[X] My Assistant

[ncc1701d]

What do NASA scientists use for the "origin of longitude" when calculating orbits for Spacecraft that orbit Venus?
I assume they use the Venus Equator for the "reference plane" but what else do they use to get the "Longitude of asscending node" ?

According to Wikopedia for a geocentric orbit around earth they use earths equator as the reference plane and the "First Point of Aries" as the "origin of longitude"

My interest is in orbits around Venus though and finding the the "Longitude of asscending node" so I need to know what they use for "origin of longitude".
thanks.

[djellison]

http://lmgtfy.com/?q=origin+of+longitude+venus&l=1

The map coordinate system used for measuring longitude on Venus is different from that used on Earth. On Earth, longitude (an imaginary line stretching from pole to pole) is measured from a starting point (the prime meridian) at Greenwich, England (near London), toward the east and toward the west with increasing values in degrees until east meets west at the 180-degree point (the dateline), which is diametrically opposed to Greenwich. On Venus, longitude is measured from 0 to 360 degrees with the prime meridian centered within a small impact crater named Ariadne, located in Sedna Planitia (see Figure 3-1). There is an arbitrary convention that determines the direction of increasing longitude on planetary bodies other than Earth: longitude shall be measured in a direction opposite to that in which the planet rotates. Because Venus rotates in a clockwise direction as viewed looking down on the north pole, longitude on Venus increases in numerical value toward the east from the planet's prime meridian.

[volcanopele]

All orbits in the solar system (around the earth, around the Sun, around other planets) use "the First Point of Ares" as the origin for the longitude of ascending node.

[ncc1701d]

All orbits in the solar system (around the earth, around the Sun, around other planets) use "the First Point of Ares" as the origin for the longitude of ascending node.

When planning orbits for Venus, does Celestial Sphere Equator run through the Venus equator or stay referenced to the earth equator when planning orbits?
Looking at the diagram here:

http://en.wikipedia.org/wiki/File:Earths_o...nd_ecliptic.PNG

They have the celestial (sphere)equator lined up to the earth equator. But Venus doesnt have the same incline as earth so when planing orbital paths at venus what are they using? Are they always using earth stars and celestial sphere as shown in the diagram to plan orbits for Venus?
You follow me?
I would like in my model to create my own celestial sphere around Venus but not sure if I should align it to the Venus equator. This stuff is new to me so trying to get a hold on the concepts. Most info on the web references earth rather than from the perspective of other planets so I have to make the jump to the perspective of other planets.

Part 2 of the question:
So if I look at an image from here:
ftp://psa.esac.esa.int/pub/mirror/VENUS-E...83_0005_VI2.IMG

and it shows me this:


/* ORBITAL INFORMATION */

ASCENDING_NODE_LONGITUDE = 106.13
ORBIT_NUMBER = 1083
ORBITAL_ECCENTRICITY = 0.841
ORBITAL_INCLINATION = 89.97
ORBITAL_SEMIMAJOR_AXIS = 39381.3
PERIAPSIS_ALTITUDE = 210.21
PERIAPSIS_ARGUMENT_ANGLE = 91.79
PERIAPSIS_TIME = 2009-04-08T03:33:55.000Z


All this orbital information was used using Earths Equatorial as the "reference plane" when calculating orbits instead of Venus equatorial plane even though they are at Venus.
Is that correct?
thanks

[Phil Stooke]

Hi - I don't do this kind of work myself so I can be corrected if anyone knows better... but mission planners and data users would want to know things like what part of the planet was under the orbit at any given time, so I think any references to the spacecraft's orbit inclination etc. would have to refer to the equator of the planet itself. For instance, a polar orbit designed to give global imaging coverage needs to be polar relative to that planet's pole, not Earth's.

And if you are trying to create a 'celestial sphere' for Venus or any other world, that has absolutely got to be tied to the planet's rotation, with the celestial pole directly above the rotation pole for that body.

Phil

[volcanopele]

Absolutely, the other orbital elements are relative to the body being orbited, like inclination. But longitude of the ascending node is tied to celestial coordinates and isn't body specific (though it does vary between bodies in the solar system and those outside, like exoplanets and binary stars).

[mcaplinger]

...mission planners and data users would want to know things like what part of the planet was under the orbit at any given time, so I think any references to the spacecraft's orbit inclination etc. would have to refer to the equator of the planet itself.

True, but one has to use orbital elements with some caution because, as Jason points out, they could be given in any defined reference frame. Most often orbital elements are stated in the "Mean Equator and IAU vector of J2000" frame (see, e.g., http://naif.jpl.nasa.gov/pub/naif/toolkit_...mes.html‎ ) but the nice thing about using NAIF/SPICE SPK files is that you can state the frame you want and the software makes any needed transformation to give it to you.

Often when orbital elements are supplied it's not stated what the frame is, and in those cases one would have to make assumptions -- one reason why I don't like using elements if I can avoid it.

"mean equator" has to be used because of precession and nutation; the spin axis of a planet is always moving, albeit slowly.