Recent news articles described Mars as recently being closer than it has been in the last 60,000 years, but gave very few details.

The articles raised some questions in my mind.

It is my understanding that both Mars and Earth have elliptical orbits. Are the orbits very precise approximations to ellipses?

Suppose the major axis of the Mars orbit were projected onto the plane of the ecliptic. Would that projection be approximately on the same line as the major axis of the Earth orbit? Is the angle between that projection and the major axis of the Earth orbit approximately a constant, or does it vary a lot? If the angle changes, how fast does it change?

What you are asking for is a course in planetary/orbital dynamics. I suggest you have a read of http://seds.lpl.arizona.edu/nineplanets/nineplanets/overview.htmland links there from. As well as http://www.wikipedia.org/wiki/Orbital_elements and links there from.

In short, the orbits are as close to ellipses (differing semi-major axes though) as you can get. They are at an angle to each other (differing inclinations) that remains a constant but other aspects of the orbit do vary through precession. This is a big topic to get into, suggest you do some background reading first.

Thed: Thanx for the URL to SEDS. It is a very interesting site and led me to a forum for questions relating to astronomy. I might get some of my questions answered there.

I have a good conceptual background in orbital dynamics. I have even written N-body gravitational programs.

There seems to be little data on some of the dirty details of solar system orbits like how major axes line up with each other, the precession of points where plaents cross the plane of the ecliptic, et cetera. Often when I have found pertinent data, it is not in an understandable format.

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Originally posted by Dinosaur
Thed: Thanx for the URL to SEDS. It is a very interesting site and led me to a forum for questions relating to astronomy. I might get some of my questions answered there.

I have a good conceptual background in orbital dynamics. I have even written N-body gravitational programs.

There seems to be little data on some of the dirty details of solar system orbits like how major axes line up with each other, the precession of points where plaents cross the plane of the ecliptic, et cetera. Often when I have found pertinent data, it is not in an understandable format.

The value you are looking for as far as how the major axes line up is called the "longitude of perihelion". (This is the angle between the Earth's vernal equinox and the perhelion of the Planet.

For the Earth, this value is 103°. For Mars it is 336°.

This places Earth's perihelion as occuring right about the first of the Year. The corresponding point of Earth's orbit that Mars' perihelion occurs is right around Aug 27. Mars has a fairly eccentric orbit (.093 compard to Earth's .017) , so it swings in and out from the sun a fair bit. (If you were to draw Mars' and Earth's orbits to scale, you would easily see a variation between the distances of their orbits.) About every 15 years, Mars nears perihelion on Aug 27, and we get a close opposition. This year we got just the right timing and conditions for an unusually close one.

Here's a link to orbital parameters of the Planets. The first table are the present values, and the second the rates of change.

FYI, The "longitude of the ascending node" is the degree measurement from the vernal equinox to to the line marking the inclination of the orbit. (See the legend under the second table)

http://ssd.jpl.nasa.gov/elem_planets.html

Originally posted by Dinosaur
Thed:
I have a good conceptual background in orbital dynamics. I have even written N-body gravitational programs.

Ah, I see. Problem with the medium, you can't see the level of knowledge of the poster.

There seems to be little data on some of the dirty details of solar system orbits like how major axes line up with each other, the precession of points where plaents cross the plane of the ecliptic, et cetera. Often when I have found pertinent data, it is not in an understandable format.

I've never seen details on the semi-major axes lining up but working out the others should be doable with the right maths. To be honest it's been mumblety years since I've with that, I'm a bit (c)rusty.

Some web pages that may be usefull though,

http://ssd.jpl.nasa.gov/iau-comm4/README --- About JPL's Planetary Ephemerides

http://xml.gsfc.nasa.gov/archive/catalogs/6/6087/index_long.html -- more ephemerides

http://www.stargazing.net/kepler/newlink.html --- Paul Schlyter is recognised as an expert in computing Ephemerides. He maintains a number of C routines for use by all.

It may be out of print but Roy and Clarke used to have a very good book on Astronomy and using Ephemerides.

Also, http://www.site.uottawa.ca:4321/astronomy/index.html#longitudeoftheascendingnode

Includes a description of terms. I think one you are after is Longitude of Ascending Node.

http://www.phy6.org/stargaze/Smotion.htm -- A bit on Spherical Trigonometry.

Sorry if you know this, you didn't say what exactly you found non-understandable in the data.

Originally posted by Janus58
The value you are looking for as far as how the major axes line up is called the "longitude of perihelion".

Aha, that's it. Dept. of Information Retrieval was a bit slow today. Time to send them the Dept. of Information Recovery.

th "brazil" ed

Thanx much. The posts here have really helped.