APOD: Mars in the Loop (2012 Aug 09)

[APOD Robot]

Mars in the Loop

Explanation: This composite of images spaced some 5 to 7 days apart from late October 2011 (top right) through early July 2012 (bottom left), traces the retrograde motion of ruddy-colored Mars through planet Earth's night sky. To connect the dots in Mars' retrograde loop, just slide your cursor over the picture (and check out this animation). But Mars didn't actually reverse the direction of its orbit. Instead, the apparent backwards motion with respect to the background stars is a reflection of the motion of the Earth itself. Retrograde motion can be seen each time Earth overtakes and laps planets orbiting farther from the Sun, the Earth moving more rapidly through its own relatively close-in orbit. On March 4th, 2012 Mars was opposite the Sun in Earth's sky, near its closest and brightest at the center of this picture. Just arrived on the surface of the Red Planet, the Curiosity rover was launched on November 26, when Mars was near the crossover point of its retrograde loop. Of course, Mars can now be spotted close to Saturn and bright star Spica, near the western horizon after sunset.

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[Beyond]

Be prepared to wait a while for Curiosity's newest pictures. The picture it took of itself finally came up. The 360' panorama gave a server error, JRun.

[Boomer12k]

That must be one of those epicycles they used to talk about....LOL...


:---[===] *

[ta152h0]

gads, how am I going to explain this to a 9 year old tomorrow, err today this afternoon...............I am going to need a parking lot and about three people with flashlights.

[henrystar]

I guess that is Leo in the background?

[Ann]

I guess that is Leo in the background?

Indeed!

Interestingly, it was the orbit of Mars (as measured by Tycho Brahe) that made Johannes Kepler realize that the planets follow elliptical orbits around the Sun. The orbit of Mars is, incidentally, more elliptical than the orbit of the Earth.

Ann

[zbvhs]

Actually, Ptolemy's models mapped things like that pretty well using epicycles for curve-fitting. Copernicus improved things somewhat but he was up against a new bugaboo: orbital ellipticity. It was Newton who finally got the physics right and everything since has been Newtonian with a smattering of Einstein in the details.

[RJN]

Be prepared to wait a while for Curiosity's newest pictures. The picture it took of itself finally came up. The 360' panorama gave a server error, JRun.

Yes. Unexpected. Thanks! I have located alternative links for "360 panorama" and "rover self portrait" which now appear to be quicker to respond. Please try them now. - RJN

[emc]

Another excellent and educational APOD with a likewise explanation.
.noitanalpxe esiwekil a htiw DOPA lanoitacude dna tnellecxe rehtonA

[Chris Peterson]

Interestingly, it was the orbit of Mars (as measured by Tycho Brahe) that made Johannes Kepler realize that the planets follow elliptical orbits around the Sun. The orbit of Mars is, incidentally, more elliptical than the orbit of the Earth.

Although your meaning is clear, it would be better to say "more eccentric" than "more elliptical", as eccentricity is the measure of how out-of-round an ellipse is (and even a circle is actually an ellipse).

[Beyond]

Be prepared to wait a while for Curiosity's newest pictures. The picture it took of itself finally came up. The 360' panorama gave a server error, JRun.

Yes. Unexpected. Thanks! I have located alternative links for "360 panorama" and "rover self portrait" which now appear to be quicker to respond. Please try them now. - RJN

Works 'normal' now. Thanks RJN.

[waterfeller]

Why is the track of the retrograde motion a loop? Doesn't Mars stay in the same plane? I would think the track would be a straight line with overlap during the retrograde period...

[Chris Peterson]

Why is the track of the retrograde motion a loop? Doesn't Mars stay in the same plane? I would think the track would be a straight line with overlap during the retrograde period...

Mars stays in the same plane, and Earth stays in the same plane... but they are different planes. So our relative "up and down" positions change with respect to each other.

[eturc]

Why is the track of the retrograde motion a loop? Doesn't Mars stay in the same plane? I would think the track would be a straight line with overlap during the retrograde period...

Mars stays in the same plane, and Earth stays in the same plane... but they are different planes. So our relative "up and down" positions change with respect to each other.

I had wondered the same thing and whether it had anything to do with where the photographer was on Earth? Are the different planetary planes in our solar system changing/shifting their angles with respect to one another over time?

[Chris Peterson]

I had wondered the same thing and whether it had anything to do with where the photographer was on Earth? Are the different planetary planes in our solar system changing/shifting their angles with respect to one another over time?

The position of the imager on Earth doesn't change the apparent path of Mars.

The planes that planets orbit on change due to precession and perturbations, but these changes have cycles on the order of tens or hundreds of thousands of years, and are thus insignificant in the context of images like today's.

[StarCuriousAero]

The two links "retrograde motion" and "can be seen" REALLY helped me visualize what was going on here exactly, I never quite understood when/for how long the retrograde motion actually occurred (one month didn't seem like long enough). Thanks for those!

[Anthony Barreiro]

This is an awesome picture. The background stars are so clear and sharp. And you can really see how Mars brightened and grew in apparent diameter as it approached opposition and faded and shrank afterwards. The overlay with the dates and star names is helpful, too.

I've been enjoying pointing out Mars, Saturn, and Spica to unsuspecting passersby and encouraging them to watch Mars move in the sky over the next couple of weeks. You can see Mars moving from one evening to the next. It's a teachable moment, how each star stays in the same place in its constellation, while the planets move through the constellations of the zodiac. And Mars, being closer to the Sun than Saturn, moves through space and through the sky much faster. Next up: Ares and Antares!

[ta152h0]

It would be a perfect haperstance if one of your " unsuspecting passerby"s " was a certain Mr Peterson.

[waterfeller]

Why is the track of the retrograde motion a loop? Doesn't Mars stay in the same plane? I would think the track would be a straight line with overlap during the retrograde period...

Mars stays in the same plane, and Earth stays in the same plane... but they are different planes. So our relative "up and down" positions change with respect to each other.

Of course. With the amazing star background, it is easy to forget that the camera was pointed in a different direction for each picture so we are seeing a different part of the Mars plane, making the "up and down" motion understandable. Out of curiosity, what is the angle between the two planes and when during our year do they intersect?

[Anthony Barreiro]

It would be a perfect haperstance if one of your " unsuspecting passerby"s " was a certain Mr Peterson.

I'm sure I would say something stupid, like "elliptical" rather than "eccentric", and need to be corrected.

[Chris Peterson]

It would be a perfect haperstance if one of your " unsuspecting passerby"s " was a certain Mr Peterson.

I'm sure I would say something stupid, like "elliptical" rather than "eccentric", and need to be corrected. :wink:

Yeah, and to be sure, I wasn't getting on Ann's case about that at all (I'm sure her English is orders of magnitude better than my Swedish). And of course, it's common to say it the way she did. But since my work heavily involves orbital dynamics, it's one of those little things that always catches my eye, so it seemed reasonable to point out the more technical usage.

[henrystar]

I guess that is Leo in the background?

Indeed!

Interestingly, it was the orbit of Mars (as measured by Tycho Brahe) that made Johannes Kepler realize that the planets follow elliptical orbits around the Sun. The orbit of Mars is, incidentally, more elliptical than the orbit of the Earth.

Ann

I finally noticed the rollover, and there indeed was Leo! Lucky Kepler did not try Venus or Mercury: Mars is much more elliptical. But, not THAT elliptical, it was a VERY tough job. And Kepler was nauseated by ellipses, he wanted absolutes of some kind.

[Anthony Barreiro]

Why is the track of the retrograde motion a loop? Doesn't Mars stay in the same plane? I would think the track would be a straight line with overlap during the retrograde period...

Mars stays in the same plane, and Earth stays in the same plane... but they are different planes. So our relative "up and down" positions change with respect to each other.

Of course. With the amazing star background, it is easy to forget that the camera was pointed in a different direction for each picture so we are seeing a different part of the Mars plane, making the "up and down" motion understandable. Out of curiosity, what is the angle between the two planes and when during our year do they intersect?

I poked around a little bit on the internet. The orbital inclination of Mars is only 1.85 degrees relative to the ecliptic, the plane of Earth's orbit around the Sun. (Mercury has the greatest orbital inclination to the ecliptic, 7.01 degrees; Jupiter's is 1.31 degrees.)

The nodes of Mars, i.e. the two points where the plane of the orbit of Mars crosses the plane of the orbit of Earth, occur roughly every Earth year, give or take a month or two from one year to the next. This makes sense, because Mars takes just over two Earth years to complete one orbit around the Sun. I assume the variability arises from the elliptical (eccentric?) shapes of the two planets' orbits and therefore their varying speeds at different distances from the Sun, but that's just a guess. Mars crossed Earth's ecliptic plane from north to south on 25 July 2012, and will cross the ecliptic from south to north on 25 May 2013.

[Anthony Barreiro]

Why is the track of the retrograde motion a loop? Doesn't Mars stay in the same plane? I would think the track would be a straight line with overlap during the retrograde period...

Mars stays in the same plane, and Earth stays in the same plane... but they are different planes. So our relative "up and down" positions change with respect to each other.

Of course. With the amazing star background, it is easy to forget that the camera was pointed in a different direction for each picture so we are seeing a different part of the Mars plane, making the "up and down" motion understandable. Out of curiosity, what is the angle between the two planes and when during our year do they intersect?

P.S. -- I would defer to an actual astrophotographer on this one, but it seems to me that the camera had to be pointed in the same direction relative to the background stars in order for the stars to appear as points. The composite picture reveals Mars' apparent movement against the background stars.

[Chris Peterson]

Of course. With the amazing star background, it is easy to forget that the camera was pointed in a different direction for each picture so we are seeing a different part of the Mars plane, making the "up and down" motion understandable.

Regardless of exactly where the imager had his camera aimed, all the images have been registered against the star background, so effectively we are seeing in just one direction over the entire period of time. In fact, it's because the view isn't changing that we see that up and down motion so clearly.

Out of curiosity, what is the angle between the two planes and when during our year do they intersect?

Mars is inclined 1.85° to the ecliptic. To calculate the intersection dates, you need to look at the orbital elements for both Earth and Mars, specifically the longitude of the ascending node. Of course, the Earth crosses Mars's orbital plane exactly twice a year; by my calculation around May 10 and November 11. Obviously these crossings are almost exactly 6 months apart, given the very nearly circular orbit of the Earth.