Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 December 16 
Posted on 12/16/2003 5:03:14 AM PST by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 December 16
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 in August, Mars loomed particularly 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 top of the loop, Earth passed Mars and the retrograde motion was the highest. Retrograde motion can also be seen for other Solar System planets. In fact, by coincidence, the dotted line to the right of the image center is Uranus doing the same thing.
The very obvious retrograde motion of Mars and other planets was a real sticking point for the Earth-centrists back in ancient times. They couldn't account for it by simple means, so they invented "epicycles"; a system in which the other planets each orbited a localized area as well as orbiting Earth.
Like this":
When that didn't satisfy the observed effects, epicycles within epicycles were used:
Of course this didn't work either; the accumulated errors built up over time.
 Credits: Courtesy Dr Pedro Rodríguez Pascual, XMM-Newton SOC, VilSpa, Madrid, Spain |
All bodies in our Solar System, including planets such as Earth and Mars, emit X-ray radiation. As far as we know, there are several possible sources of this radiation.
One of the main sources is thought to be 'fluorescence emission'. X-rays from the Sun hit atoms of elements such as oxygen in the atmosphere of the planet, and this radiation is re-emitted as so-called 'characteristic' radiation which identifies those specific elements.
This image from XMM-Newton, recorded as part of a study by Dr K. Dennerl (Max Planck Institute for Extraterrestrial Physics, Garching, Germany) shows X-ray fluorescence emission from the atmosphere of Mars, mainly from oxygen. All of these emissions tell us something about the interaction of radiation with the planet's atmosphere and its environment.
The study of Mars in X-ray wavelengths brings together the work of two very important ESA missions XMM-Newton and Mars Express. Both are crucial to our understanding of our nearest planetary neighbour, demonstrating the coherence of the ESA Science programme.
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(I remember that from when Mom and Dad were big into that sort of thing...)
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