Hello members
I would be very thankful if somebody could explain to me why the small magellanic cloud has an apparent diameter of over 5 degrees in our sky. It is listed as having a distance from our galaxy of about 200000 light years and a diameter of 7000 light years.
e.g.
When I stick this into a calculator it shows that it should have a diameter about 2 degrees. ie Inverse Tan 7000/200,000 (Opposite / adjacent). Other astro information sites I have checked have similar numbers for diameter and distance.
Thanks in advance.
Jim
Last edited by Jimbus; 06-09-2019 at 06:24 PM.
Reason: Clarity
You need to think in all dimensions - if you look at it as a flat trig equation, it only works for 1/4 of the picture. Breaking it down into a singular right angle triangle, the "diameter" (opposite "radius") is 3500 l.y.
So, tan-1(3500/200000) is 1.002°. Now, if we simplistically expand that to 4 right angle triangles of x,y, -x, -y, you get 4.01028°.
I have a saved version of Wiki from 12 years ago and it says the distance to the SMC is 157,000 Light years away, but even at this shorter distance 7000 light years is only 2.55° in apparent size.
I have a saved version of Wiki from 12 years ago and it says the distance to the SMC is 157,000 Light years away, but even at this shorter distance 7000 light years is only 2.55° in apparent size.
I'm not sure Lewis' method is correct (but I could be wrong).
The method I used is as follows.
Work out the circumference of the circle at 200,000 radius (pi * 200K *2)
= 1,256,637 LY, divide by 1,296,000 (arc sec in a circle) = 0.96LY/arcsec.
Divide 7000LY by 0.96 = 7291 arcsecs, or 121 arcmins or 2.02°
I agree, when I did the math it came out to 2.02° . (A different method to what you used)
P.S. You should edit the title to show SMC size confusion.
Thanks Billdan
Yes it has been changed to SMC in the title. I confused it in the title.
Rough estimate the diameter should be 20000 light years for that angular size. How can they have it so wrong?
further.
Cheers
Jim
Last edited by Jimbus; 06-09-2019 at 06:32 PM.
Reason: Spelling
Angular size for the SMC is given as 320 arc-min x 185 arc-min, which is 5.3333° x 3.08333°, which, again taking a mean, is 4.21° apparent angular size. (that's using published figures, NOT Wiki) - which is pretty close to my guesstimates.
Angular size for the SMC is given as 320 arc-min x 185 arc-min, which is 5.3333° x 3.08333°, which, again taking a mean, is 4.21° apparent angular size. (that's using published figures, NOT Wiki) - which is pretty close to my guesstimates.
Hi Lewis
Your method assumes 7000ly is the radius not the diameter.
Radius would be 3500ly.
Jim
For example: "Many deep sky objects such as galaxies and nebulas appear as non-circular and are thus typically given two measures of diameter: Major Diameter and Minor Diameter. For example, the Small Magellanic Cloud has a visual apparent diameter of 5° 20' x 3° 5'." [https://lonewolfonline.net/angular-size/]
Caltech: "Note that the field of view of the images in this gallery is 4.4 degrees on a side, or about nine times the diameter of a full Moon. While considered dwarf galaxies, their close proximity means that the Magellanic Clouds subtend a large angle on the sky" [http://coolcosmos.ipac.caltech.edu//...seum/smc.html]
And lastly:The Small Magellanic Cloud (NGC 292). Irregular Galaxy, type SB(s)m pec, in Tucana
NED data file for the Small Magellanic Cloud (9 Mar 2000)
Object Names Type Object Names Type
SMC G [RC2] A0051-73 G
Small Magellanic Cloud G [RC1] A0051 G
NGC 0292 G PGC 003085 G
ESO 029- G 021 G Nubecula Minor G
ESO 005053-7304.3 G [TSA98] J005833.42-713534.33 XrayS
For example: "Many deep sky objects such as galaxies and nebulas appear as non-circular and are thus typically given two measures of diameter: Major Diameter and Minor Diameter. For example, the Small Magellanic Cloud has a visual apparent diameter of 5° 20' x 3° 5'." [https://lonewolfonline.net/angular-size/]
Caltech: "Note that the field of view of the images in this gallery is 4.4 degrees on a side, or about nine times the diameter of a full Moon. While considered dwarf galaxies, their close proximity means that the Magellanic Clouds subtend a large angle on the sky" [http://coolcosmos.ipac.caltech.edu//...seum/smc.html]
And lastly:The Small Magellanic Cloud (NGC 292). Irregular Galaxy, type SB(s)m pec, in Tucana
NED data file for the Small Magellanic Cloud (9 Mar 2000)
Object Names Type Object Names Type
SMC G [RC2] A0051-73 G
Small Magellanic Cloud G [RC1] A0051 G
NGC 0292 G PGC 003085 G
ESO 029- G 021 G Nubecula Minor G
ESO 005053-7304.3 G [TSA98] J005833.42-713534.33 XrayS
Agree Lewis I think you are flogging a dead horse. Diameter is diameter and nothing more. Adding 4 radii does not make an angular dimension. Dimensions quoted as 5x 3.5 degrees. No way you can turn 2 degrees dimebsions into 5x 3.5 degrees. Otherwise I am 7 foot six tall based on my width and height.
I think error is in quoted diameter in ly.
Error may come from unquestioned NASA figures.
I don't think NASA would take me seriously enough though.
Best
Jim
Astronomy is an odd physical science in a couple of respects.
Some measurements and calculations can be done to stunningly high orders of precision - measurements of the moons shape and distance to centimetres, or Suns diameter, the NASA/JPL ephemeris of the solar system and Astro-navigation by space craft.
Yet other measurements are quite imprecise - uncertainties of a factor of 2 are not uncommon regarding the dimensions of DSOs where the deeper the image the more to be seen.
For physics it's also the test lab for the very biggest of things (planets, stars, black holes, galaxies) while also the test lab for the very smallest (extremely high energy particles beyond what can be produced on earth). Then mix in relativity and quantum electrodynamics just for fun.
Agree Lewis I think you are flogging a dead horse. Diameter is diameter and nothing more. Adding 4 radii does not make an angular dimension. Dimensions quoted as 5x 3.5 degrees. No way you can turn 2 degrees dimebsions into 5x 3.5 degrees. Otherwise I am 7 foot six tall based on my width and height.
I think error is in quoted diameter in ly.
Error may come from unquestioned NASA figures.
I don't think NASA would take me seriously enough though.
Best
Jim
This would mean that SIMBAD, Caltech et al are all wrong, as I got my figures from the official database. If they say the angular dimension translates to 5° x 3.5°, I think they have a better astrophysics knowledge than all of us here. The Messier catalogue dimensions in arc min are more than enough - just divide them by 60 to get degrees and you see there is simply no way the SMC subtends 2° apparent.
The SMC isn't circular - far from it (thus the 5x3.5). If you interpolate that a bit, an approximate 4° circle is produced - FAR from ideal since the shape is more elliptical.
You simply cannot use sin, cos or tan trigonometrically outside of a right angle triangle, so you need to use 3500 LY as the radius (you cannot use a 7000 LY diameter), as the RA triangle is opp 3500, adj 200,000.
But anyway. I'll stick with Messier, NASA, Caltech, Wiki and even Colin
This would mean that SIMBAD, Caltech et al are all wrong, as I got my figures from the official database. If they say the angular dimension translates to 5° x 3.5°, I think they have a better astrophysics knowledge than all of us here. The Messier catalogue dimensions in arc min are more than enough - just divide them by 60 to get degrees and you see there is simply no way the SMC subtends 2° apparent.
The SMC isn't circular - far from it (thus the 5x3.5). If you interpolate that a bit, an approximate 4° circle is produced - FAR from ideal since the shape is more elliptical.
You simply cannot use sin, cos or tan trigonometrically outside of a right angle triangle, so you need to use 3500 LY as the radius (you cannot use a 7000 LY diameter), as the RA triangle is opp 3500, adj 200,000.
But anyway. I'll stick with Messier, NASA, Caltech, Wiki and even Colin
Thanks Lewis
Angular dimension is there for all to see. Error must be in stated diameter or distance as stated in ly not in angular size as seen from Earth. For a three dimensional object the maximum length as seen from any angle cannot exceed root 3 x maximum side length. Even if SMC was a cube of side length 7000 ly it should have no more than 3.5 degrees angular at dimension as seen from Earth. If it is a sphere of diameter 7000 ly then it should be no more than 2 degrees at maximum. An ellipse or circle cannot exceed that.
No argument with what can be observed ie angular size, only with what is inferred ie distance or diameter in ly. Quoting Caltech etc on angular dimension does not prove actual diameters in ly.
Don't forget NASA sent Hubb lie into orbit with a spherical mirror by accident.
Cheers
Jim
