APOD: Retrograde Mars (2010 Jun 13)

[APOD Robot]

Retrograde Mars

Explanation: Why would Mars appear to move backwards? Most of the time, the apparent motion of Mars in Earth's sky is in one direction, slow but steady in front of the far distant stars. About every two years, however, the Earth passes Mars as they orbit around the Sun. During the most recent such pass late last year and early this year, Mars as usual, loomed large and bright. Also during this time, Mars appeared to move backwards in the sky, a phenomenon called retrograde motion. Pictured above is a series of images digitally stacked so that all of the stars images coincide. Here, Mars appears to trace out a loop in the sky. At the center of the loop, Earth passed Mars and the retrograde motion was the highest. Retrograde motion can also be seen for other Solar System planets.

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[biddie67]

Amazing picture - what an effort went into capturing the complete sequence!

While I understand the cause and effect of the retrograde motion, what I don't understand is why the "loop" isn't in the same plane as the rest of Mars' path .....

[Ann]

Thanks to Tycho Brahe's careful observations of the path of Mars across the sky, Johannes Kepler was able to show that Mars follows an elliptical orbit around the Sun.



Tycho Brahe. Note the fake nose. Tycho Brahe wore a prosthetic nose after fighting a duel in his youth.



Johannes Kepler. Lately there has been some speculation that Kepler may have murdered Tycho Brahe in order to get his hands on Brahe's documentations of his observations, which the Danish master didn't want to part with.

Ann

[rwarner]

I love this picture. The amount of work that went into it is obvious to those of us who have attempted such things. But the best thing to me is how it immediately teaches the concept.

[rwarner]

Amazing picture - what an effort went into capturing the complete sequence!

While I understand the cause and effect of the retrograde motion, what I don't understand is why the "loop" isn't in the same plane as the rest of Mars' path .....

I always assumed it to be due to the inclination of Mars' orbit vs ours, but would like to hear from the pros on that subject also.

[hstarbuck]

It is my humble opinion that if one were to remove all prior knowledge of the solar system and look to the skies regularly, that discerning the characteristics of the actual motion of the planets around the sun from apparent motion, even with cameras and telescopes, is amazingly difficult and beyond a single person. As a possible starting point one would not necessarily know the nature of the solar system initially but maybe starting with planets being wandering stars.

[NeoPod]

Why does there seem to be a slight "zoom blur" on the other stars in the sky? They all seem to be moving/blurred radially from the center of the image.

[orin stepanek]

Why does there seem to be a slight "zoom blur" on the other stars in the sky? They all seem to be moving/blurred radially from the center of the image.

I noticed that also; when I clicled on the larger picture. It almost looks like all the stars have comet tails. I'm guessing it is caused by the multi-exposure while following Mars; but I would have expected star trails instead. Actually as the stars location changes from day to day; there shouldn't be trails but extra stars in the picture.

[rstevenson]

Why does there seem to be a slight "zoom blur" on the other stars in the sky? They all seem to be moving/blurred radially from the center of the image.

I noticed that also; when I clicled on the larger picture. It almost looks like all the stars have comet tails. I'm guessing it is caused by the multi-exposure while following Mars; but I would have expected star trails instead. Actually as the stars location changes from day to day; there shouldn't be trails but extra stars in the picture.

There are 42 images of Mars as it moved across the sky, taken on 42 nights from Oct 2nd/09 to June 5th/10. The stars, over such a short period of time, are an unmoving backdrop so they won't show trails. Except the camera is down at the bottom of a jiggly atmosphere and on a moving platform (the Earth), so I'd say it's bloody amazing that the stars are as cleanly imaged as they are. Great image!

Rob

[Ann]

Rob said:

I'd say it's bloody amazing that the stars are as cleanly imaged as they are. Great image!

I agree!

As a color freak, I'd like you to pay attention to the little "ultra-red" star below the loop of Mars (just below the 11/11 position of Mars, if you look at the annotated image (slide your cursor over the image and there it is). The little red star is a carbon star, reddened by the carbon it has dumped in its own atmosphere. It's like looking at the Sun as it's hanging just above the horizon and being reddened because its rays have to travel an extra-long distance through the atmosphere.

http://blog.lib.umn.edu/straw020/test/sunset.jpg

A reddened Sun on the horizon - it looks just like a carbon star!

Ann

[rwarner]

I'd say it's bloody amazing that the stars are as cleanly imaged as they are. Great image!

The stars are so clean that I think he used a single background image and worked in multiple layers to place Mars. That's how I have done it while photographing meteor showers and time-lapse imaging of planetary motion. Ronnie

[orin stepanek]

There are 42 images of Mars as it moved across the sky, taken on 42 nights from Oct 2nd/09 to June 5th/10. The stars, over such a short period of time, are an unmoving backdrop so they won't show trails. Except the camera is down at the bottom of a jiggly atmosphere and on a moving platform (the Earth), so I'd say it's bloody amazing that the stars are as cleanly imaged as they are. Great image!

Rob

OK but: as the Earth orbits the sun the backdrop of stars are different from our view. October to June is 7 months. So shouldn't the backdrop of stars change between snapshots? Just follow Orion; a winter constellation; You can see it change positions in the sky as the season passes until it is on the other side of the sun.

[orin stepanek]

The stars are so clean that I think he used a single background image and worked in multiple layers to place Mars. That's how I have done it while photographing meteor showers and time-lapse imaging of planetary motion. Ronnie

I believe that is how it had to be done. Thanks.

[Chris Peterson]

It is my humble opinion that if one were to remove all prior knowledge of the solar system and look to the skies regularly, that discerning the characteristics of the actual motion of the planets around the sun from apparent motion, even with cameras and telescopes, is amazingly difficult and beyond a single person. As a possible starting point one would not necessarily know the nature of the solar system initially but maybe starting with planets being wandering stars.

I think history demonstrates that the actual motion of the planets is not beyond the grasp of a single person. In fact, the concept isn't difficult at all, but it required a long time before somebody made the jump. The basic system is easily understood once you decide to place the Sun in the center of the system, and once the scientific method was adopted, this followed quickly. Also not terribly difficult, but requiring more detailed observations, are the basic orbital laws derived by Kepler (and others).

The length of time it took for the nature of the Solar System to be worked out is a testament to the power of the scientific method. In the absence of that technique, people were simply locked into a dogmatic rut. Once science was applied to the problem (around Galileo's time), the rest fell into place very quickly. It didn't require better minds, just a better way of thinking.

[Beyond]

I think that this Apod picture could be called -- Strand of Pearls.

Ann, how do you know that the red spot under 11/11 is a carbon star and not just the redshift of a star that is going away from us?
Are there a lot of carbon stars that would be mistaken for stars traveling away from us?

[zbvhs]

The ancients discovered retrograde motion using simple instruments. It's what you see when the Earth is the basis for measurement. Ptolemy devised an empirical method for predicting the motion of the planets using epicycles. It was quite accurate and was still in use at the time of Johannes Kepler. Copernicus switched to a Sun-based system and still had to use epicycles in his theory because he didn't know about orbital eccentricity - which Kepler later discovered.

[Chris Peterson]

Ann, how do you know that the red spot under 11/11 is a carbon star and not just the redshift of a star that is going away from us?

The star is SAO 98230, a magnitude 6 C6II (carbon) star (variable from 5.6 to 7.5 with a 195 day period). There are no stars which are redshifted enough to appear a different color to our eyes. That's because the only stars we can see are far too close for cosmological redshift to be a factor, and none are moving more than a few hundred kilometers per second relative to us, so Doppler redshift is also very small. Even if we could see redshifted stars, there's no assurance they would appear red, or more red than their unshifted appearance. It would all depend on their spectral characteristics. While there would be energy shifted into the red range, there might be even more blue or UV shifted into the middle or short wavelengths. So a redshifted star might appear bluer or whiter.

Are there a lot of carbon stars that would be mistaken for stars traveling away from us?

No, for the reasons given above. There are many carbon stars, but not so many as bright as this one- bright enough to appear intensely red through a telescope. BTW, the red of carbon stars is only partly related to the scattering of shorter wavelengths (as in a red sunset). These stars are very cool (<3000K), so their blackbody peak is in the red or IR, and the C2 component of their atmosphere has some strong peaks in the middle and long wave parts of the visible spectrum. So you are starting with something quite red, and then passing it through a sooty atmosphere (and likely a dusty shell, as well) further reddens the light. These stars are very striking telescopically.

[Beyond]

Thanks chris.
So IF i ever get rich enought to get a decent telescope and look through it, i will be able to tell by the red color which stars are the carbon stars and i will have to leave the red-shift stuff to the guys with the computers and BIG telescopes. OK, i can do that.

[Chris Peterson]

So IF i ever get rich enought to get a decent telescope and look through it, i will be able to tell by the red color which stars are the carbon stars and i will have to leave the red-shift stuff to the guys with the computers and BIG telescopes. OK, i can do that.

No need to get rich. A carbon star like the one in this image is easily seen with nothing more than binoculars. Just look for the Beehive cluster (M44) and the carbon star will be in the same field. It's getting pretty far west, so you need to do this just after it gets reasonably dark, or wait a few months to see it again in the predawn sky.

Even the biggest telescopes can't see individual stars at cosmological distances, except for the brief time they supernova. Cosmological redshift measurements are generally reserved for entire galaxies, quasars, and similar bright objects.

[Beyond]

Thanks chris. It will probably take me a few months to locate the beehive cluster anyway. Except for the BIG Dipper and the three stars in a row that i think are Orion's Belt, i just do not see things in the sky like other people do.

[Ua41]

There are 42 images of Mars as it moved across the sky, taken on 42 nights from Oct 2nd/09 to June 5th/10. The stars, over such a short period of time, are an unmoving backdrop so they won't show trails. Except the camera is down at the bottom of a jiggly atmosphere and on a moving platform (the Earth), so I'd say it's bloody amazing that the stars are as cleanly imaged as they are. Great image!

Rob

OK but: as the Earth orbits the sun the backdrop of stars are different from our view. October to June is 7 months. So shouldn't the backdrop of stars change between snapshots? Just follow Orion; a winter constellation; You can see it change positions in the sky as the season passes until it is on the other side of the sun.

The images are stacked onto each other so that the stars are all aligned. They are the reference point for the retrograding mars

[orin stepanek]

Why does there seem to be a slight "zoom blur" on the other stars in the sky? They all seem to be moving/blurred radially from the center of the image.

I noticed that also; when I clicled on the larger picture. It almost looks like all the stars have comet tails.

The images are stacked onto each other so that the stars are all aligned. They are the reference point for the retrograding mars

Hi! Ua41 How are you? Actually what started all this was what caused the fuzziness when the enlarged photo came up! I'm pretty satisfied with the answer rwarner gave about how the picture was made; unless that isn't correct either.

I'd say it's bloody amazing that the stars are as cleanly imaged as they are. Great image!

The stars are so clean that I think he used a single background image and worked in multiple layers to place Mars. That's how I have done it while photographing meteor showers and time-lapse imaging of planetary motion. Ronnie