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Cardinal directions - celestial vs. lunar & planetary

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#1 ascii

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Posted 16 August 2018 - 08:56 PM

The conventions for cardinal directions in observing has come to mind several times, and I want to make sure that I have them straight.  My searches have not found a complete accounting of the conventions, so my apologies if this has been answered elsewhere.

 

I understand that for most observations, we use celestial east and west.  For example, Sagittarius is east of Scorpius.

 

My uncertainty would be for describing an object's surface features.

  1. For lunar and planetary surface features, would we use the cardinal directions that apply on the object itself?  For example, would Oceanus Procellarum be near the western edge of the Moon's face?
  2. What about the edge itself?  Would the limb of the Moon near Oceanus Procellarum be the western limb?

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#2 Sketcher

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Posted 16 August 2018 - 09:39 PM

1 and 2:  I use "preceding" and "following", leaving it up to others to decide which they want to call east and west.

 

When I started out observing the moon, the western limb of the moon was the preceding limb.  Later, someone decided to change that convention.  Astronauts were going to go to the moon.  They couldn't deal with the sun rising in the west and setting in the east.  So they reversed the earlier conventions and made the preceding limb the eastern limb.  This, naturally, confused me.  After all, I had ended up with some maps that had Mare Crisium near the moon's western limb and others with Mare Crisium near the moon's eastern limb.  What was I to do?  So I gave up on east and west and started using preceding and following for solar, lunar, and planetary observations.


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#3 Special Ed

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Posted 17 August 2018 - 11:05 AM

Sid,

 

You can infer from the lack of response to your question that this is perplexing for a lot of people.  

 

Sketcher is correct about the change of convention for the Moon.  In 1961, the IAU dropped the old convention and adopted the new one--east and west on Luna are the same as on Earth.  So if you are standing on Luna, the Sun rises in the east and sets in the west.  If you are in the northern hemisphere looking at Luna naked eye or with binoculars (the correct image view), east is on the right and west is on the left.  The same is true of the limbs.  Celestial north and south is used for the poles, of course.

 

I believe this same convention is used on Mars because we have rovers on Mars.

 

Most planetary observers use preceding and following on the planets (celestial west and east) to indicate the direction of rotation.  If you do this then it doesn't matter whether the image, sketch, or observation is done with an inverting, mirror-reversed, or correct image instrument--you (and your audience) can stay oriented.

 

 I've never heard of using preceding and following for the Moon, probably because it rotates so slowly.  You can stay oriented when looking at the Moon through a telescope by noting how the Sun is casting shadows.  The sunny side of a crater is on the east before full moon and on the west after.

 

Celestial north-south-east-west is used for Venus even though it's upside down and Uranus even though it's on its side.  Nice, huh?  smile.gif

 

The convention for the Sun is celestial N-S-E-W.  It rotates slowly but if any sunspots are visible you know that they are rotating in the direction of the drift, i.e., to the west.

 

HTH.  grin.gif


Edited by Special Ed, 17 August 2018 - 12:23 PM.

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#4 mikerepp

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Posted 17 August 2018 - 11:50 AM

shut off tracking, the direction the object drifts is West for me.   So if I sketch anything its shown as I see it in the EP with a Diagonal.  So west is on the left side of the sketch as it is in the EP.   When it comes to the moon and planets I use that convention to try and keep things simple for myself.   I do use a reverse image moon map along with books that show conventional luna directions, moon globe, mars globe etc.

 

ps. my sketches are definitely nothing to write home about.   My Solar sketches are pretty good in my eyes, for some reason I can sketch proms and other features pretty well.  I cant sketch Saturn for the life of me! lol.gif  Pretty fair on Jupiter and Mars.


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#5 Alex McConahay

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Posted 17 August 2018 - 12:00 PM

On a celestial object, north is defined by the right hand rule and rotation (with exceptions).

 

Figure out which way an object is rotating. Point that way with your index finger on your right hand. Raise your right thumb. Your thumb is pointing north and your index finger is pointing east.

 

That held true for all rotating objects until recently. Then it was discovered that some solar system objects (Venus, for instance) have a north celestial pole that points opposite (generally) to where earth's points. This did not sit well. So, scientists (the same people who reclassified Pluto, if I recall correctly) changed the "right hand rule" for solar system objects such that all "north celestial poles" for all solar system objects are in the same celestial hemisphere as the earth's north celestial pole.

 

Outside the solar system, the right hand rule rules...…

 

Alex


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#6 ascii

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Posted 18 August 2018 - 08:00 AM

Thanks for the replies.  I guess I'll try to remember the preceding/following terminology.



#7 Sketcher

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Posted 18 August 2018 - 03:57 PM

Since I nearly always use my scopes 'manually', no drive motors, it's pretty easy to tell which direction is preceding.  It's the direction in which everything appears to drift across the field.  So on sketches I draw a little arrow and label it "P".  Then I nudge the scope toward Polaris and draw another little arrow and label it "N".  I sometimes use scopes with 'correct' images and at other times scopes with mirror-reversed images.  This system works just as well either way, so I don't have to stop to think about directions.

 

One of the nice things about this approach is that I can label all sketches in the same manner regardless of whether they're deep-sky, lunar, double star, etc. -- less to remember -- less room for error.

 

Sometimes, later, while inside, I'll adjust for the tilt of an object (the sun for instance, whose pole appears to wobble to and fro in the course of the year).  Also, after the fact, I'll sometimes use east and west when describing a lunar observation:  "This feature was west of that one, etc."  I've labeled selenographic east and west on some of my moon maps to make that task easier.  Oddly enough, some maps don't have those directions printed on them. . .  perhaps due to that historic reversal?


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