Celestia Forums

I noticed that in the Venus_locs.ssc that Maxwell Montes is shown there to be at
LongLat[ -3.3 -65.2 0 ] and yet on WikiPedia and other sites
http://en.wikipedia.org/wiki/Maxwell_Montes
it says its Coordinates are 65.2° N, 3.3° E

I started getting confused when I looked at the Venus texture and noticed
Maxwell Montes at the bottom or in the negative/south degrees lat.
Can someone clarify with me where I am mixed up? Is Celestia showing Venus upside down somehow? and is this correct? Iam getting mixed up.

thanks
Steve

65.0333° N, 3.0500° E = 65°2'N, 3°3'E

Location "Maxwell Montes" "Sol/Saturn"
{
LongLat [ 3.05 65.0333 0 ]
Importance 10000
Type "XX"
}

interesting information symaski62 but can you explain to me in words what your trying to tell me? Iam new to Celestia.
I am not sure what Sol/Saturn means? I am talking about Venus.

I suppose, all though I am not sure about undertaking it, is to flip the texture and change Montes Label to North Latitudes etc but then I dont know if thats going to be correct relative to the whole solar system and spinning direction and all that.
steve

ncc1701d wrote:interesting information symaski62 but can you explain to me in words what your trying to tell me? Iam new to Celestia.
I am not sure what Sol/Saturn means? I am talking about Venus.

I suppose, all though I am not sure about undertaking it, is to flip the texture and change Montes Label to North Latitudes etc but then I dont know if thats going to be correct relative to the whole solar system and spinning direction and all that.
steve

Do you speak French?

F.

The following is what my Venus locations file shows as the location (as provided with Celestia 1.5.1 Final) from Doctor Schrempp's venus_locs.ssc file:
# Processed 2004-7-11 21:11:4 UTC
# by Dr. Fridger Schrempp, fridger.schrempp@desy.de
# ------------------------------------------------------

Location "Maxwell Montes" "Sol/Venus"
{
LongLat [ -3.3 -65.2 0 ]
Size 797.00
Importance 98.24
Type "MO"
}

The Maxwell Montes location shows up at the Southern tip of Venus on my system.

Perhaps the retrograde rotation has some effect? Just FYI. NOTE: I noticed also that
NONE of the mountains on that wiki page feature negative latitude/longitude numbers.
Any reason for this?

Thanks, Bob

Is this just an effect of the two different ways to define which of the poles is the north pole - one way is to take the pole which is above the ecliptic plane as the north pole, the other is to choose the pole around which the planet appears to rotate anticlockwise (looking down on the pole). For most of the planets in the solar system, the two are the same, however on Venus and Uranus the rotation is retrograde and so the two methods give opposite definitions of the north pole. The IAU supports the ecliptic plane defintion (personally I believe this is a dubious choice, if only for the reason that things get really amusing if an object precesses so that the poles alternately move above and below the ecliptic plane - IIRC there is a known asteroid where this actually occurs). Celestia uses the rotation definition.

ajtribick wrote:Is this just an effect of the two different ways to define which of the poles is the north pole - one way is to take the pole which is above the ecliptic plane as the north pole, the other is to choose the pole around which the planet appears to rotate anticlockwise (looking down on the pole). For most of the planets in the solar system, the two are the same, however on Venus and Uranus the rotation is retrograde and so the two methods give opposite definitions of the north pole. The IAU supports the ecliptic plane defintion (personally I believe this is a dubious choice, if only for the reason that things get really amusing if an object precesses so that the poles alternately move above and below the ecliptic plane - IIRC there is a known asteroid where this actually occurs). Celestia uses the rotation definition.

There is definitely a systematic /sign/ deviation between the long-lat
coordinates of planetary features for Venus from the ones given by IAU. I will investigate. since I generated these location data 4 years (!) ago, it will take some "digging" first.

F.

Here is the proof that my venus_locs.ssc files take correctly into account the Venus pecularities and Celestia conventions, despite apparent sign differences in long_lat relative to the IAU official numbers:

The first image is the official map of the Maxwell Montes feature according to IAU/USGS, with the red dot being the center location



The next image shows the SAME feature with the associated name (Maxwell Montes) in Celestia 1.5.1SVN.



I think there is no doubt that this is all correct! Certainly North and South were NOT swapped

F.

It's a coordinate thing, as Andrew says.
The USGS designate the north pole of retrograde rotators (like Venus) as being the pole that lies in the north celestial hemisphere. Viewed from above that pole, Venus rotates clockwise (whereas a direct rotator, like Earth, rotates anticlockwise). The USGS flags the direction of rotation by giving longitudes east of the prime meridian for retrograde rotators, and west of the prime meridian for direct rotators. (There's a slight complication for Mars, which has two different mapping conventions.)

Now, Celestia uses a different pole definition internally: the north pole is whichever pole gives an anticlockwise rotation. So this convention is the same as the USGS for direct rotators, and the opposite of the USGS for retrograde rotators. This is why the signs of the USGS coordinates are reversed in the locations files for Venus, and why the Venus texture is rotated 180 degrees to put USGS north at the bottom of the texture.

But Celestia provides USGS coordinates on-screen. If you go to Maxwell Montes, you'll find the on-screen coordinates are given as 65.2N 3.3E.

Grant

Simply amazing what one can learn from a very simple question.

Thanks very much Good Doctor, and of course Mr. Hutchison. I'm getting educated again.

BobHegwood wrote:Simply amazing what one can learn from a very simple question.

Thanks very much Good Doctor, and of course Mr. Hutchison. I'm getting educated again.

You'll find the same thing going on if you check out the satellites of Uranus.

Grant

I wonder when I look at Venus Express UV images where you cant see the ground as reference points, what system are they using?
the pic below if I attached correctly looks like you looking underneath and it uses negative numbers.
I guess Maxwell montes would or would not be at these lower latitudes underneath these clouds?
Iam going to assume I think maxwell would not be under theses clouds in lowerer latitudes as scene here.
steve

ncc1701d wrote:I wonder when I look at Venus Express UV images where you cant see the ground as reference points, what system are they using?

The ESA's website describes Venus as a 'clockwise, or retrograde' rotator, which would imply that they define the north pole wrt the celestial sphere, not rotation. They also state that on Venus the Sun rises in the west and sets in the east, which supports the implication.

I couldn't find any 'official' definition though.

Yeah, that picture shows the hemisphere that Maxwell Montes is not on. Compare the orientation of the Y feature of the clouds to Celestia. The Y opens towards the direction of rotation.

Brendan

Since you seem to understand the y phenom I am wondering.
Does the opening of the y always face the sun in the same way a comets tail is always pointing out and away from direction of sun?
You know of good website discussing specifically the y phenom?
thnx
steve