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Ecliptic Coordinate System

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#1 DoctorTR

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Posted 26 September 2020 - 01:27 PM

Hi!

 

For an article about the Ecliptic Coordinate System, please let me know if the following is correct:

 

In the Ecliptic Coordinate System, horizontal distances are measured in degrees of longitude, where zero degrees of longitude is the Vernal Equinox, when the Ecliptic Plane crosses the Equatorial Plane in the Spring and vertical distances are measured in degrees of latitude.

 

Thank you!

 

Doctor TR



#2 sg6

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Posted 26 September 2020 - 02:51 PM

By rights they are times, we find it "easier" to work/think in degrees, however I suspect it is wrong.

Which then raises the problem that 1 day, as in one 360 degree rotation of the earth, is 23 hours and 56 minutes long not 24 hours.

23 hours 56 minutes is the time the earth takes to complete one 360 degree rotation, 24 hours is the time it needs to rotates once and then about 1 degree more to place the sun back on your meridian.

 

One is termed Celestrial and the othe Solar, our clocks, watches etc work on Solar. Stars work on Celestrial. And that is where the problems start as both are termed "a day".

 

The difference in 1 "day", ~4 minutes, is why a star on your meridian at midnight is then on your meridian at midday 6 months later.

 

Will say a real pain to explain, and I haven't I suppose.

I get asked this a lot at outreach as the place has a "Clock" that shows the RA of an object/scope. So at say 20:30 at night a big illuminated clock is displaying 16:42 if aimed at M13 and the "time" isn't changing, whereas their watches are. At least 5 times a night I get it.

 

At the exact moment of the equinox, whereever you are you "look" along your specific meridian and record stars on it.

Your Meridian is on a given Longitude.

This Longitude is converted to the Celestrial time and that Time is the RA of the object.

 

Traditionally the Spring Equinox is used and the 2021 Spring equinox will be at March 20 at 09:37 UTC, not sure of the seconds but you can find it. The object position is "At the exact time" not on that day.

 

So Longitude is "easy" but not exactly right as something viewed from Greenwich at 1 degree Longitude, isn't in view 6 months later at 1 degree Longitude.


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#3 Andrekp

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Posted 26 September 2020 - 03:29 PM

Hi!

 

For an article about the Ecliptic Coordinate System, please let me know if the following is correct:

 

In the Ecliptic Coordinate System, horizontal distances are measured in degrees of longitude, where zero degrees of longitude is the Vernal Equinox, when the Ecliptic Plane crosses the Equatorial Plane in the Spring and vertical distances are measured in degrees of latitude.

 

Thank you!

 

Doctor TR

Maybe you should do your OWN homework?  If you don’t know this, or can’t find it to understand it for yourself, why are you writing an article about it?  Seriously.  


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#4 beggarly

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Posted 26 September 2020 - 03:44 PM

The explanation in Wikipedia is correct! https://en.wikipedia...ordinate_system

 

http://ircamera.as.a..._518/ametry.pdf


Edited by beggarly, 26 September 2020 - 03:48 PM.

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#5 kathyastro

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Posted 27 September 2020 - 07:03 AM

This is conceptually correct but wrong in detail. In the ecliptic system, longitude and latitude are called right ascension and declination, respectively. And, confusingly, right ascension (RA) is sometimes measured in degrees (especially in professional as opposed to amateur publications) but more often in hours, where 24 hours = 360 degrees.

RA and dec are equatorially-based.  The OP is asking about a coordinate system based on the ecliptic, not the equator.


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#6 DoctorTR

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Posted 27 September 2020 - 07:17 AM

Hey SG6!

 

Thank you for your detailed reply!

 

I realize that the six motions of the Earth (Rotation, orbiting, precession, the motion around the baricenter, the change in ellipticity, and the change in the tilt) make describing the three Celestial Coordinate Systems (Ecliptic, Equatorial, and Horizon) challenging.

 

I am trying to craft a single statement that describes the Ecliptic Coordinate System at an introductory level.

So my query stands.  Is the following statement correct?

 

“In the Ecliptic Coordinate System, horizontal distances are measured in degrees of longitude, where zero degrees of longitude is the Vernal Equinox, when the Ecliptic Plane crosses the Equatorial Plane in the Spring and vertical distances are measured in degrees of latitude from the Ecliptic Plane.”

 

Dr. T

.

 



#7 DoctorTR

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Posted 27 September 2020 - 07:20 AM

Maybe you should do your OWN homework?  If you don’t know this, or can’t find it to understand it for yourself, why are you writing an article about it?  Seriously.  

Hey Andrekp!

 

This is NOT homework.  I hold a doctoral degree and, although now I am retired, I was a college professor.  I am trying to learn astronomy.  And, in academia, I learned that the best way to learn a subject is to write about it.

 

Please do not waste precious bandwidth making frivolous comments.

 

Dr. T



#8 DoctorTR

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Posted 27 September 2020 - 07:29 AM

Hey Beggarly!

 

Thank you for your reply!

 

I had reviewed the Wikipedia article and I believe that I interpreted correctly.  I am looking for confirmation. 

 

So my query stands.  Is the following statement correct?

 

“In the Ecliptic Coordinate System, horizontal distances are measured in degrees of longitude, where zero degrees of longitude is the Vernal Equinox, when the Ecliptic Plane crosses the Equatorial Plane in the Spring and vertical distances are measured in degrees of latitude from the Ecliptic Plane.”

 

Dr. T



#9 DoctorTR

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Posted 27 September 2020 - 07:32 AM

This is conceptually correct but wrong in detail. In the ecliptic system, longitude and latitude are called right ascension and declination, respectively. And, confusingly, right ascension (RA) is sometimes measured in degrees (especially in professional as opposed to amateur publications) but more often in hours, where 24 hours = 360 degrees.

Hey Tony!

 

Thank you for your reply!

 

I thought the terms Right Ascension (RA) and Declination are used in the Equatorial Coordinate System.  No?

 

Dr. T



#10 DoctorTR

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Posted 27 September 2020 - 07:34 AM

RA and dec are equatorially-based.  The OP is asking about a coordinate system based on the ecliptic, not the equator.

Hey KathyAstro!

 

Thank you!

 

Here's what I replied to Tony:

 

I thought the terms Right Ascension (RA) and Declination are used in the Equatorial Coordinate System.  No?

 

Dr. T



#11 Voyager 3

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Posted 27 September 2020 - 07:47 AM

This is easy to remember... Remember the right ascension as our own earth's geographical longitude. We measure it in hours ahead or behind GMT . Remember the declination as our earth's latitudes which we also measure in degrees . The celestial equator is the meridian as seen from the equator ( am I correct?) . The north celestial pole is the north pole :-) and same for South pole . I haven't heard of longitudes measured in degrees yet . Also the 0:00 hours is presently at pisces I believe .

#12 Eddgie

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Posted 27 September 2020 - 09:45 AM

Ecliptic longitude is indeed expressed in degrees from the Vernal equinox.. 

 

Search "Positional Astronomy" and you should find works out there that will confirm this.


Edited by Eddgie, 27 September 2020 - 09:48 AM.


#13 Eddgie

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Posted 27 September 2020 - 10:16 AM

And anyone with Sky Safari Pro can see this for themselves. In Sky Safari Pro, if you look up a planet, in the data panel it will give three positions but specific to the OPs question, one of those positions in in Ecliptic Longitude and Ecliptic Latitude and the Longitude will be provided in degree (~ 288 as I write this).

 

My guess is that the software constantly uses the spherical triangle formula to convert the Ecliptic coordinates to RA and DEC.

 

Anyway, here is another reference:

 

http://www-star.st-a...es/chapter9.htm



#14 KBHornblower

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Posted 27 September 2020 - 01:02 PM

I think the practice of using time units rather than degrees in equatorial right ascension is to simplify the calculation of the east-west component of the current position relative to the local meridian, as that component depends on the local sidereal time.



#15 Dave Mitsky

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Posted 27 September 2020 - 03:22 PM

Stellarium also supplies ecliptic coordinates and galactic coordinates as well.



#16 beggarly

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Posted 27 September 2020 - 03:52 PM

Hey Beggarly!

 

Thank you for your reply!

 

I had reviewed the Wikipedia article and I believe that I interpreted correctly.  I am looking for confirmation. 

 

So my query stands.  Is the following statement correct?

 

“In the Ecliptic Coordinate System, horizontal distances are measured in degrees of longitude, where zero degrees of longitude is the Vernal Equinox, when the Ecliptic Plane crosses the Equatorial Plane in the Spring and vertical distances are measured in degrees of latitude from the Ecliptic Plane.”

 

Dr. T

I wouldn't use the words 'horizontal' and 'vertical'. To perform calculations in spherical astronomy you rely on spherical trigonometry, you don't measure distances but angles. So don't use the word 'distances' either. In fact, if you would measure positions of a comet and use an algorithm (Gauss's method) to calculate its orbital elements you would also obtain it's distance (in astronomical units) from the Sun and from Earth.

 

Edit.

The vernal equinox or 'First Point of Aries' is the point at the intersection of the celestial equator and the ecliptic. The intersection of two planes (ecliptic plane and equatorial plane) is a line, in this case the line of nodes. The vernal equinox is also called the ascending node and the autumn equinox is called the descending node.

Ecliptic longitude is measured in degrees along the ecliptic in the eastern direction.

 

I don't know what else is in the article. What comes before and after the statement? I think you're trying to squeeze too much in one sentence.

 

Ecliptic.jpg

 

In this figure the J2000 ecliptic longitude of Mars is 25°29'42.8", ecliptic latitude is -3°50'00.4". Distance from the Sun 1.403AU and from the Earth 0.42 AU.

 


Edited by beggarly, 28 September 2020 - 03:32 AM.

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#17 Andrekp

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Posted 27 September 2020 - 04:32 PM

Hey Andrekp!

 

This is NOT homework.  I hold a doctoral degree and, although now I am retired, I was a college professor.  I am trying to learn astronomy.  And, in academia, I learned that the best way to learn a subject is to write about it.

 

Please do not waste precious bandwidth making frivolous comments.

 

Dr. T

no frivolity involved.  Internet fora are full of students who come onto fora just to get people to do their homework for them. I see it all the time.  It is especially telling when it is their very first post, as it was yours.  Many times, “it’s for an article.”  Seriously - you fit the profile.  Sorry if I assumed what seemed apparent.  I’ll leave it at that.

 

I also have an advanced degree, as, you will find, many here do.  Outside of academia, it’s not really something that is brought up in conversation.



#18 Tony Flanders

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Posted 27 September 2020 - 04:48 PM

RA and dec are equatorially-based.  The OP is asking about a coordinate system based on the ecliptic, not the equator.

Whoops, my mistake! I will delete my erroneous post.

 

Ecliptic latitude and longitude are indeed called just that, and almost always measured in degrees. The ecliptic coordinate system is useful for many purposes -- not least being the study of eclipses! Conjunctions between two solar-system objects are also usually defined to take place when their ecliptic longtiudes are equal, which is usually a bit off from the time when their RAs are equal.



#19 Voyager 3

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Posted 28 September 2020 - 08:56 AM

Tony then if the Venus is in Gemini and the Jupiter is in Sagittarius and they both share the same RA at a time. Would this be called a conjunction?

#20 KBHornblower

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Posted 28 September 2020 - 09:01 AM

Tony then if the Venus is in Gemini and the Jupiter is in Sagittarius and they both share the same RA at a time. Would this be called a conjunction?

They do not have the same RA.  They differ by 12 hours.  In sidereal timekeeping for this purpose we use a 24-hour clock.  I would say they are in opposition to each other.



#21 Tony Flanders

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Posted 29 September 2020 - 04:28 AM

Here is some more information about the ecliptic coordinate system.

 

This is actually the oldest coordinate system; it is the one that was used by Claudius Ptolemy and his mighty predecessors. It is in many ways more natural than the equatorial system currently in use. The latter was adopted because it is convenient for equatorial mounts with setting circles -- a choice that is beginning to look rather dated now that hardly anyone uses equatorial setting circles.

 

In the equatorial system, both the RA and the Dec of each star vary throughout the 26,000-year precession cycle. In the ecliptic system, the longitude varies due to the fact that the zero point is defined by the intersection of the ecliptic and celestial equator. But the latitude remains constant to a good first approximation. This is, by the way, one of Ptolemy's propositions; he knew all about precession.

 

Ecliptic coordinates are also more meaningful for describing the motions of the Moon and planets, which never stray far from the ecliptic.

 

Ptolemy measured ecliptic latitude in degrees, as we do now. But his primary unit for longitude was the sign, where 12 sings = 360 degrees. The signs are named after the constellations that were in them when Ptolemy was alive, but they are in fact strictly mathematical constructs. The signs precess along with the equinoxes, whereas the constellations stay put. So the sign of Aries now contains the constellation Pisces. Ptolemy was well aware that this would happen, but he didn't really stress over the fact that the nomenclature would look strange 2,000 years after he died.


Edited by Tony Flanders, 29 September 2020 - 01:16 PM.

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#22 Voyager 3

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Posted 29 September 2020 - 07:52 AM

They do not have the same RA. They differ by 12 hours. In sidereal timekeeping for this purpose we use a 24-hour clock. I would say they are in opposition to each other.

Thank you a lot I really meant dumb . I didn't think and foolishly posted 😳 . Thank you for your help . I didn't express it well . I actually should have asked whether the declination matters in a conjunction . I chose (wrongly) Gemini and Sagittarius as they lie far . Consider both planets are in the same RA , will the declination matters for the conjunction?

#23 Ulmer Spatz

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Posted 29 September 2020 - 08:35 AM

Consider both planets are in the same RA , will the declination matters for the conjunction?

In the ecliptic coordinate system, there is no right ascension (RA) and no declination {Dec). Mentioning those two terms brings the equatorial coordinate system back into this thread. This might confuse beginners carefully reading the title of this thread, which is "ecliptic coordinate system."

 

With that out of the way, I don't have a precise answer to your question. If you define planetary conjunction as  "any planets appearing to be close together in the sky as observed from Earth," I suppose "about the same RA and not too far apart in Dec" could be the answer. It's possible that declination is not mentioned when talking about planetary conjunctions because the planets all hang around the ecliptic, so their declinations are never all that far apart.



#24 KBHornblower

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Posted 29 September 2020 - 09:16 AM

Here is some more information about the ecliptic coordinate system.

 

This is actually the oldest coordinate system; it is the one that was used by Claudius Ptolemy and his mighty predecessors. It is in many ways more natural than the equatorial system currently in use. The latter was adopted because it is convenient for equatorial mounts with setting circles -- a choice that is beginning to look rather dated now that hardly anyone uses equatorial setting circles.

 

In the equatorial system, both the RA and the Dec of each star vary throughout the 26,000-year precession cycle. In the ecliptic system, the longitude varies due to the fact that the zero point is defined by the intersection of the ecliptic and celestial equator. But the latitude remains constant to a good first approximation. This is, by the way, one of Ptolemy's propositions; he knew all about precession.

 

Ecliptic coordinates are also more meaningful for describing the motions of the Moon and planets, which never stray far from the ecliptic.

 

Ptolemy measured ecliptic longitude in degrees, as we do now. But his primary unit for latitude was the sign, where 12 sings = 360 degrees. The signs are named after the constellations that were in them when Ptolemy was alive, but they are in fact strictly mathematical constructs. The signs precess along with the equinoxes, whereas the constellations stay put. So the sign of Aries now contains the constellation Pisces. Ptolemy was well aware that this would happen, but he didn't really stress over the fact that the nomenclature would look strange 2,000 years after he died.

My bold.  I think you accidentally interchanged latitude and longitude.



#25 Tony Flanders

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Posted 29 September 2020 - 01:16 PM

My bold.  I think you accidentally interchanged latitude and longitude.

Right you are. Error corrected.




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