Zero degrees longitude on other planets

[Zamedy]

Main Question or Discussion Point

Does anyone know if there is any internationally agreed method for determining where zero degrees longitude is when mapping other planets etc.?

 

[SteamKing]

Does anyone know if there is any internationally agreed method for determining where zero degrees longitude is when mapping other planets etc.?

For some planets, like the gas giants, it's not clear that you could map them in a fashion similar to what has been done on earth, because there are no fixed features or other reference points to use.

Most of the probes sent to other planets in the solar system were designed to photograph these bodies as they flew by. There have been probes sent to the moon, Mars, Venus, and Mercury which went into orbit around those bodies. Because actual exploration of the surface may be impossible or very remote in the future, the concentration of data gathering for those missions was to collect physical data about each body, like whether there was a magnetic field, etc., rather than setting up a surface navigation system.

https://en.wikipedia.org/wiki/Exploration_of_Mercury#Missions

Perhaps in the future, it may be appropriate to devise some sort of mapping convention, but for now, just getting a glimpse of these bodies up close and taking photographs has been sufficient.

In the meantime, you can access Google Mars and Google Moon, which are similar to Google earth.

 

[D H]

Yes and no.

The no part first: Different parts of NASA (and sometimes, different parts of one NASA center) agree to disagree on how to specify coordinates for the Moon. For example, maps of the Moon from the Clementine mission used Principal Axis (PA) coordinates while maps of the Moon from the Lunar Reconnaissance Orbiter uses Mean Earth / Polar Axis (ME) coordinates. Advocates of ME coordinates argue that these are the coordinates that have been used since the invention of the telescope about 400 years ago, and there's a lot to be said for maintaining continuity with the past. Advocates of PA coordinates argue that we've learned a lot in the last 400 years, and that PA coordinates make much more sense. For example, the ME z axis is *not* the Moon's rotation axis. It's off by about a minute of arc. The advocates of the two sides agree to disagree, and also apparently agree that advocates of the other side are a bunch of morons. (There are ways of writing things in scientific journals and saying things in hallways that indicate that the authors / speakers think their opponents are a bunch of morons without using a single derogatory term.)

The yes part: The International Astronomical Union (IAU) has a working group on defining coordinates for planets, asteroids, moons, etc., the IAU Working Group on cartographic coordinates and rotational elements. One of the reasons this working group was established 40 years ago was "to avoid a proliferation of inconsistent cartographic and rotational systems, there is a need to define the rotational elements of the planets and satellites on a systematic basis and to relate the new cartographic coordinates rigorously to the rotation elements." The current website for this working group is http://astrogeology.usgs.gov/groups/IAU-WGCCRE. Note however that this working group does not have any enforcement capabilities.

 

[Vanadium 50]

There are ways of writing things in scientific journals and saying things in hallways that indicate that the authors / speakers think their opponents are a bunch of morons without using a single derogatory term

The admirable, almost Emersonian, consistency of the other group...

 

[Zamedy]

Thanks folks.

It was this set of images of Pluto that got me thinking.
http://www.nasa.gov/sites/default/files/thumbnails/image/nh-lorri-images-6-1-15.jpg

The images are annotated with a longitudinal value to give an idea of which face we're looking at. However at the time these were taken the images available were blobby at best. Which got me to thinking about what criteria had been used to set a zero longitudinal reference point. Was it maybe a sharp change in the overall brightness measurements? That then led to me pondering whether or not this was internationally recognised or just a system used internally by NASA.

I'm guessing from DH's answer that it will at least be taken into consideration by the IAU-working group, as is the case for the naming of surface features e.g. the presently informally named regions like Sputnik Planum.

I understand that the rotational axis is used to determine latitudinal coordinates, and that gaseous planets/bodies are not so easily dealt with.

As for the moon coordinates - it beggars belief that these well-educated logical creatures still can't decide one way or the other.

A minute arc would be about 180km on the surface of the moon - that's quite a hike if they get it mixed up!

 

[Zamedy]

For some planets, like the gas giants, it's not clear that you could map them in a fashion similar to what has been done on earth, because there are no fixed features or other reference points to use.

Most of the probes sent to other planets in the solar system were designed to photograph these bodies as they flew by. There have been probes sent to the moon, Mars, Venus, and Mercury which went into orbit around those bodies. Because actual exploration of the surface may be impossible or very remote in the future, the concentration of data gathering for those missions was to collect physical data about each body, like whether there was a magnetic field, etc., rather than setting up a surface navigation system.

https://en.wikipedia.org/wiki/Exploration_of_Mercury#Missions

Perhaps in the future, it may be appropriate to devise some sort of mapping convention, but for now, just getting a glimpse of these bodies up close and taking photographs has been sufficient.

In the meantime, you can access Google Mars and Google Moon, which are similar to Google earth.

Thanks, I have both of these and NASA's 'Eyes on the Solar System'. I like being able to visualise where things are when reading about different missions.

 

[D H]

As for the moon coordinates - it beggars belief that these well-educated logical creatures still can't decide one way or the other.

A minute arc would be about 180km on the surface of the moon - that's quite a hike if they get it mixed up!

Your 180 km figure corresponds to roughly six degrees. A minute of arc is 1/60th of a degree. A minute of arc represents about 500 meters on the surface of the Moon. (I wrote "about a minute of arc." The actual discrepancy between Moon ME and Moon PA coordinates is about 860 meters at the surface.)

 

[Zamedy]

Your 180 km figure corresponds to roughly six degrees. A minute of arc is 1/60th of a degree. A minute of arc represents about 500 meters on the surface of the Moon. (I wrote "about a minute of arc." The actual discrepancy between Moon ME and Moon PA coordinates is about 860 meters at the surface.)

I should have put a bit more thought into that - my bad - not such a hike then.