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The Milky Way in the Southern Hemisphere

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#1 City Kid

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Posted 26 January 2020 - 06:55 AM

During the course of a year is the entire plane of the Milky Way visible in the Southern Hemisphere? Are any sections of it unavailable?



#2 quazy4quasars

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Posted 26 January 2020 - 09:25 AM

 Since you'e in Indiana, I wonder:  May I, based on the context, ask if what you meant was  " Is the entire Milky Way visible from here over the course of a year?  (No)  Is some part of it too far South to clear the Southern Horizon (from here)?"  (Yes...a lot.)

 

The Northern and Southern Hemispheres each contain a 180 degree portion of the Galactic Plane, and since the Galaxy is tilted about 60 degrees from the Celestial Equator,  That means a rather long section of the MW never clears your Southern Horizon, just as a fairly good chunk of the Northern  MW will be found to be somewhere above your Northern horizon at any given time.

 

So, inversely, subtracting 60 from 90, it follows you have to be South of 30N for the entire Galactic plane to even clear your horizon over a day, over a year, ... although over many (Millenia the relation changes somewhat due to precession: the long, slow wobble or drift of the Earth's axis around the ecliptic axis;  But anyway,  as a rule you'd want to be down near the Equator to ever be able to see the whole Galaxy at least decently well- Though not all at the same time, of course.

 

Hope that helps


Edited by quazy4quasars, 26 January 2020 - 09:40 AM.


#3 City Kid

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Posted 26 January 2020 - 10:19 AM

No, I’m asking if from Australia, for example, is the entire plane of the MW visible over the course of a year.



#4 quazy4quasars

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Posted 26 January 2020 - 06:27 PM

No, I’m asking if from Australia, for example, is the entire plane of the MW visible over the course of a year.

Ah, OK. Well, as for the Southern Hemisphere, similarly, for anywhere N of about 30 South (i.e most of Austalia), The entirety of the Plane would clear the N horizon, However, again, Air-mass extinction would render it far below what one could consider "visible".  and the galactic area Northward  of the plane would be lower and/or completely below the Horizon. So, Again, to see the Entire Milky way decently over the course of a year, you really need to be well within the Tropic zone,  preferably observing at a higher altitude.

 

I have often wished I could spend a whole night on the Equator watching the progression of the Milky Way from Sunset Till Dawn: While we know that the Plane of the Milky Way truly* describes a Great Circle (a straight line completely bisecting two hemispheres) over the hours we would see the rising stream arcing back and forth, an encircling sine wave more like what you see when you look at an all-sky starmap, (such as Orion's DeepMap) though you are only seeing half at any one moment, You can always see the Milky Way as a plane if you are looking directly at any point along the plane:  If you look away, toward  the Galactic cap, The Galaxy will appear to curve around your center of view. (Note:- the straight line of the galactic plane continues - all the way around -YOU!)  The illusion of it's curving is due solely to the nature of the eye: a flat 2D projection of the inside of a sphere. In fact it is the same as the illusion presented by the venerable flat all-sky map: Accurate enough but only relative to nearby references. 

 

*never mind.


Edited by quazy4quasars, 26 January 2020 - 08:10 PM.

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#5 Tony Flanders

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Posted 27 January 2020 - 06:57 AM

No, I’m asking if from Australia, for example, is the entire plane of the MW visible over the course of a year.

Any two great circles on a sphere intersect each other at points on precisely opposite sides of the sphere. Examples of great circles on the celestial sphere include:

 

The horizon

The celestial equator

The ecliptic

The galactic equator

 

So, surprisingly enough, from any point on Earth, at any moment, half of the Milky Way is above the horizon. However, much of it may be so close to the horizon that it's quite hard to make out.

 

The galactic plane is tilted at 63 degrees to the celestial equator. That means that anywhere between 90-63=27 degrees north latitude and 27 degrees south latitude, every point on the galactic plane is above the horizon at some time of year. The closer to the equator, the higher it is. There is no bias toward the Northern or Southern Hemisphere.

 

For what it's worth, Australia is a sub-tropical country. Cape York, at latitude 11S, is deep in the tropics. The southernmost point on the Australian mainland is 39S. Sydney is at 34S, so a good chunk of the northern Milky Way is invisible there.

 

Where there is bias is in which parts of the Milky Way are visible when you're north of 27N or south of 27S. The southernmost stretch of the Milky Way, from Alpha Cen to Eta Carinae, is arguably its most spectacular section. But the northernmost stretch, in Cassiopeia, is no slouch either.


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#6 Araguaia

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Posted 27 January 2020 - 08:22 AM

I am at 9 degrees south of the Equator.  Here, in theory, we can see almost the whole plane of the Milky Way.  However, in practice we can't, because even on the best day you can't see the Milky Way clearly all the way to the horizon, even if there were no trees around.  The faint bits near the north celestial pole never really come into clear view, especially as they rise in the rainy season, when even on "good" days there is a bit of murk down low.



#7 Allan Wade

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Posted 28 January 2020 - 06:49 AM

My dark site is at 31S, so its the small portion of Milky Way around Cassiopeia that I can't see from there.

 

One of my favourite Milky Way periods during the year is the southern winter, when it is in all its glory with Sagittarius passing through zenith and the arms spreading out full length of the sky from north to south. It almost feels like daytime with the bright galaxy casting shadows.

 

The other is late in the year, around October and November when the Milky Way lays flat on the horizon. That's when I get SQM 22 readings. It always amazes me to see the sky get that dark compared to the 21.6 when the Milky Way is overhead.


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#8 Tony Flanders

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Posted 28 January 2020 - 07:46 AM

By the way, just in case it's not obvious to everyone, the biggest difference between Earth's hemispheres as far as Milky Way viewing is concerned is the location of the galactic center, near the Sagittarius/Scorpius border. At mid-northern latitudes, we get to see it briefly, low on the horizon for a few hours, during the short summer nights. At mid-southern latitudes, it's up all night at that same time of year, and visible at some point during the night pretty much throughout the year. M7, which is quite close to the center, is an unmistakable beacon in the Southern Hemisphere's sky.

 

We northerners get a correspondingly privileged view of the anticenter, near the Auriga/Taurus/Gemini corner. Fortunately for us, that area does have some splendid clusters and nebulae. But as far as the Milky Way itself is concerned, it's pretty thin and dim there.


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#9 birger

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Posted 10 February 2020 - 03:15 PM

By the way, just in case it's not obvious to everyone, the biggest difference between Earth's hemispheres as far as Milky Way viewing is concerned is the location of the galactic center, near the Sagittarius/Scorpius border. At mid-northern latitudes, we get to see it briefly, low on the horizon for a few hours, during the short summer nights. At mid-southern latitudes, it's up all night at that same time of year, and visible at some point during the night pretty much throughout the year. M7, which is quite close to the center, is an unmistakable beacon in the Southern Hemisphere's sky.

 

We northerners get a correspondingly privileged view of the anticenter, near the Auriga/Taurus/Gemini corner. Fortunately for us, that area does have some splendid clusters and nebulae. But as far as the Milky Way itself is concerned, it's pretty thin and dim there.

Currently, the center of the Milky Way is located very close to the 18h right ascension, meaning it is as far south as it will ever get. Southerners have the best view of it. Some 13,000 years ago, however, the center of the Milky Way was at 6h RA, meaning us northerners got a very good look at it. This will happen again in about 13,000 years. :)

 

This, of course, will give us northerners a much poorer view of Orion...


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