Mars is on the other side of the Sun from us, it can be as far as 400 million km. And currently (August 2008), it is about 360 million km away.

Naturally, if an object is nearer to us, it’ll look larger. So, during Mars historical closest approach to Earth on 27 August 2003, it, of course, will look largest than any other time. And it’s true that “no one alive today will ever see Mars this large again” because the next time Mars is going to come this close to Earth will be in year 2287.

So, I said large. But how large? As large as the Moon? No, no, no, nooooo… Simple maths will tell us that it is impossible. Mars is only twice the size of our Moon, but at closest approach back in 2003 it was 144 times further away. Since the size of an object seen in the sky is inversely proportional to its distance, Mars will have to be at least 144 times larger than the Moon to look as large.

A very good example is our Sun. Our Sun is the only natural object in the sky that will look as large as the Moon. Although the Sun is about 400 times much further away than the Moon, it has a diameter 400 times of the Moon, therefore the Sun will appear about the same size as the Moon in the sky. And that’s why we can have solar eclipse.

Solar Eclipse on 1 August 2008 taken in China. The black disc is our Moon, blocking the Sun at the back. The size of our Sun and Moon is almost the same in the sky. Credit: William Chin (APGM)

On 27 August 2003, Mars angular size in the sky was 25.11 arc-second. This was very “large” if compared to “normal time” Mars, which can be as small as 10 arc-second or even smaller.

Maybe someone was playing a joke by sending out that email; or maybe someone mess up the figure 25.11 arc-second and the size of a full moon, which is 30 arc-minute. One look at these two figures one may jump into conclusion that since the difference between 25.11 and 30 is small, then Mars should be almost as large as the Moon. Wrong here! Just be careful of the unit, 25.11 arc-SECOND and 30 arc-MINUTE! 1 arc-minute is equal to 60 arc-second. This is just like trying to say 25 mm are almost the same length as 30 cm!

Mars closest approach to Earth happens around Mars Opposition. This is the time, as seen from Earth, when Mars appears at the opposite side of the Sun. And this is also the time when Mars, in its orbit around the Sun, comes close to Earth. And this is also the time when Mars is best placed for observation. Mars Opposition occurs every 2 years and 2 months or every 26 months.

Due to the fact that the orbits of Earth and Mars are not circular and they do not centre exactly on the Sun, the distance between Earth and Mars, and hence the size of Mars, can vary for each opposition. The diagram below from Hubble clearly illustrates this.

Mars at Opposition: This illustration shows the relative positions of Earth and Mars at the last seven oppositions (1995-2007), when the Sun and Mars are on exact opposite sides of Earth. The images of Mars show the planet’s apparent relative size at each opposition, as viewed by Hubble Space Telescope. Illustration credit: Z. Levay (STScl)

Another two Mars Oppositions have past since the “Great Mars Opposition” in August 2003. The next Mars Opposition will be in January 2010.

Hopefully the next time when you receive the Mars Hoax email again, you are confident to sent it back and tell the sender that he/she is wrong and this is only a joke.

You can also read the article I wrote 3 years back on this. The hoax email then was more “details” than the one above.

~ by thChieh on August 22, 2008.

Posted in General Astro Talk
Tags: Mars Hoax