Earth observed from Mars...

After I had published my article on Earth's cloud bands, a fellow astronomer approached my with an interesting thought. "I now know how an observer on Mars would see earth". As far as the cloud bands are concerned, that is correct. However, when observing from Mars, Earth is an inferior planet. It travels between Mars and Sun. Consequently, there are "earth phases" - just like we on earth observe them on mercury and venus. "Full Earth" would only be visible for a view days from Mars (more precisely, it would not be visitble at all, because it would be hidden by the Sun).

I like things I can see. Thus, I have create an animation with the astronomy-program "Redshift 6". It tells  how earth phases look to our Mars based observer. Unfortunately, I could not include the actual cloud bands: first, I do not have the satellite imagery necessary and secondly I have no program to do the job. Redshift itself supports a cloud simulation. However, I noticed that this "simulation" is as far from reality as it could be. So I decided to simply provide an earth view without any cloud cover. If you are interested in the cloud bands, I recommend opening the actual satellite picture animation side-by-side (click here to open it in a new browser window).

Some data on the animation: It starts on December, 1st 2006 and ends on December, 2nd 2008. Thus, it covers roughly a martian year. A picture was created every 24 (earth) hours at exactly 11a UTC. Ten pictures are combined into 1 second of animation. We are looking from the martian equator. The slight "shaking" of mars results from the movements of mars as well as earth's rotation, which results in a slight angular difference in view. I have not tried to remove that - not the least because I have to admit I do not know how I could remove it ;).

If you follow the animation, you see that Earth gets bigger and smaller. This is realistic. It is a result of the vast distance difference betwen Mars and Earth between inferior or superior conjunction (or opposition and conjuction in "earth view terms") . We see this effect very well when we observe Venus. As "full Venus", she is directly on the other side of the sun. Thus, the full venus is observable, but it is very distant from earth. Consequently, it appears relatively small. The "new Venus", which we can not directly observe, is very close to Earth. If we could see it, it would appear much larger than the "full Venus". In a telescope, you can observe the "big Venus" best the days before and after inferior conjunction. The Venus cresecent is then faint, but you can clearly see how large the disc appears.

I have created an animation to show that, too. Now the observer is located in a space ship and looking into our solar system from a point "high" above the ecliptic plane. I have focused on Earth and Mars and removed all other planets and celestial bodies from the animation. Both planets are 5,000 times magnified. The animation looks like Mars rotates around itself, while Earth does not do that. However, this is an artifact of animation parameters: I have used the same parameters as in the previous animation, thus pictures are taken every 24 hours. Consequently, Earth seems to not rotate (if you watch closely, you'll notice that Earth actually rotates once per earth-year, that's because I have not used sideral days - but that explanation goes to far here). Mars has a different rotational period (roughly 24.6 earth-hours), so its spin is visible. In reality, of course, both Earth and Mars spin around themselves.

The animation shows both the different "Earth phases" as well as the apparent size difference of the visible Earth. Now, do a space trip to Mars in your thoughts. Think about how Earth looks when it comes closer to Mars and then departs from it.

I hope these animations are useful. Please feel free to post any comments, corrections or questions to the forum. I promise to follow up on each.

Copyright © 2007 Rainer Gerhards
Animation: Rainer Gerhards (based on astronomy software RedShift 6)
Last Updated: 2007-07-02