by Anthony Barreiro » Thu Aug 09, 2012 9:28 pm
waterfeller wrote:Chris Peterson wrote:waterfeller wrote: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.
[quote="waterfeller"][quote="Chris Peterson"][quote="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...[/quote]
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.[/quote]
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?[/quote]
I poked around a little bit on the internet. The [url=http://en.wikipedia.org/wiki/Orbital_inclination]orbital inclination[/url] 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 [url=http://spider.seds.org/spider/Mars/mars2012.html]nodes of Mars[/url], 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.