northern vs southern hemisphere night sky.
1.0 Why is there a difference?
2.0 The Milky Way
2.1 The Northern Milky Way
2.2 The Southern Milky Way
3.0 Stars and Constellations
4.0 Telescope Objects
4.1 Planets
4.2 Nebulae
4.3 Globular Clusters
4.4 Open Clusters
5.0 Galaxies
0.0 Preface
While some may be familiar with the fact that your view of the night sky is highly dependant on your latitude, to the majority of people this is probably underappreciated.
Not only do the constellations appear to exist upside down (relative to your hemisphere of origin), but certain parts of the night sky will disappear from view entirely as you travel further towards the poles.
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This entry will aim to explain why the difference exists, and compare the features exclusive to each hemisphere.
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Also note the number in parentheses following an astronomical object refers to its apparent magnitude. This is a measure of how bright the object appears to an observer on earth (without an atmosphere). The higher the number, the dimmer the object (intuitively of course), with negative numbers being the brightest. The brightness changes logarithmically with each number representing an approximate ~2.5 times change in brightness (or more accurately - a 100 fold change in brightness for every 5 numbers).
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1.0 Why is there a difference?
The milky way is our home galaxy, a barred spiral galaxy (which I talk more about here). We exist in a spiral arm of the milky way (called the orion arm) some ~25,000 light years from the center, therefore our view of the universe contains the milky way core on one side, and the adjacent perseus spiral arm on the other.
However the earth's axis is not aligned with the galaxy, and as such the south pole points about ~30 degrees toward the milky way core, with the north pole therefore pointing ~30 degrees away from it.
This asymmetry in the alignment of our rotational axis with respect to our home galaxy, is what gives the earth a lopsided view of the milky way, and combined with a large circumpolar region at higher latitudes, causes a significant portion of the night sky to be hidden for the majority of people on earth.
Combine this with the fact that people on opposite sides of the planet quite literally stand upside down relative to each other, and therefore see the sky upside down too, and you have some significant differences between a northern and a southern night sky.
2.0 The Milky Way
2.1 The Northern Milky Way
I recently had the opportunity to view the northern milky way in its entirety, as I was visiting some dark locations in north america during a new moon in late april. While this did mean I had to wake up at around 4 am due to the time of year, the experience was certainly worth it.
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The celestial equator (aka the region of the night sky directly above the equator) passes through the constellations of orion and aquila, therefore everything to the north of these constellations belongs to the northern hemisphere. This includes the regions of the milky way such as cygnus and cassiopeia, as well as the summer triangle and winter hexagon (at least most of it anyway).
Because the milky way center sits at around ~30 degrees south declination, it will always appear in the southern sky to those in the northern hemisphere. The way to determine the angular height of the core above the horizon (for those north of the equator) is simply to subtract your latitude off 60, and this will give you the maximum height attained by the core. For this reason, those above 60 degrees north will never see the core.
As the core makes its way across the southern horizon, the great rift (containing aquila) and cygnus, will rise high into the sky. This orientation frames the classic northern composition of the milky way shooting down into the horizon, growing in size and brightness as it does so (see image 3).
Later into milky way season, the core will set in the south west, leaving the great rift and cygnus standing vertically before slowly disappearing below the horizon. While I haven't seen this orientation of the milky way in person before, it's something I'd like to photograph in the near future.
image 1: the northern night sky (from Utah)
image 2: the southern night sky (from New Zealand)
image 3: the milky way split in half (approximate)
southern sky
northern sky
celestial equator
image 4: scorpius to cygnus as seen from the northern hemisphere
image 5: aquila to cassiopeia and the summer triangle
2.2 The Southern Milky Way
Having grown up in the southern hemisphere, the southern milky way feels much more familiar to me. Regions like crux and vela are some of the most star dense areas of the sky, and the milky way extends from edge to edge during the winter months.
Because the milky way center sits below the celestial equator, the southern hemisphere has a better view directly into the heart of our galaxy, with the core passing close to the zenith for people between -10 degrees and -50 degrees latitude. As new zealand sits at around -40 degrees south, I can personally attest to how incredible this view is, with the core distracting you from looking elsewhere. That being said, the core is of course visible to most people in the north too, just not quite at this neck craning angle.
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In the south, the milky way season starts with the core rising near vertically (depending on latitude) in the eastern sky, with the crux and carina regions almost right overhead. While crux is high, the distinctive coalsack nebula appears as a dark hole in the top of the sky (see image 7). As the season progresses, the core sweeps across the zenith, giving a view of cygnus and the great rift to the north (see image 6) and carina to the south. Towards the end of the year, the core is low in the west, with the milky way sitting horizontally above the horizon before disappearing again until early february.
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3.0 Stars and Constellations *
Given that 90% of the population lives in the northern hemisphere, it's no surprise that the northern constellations are much better known. Some iconic constellations of the north include:
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- ursa major (the big dipper)
- ursa minor (the little dipper)
- cassiopeia
- cygnus
- gemini
- taurus
- auriga
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Down south the constellations are quite underwhelming. It's not that there aren't bright stars or patterns in the sky, but rather they were just assigned confusing animals/shapes (such as musca - the house fly??), also for some reason there are a slew of crosses. Regardless, there are still a few notable ones down here too such as:
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- crux (the southern cross)
- the pointers (asterism within centaurus)
- scorpius
- canis major
- the diamond + false cross (asterisms within vela and carina)
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Orion is an honorable mention for both hemispheres as it sits right on the celestial equator, and as such is seen equally from both hemispheres. That said, due to its flipped orientation in the southern hemisphere, several countries here refer to it as some variation of the pot/saucepan. Not quite as graceful as "orion the hunter" but in fairness the resemblance to the kitchen utensil is uncanny - more than I can say about most of the constellations down here.
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When it comes to stars, both the northern and southern hemisphere were given their share of interesting ones. Stars of interest in the northern hemisphere include:
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- polaris (the north star)
- arcturus
- vega
- capella
- betelgeuse
- deneb
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In the south, the bright and famous stars are:
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- alpha centauri (closest neighboring star**)
- sirius (brightest star)
- canopus
- rigel
- acrux
- achernar
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When it comes to the brightest stars, the south contains the brightest three, sirius (-1.46), canopus (-0.74) and alpha centauri (-0.27***). The next three belong to the north however, with arcturus (-0.05), vega (0.03) and capella (0.08). One star unique to the northern hemisphere is polaris (1.98), a star about a degree away from the northern celestial pole. This acts as celestial compass as it sits almost directly above the north pole at all times. While the southern hemisphere does have sigma octantis (5.42), it's so faint that it's hardly worth mentioning. Instead we use the midway point between crux and achernar (0.46) to find south. Achernar is also the flattest known star, due to its fast rotation causing an intense centripetal force at the equator.
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* technically a constellation refers to a region of the night sky, whereas the line between stars (forming the shape people think of as a constellation) is called an asterism. In casual conversation the two are often conflated.
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** alpha centauri refers to the triple star system containing the closest individual star, proxima centauri. However, this cannot be seen with the naked eye as it is a red dwarf, so the star we see as alpha centauri is a binary containing the two brighest stars in the system.
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*** alpha centauri is magnitude -0.27 due to the combined light of the binary system (rigil kentaurus and toliman), with individual magnitudes of 0.01 and 1.33 respectively. Due to this, the ranking is not always consistent as arcturus is brighter than the stars individually.
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image 6: the milky way as seen from the southern hemisphere
image 7: the milky way of the deep south
image 8: ursa major (the big dipper)
image 9: orion the hunter (aka the pot) with sirius above
image 10: crux (center) and the pointers (left)
image 11: deneb in the head of cygnus
4.0 Telescope Objects
When looking through a telescope, there are a few major object categories that all the best things fall into. These are planets, nebulae and clusters (open and globular). Galaxies too, but I'll come back to those in section 5.0.
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4.1 Planets
Planets are almost always the stars of the show, with saturn especially being the favorite for most. Currently jupiter sits at about 10 degrees into the northern hemisphere, and saturn sits about 10 degrees into the southern hemisphere, but this will constantly change as the planets orbit the sun.
There is no best hemisphere for viewing the planets, but throughout the course of their orbits, they will pass periodically through both the northern and southern sky. However, due to the tilt of the ecliptic, the planets will never really get any further than 23.5 degrees from the celestial equator (in reality they might stray slightly beyond due to their orbital paths not perfectly aligning with the ecliptic). So provided you live between -65 and 65 degrees latitude, you should be able to see the planets at some point each year. That being said, the closer to the zenith the planets are, the sharper the telescope viewing will be.
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4.2 Nebulae.
Nebulae are some of the most famous deep sky objects, largely due to the hubble images (and others from similar telescopes). Unfortunately many of the most beautiful structures like the pillars of creation or the horsehead nebula, are not visible even through a telescope. The light is simply too dim for the human eye to see detail, or the emitted wavelength is outside of the visible spectrum entirely. There are a few nebulae however that can be seen through a telescope, and some bright enough for the naked eye.
In this category the southern hemisphere was somewhat spoilt. The biggest and brightest in the night sky by far is the carina nebula, sitting at a declination of about -60 degrees. The carina nebula is easily visible to the naked eye and stunning through a telescope, although it's too big to look at in its entirety.
Next is the orion nebula sitting at around -5 degrees declination. Because it's so close to the celestial equator, this can be seen clearly from both hemispheres and therefore is only a southern object on a technicality. The orion nebula (combined with the star clusters within) makes up the middle star in orion's sword, clearly visible both with and without a telescope.
Another incredible nebula, and probably the last one in the top tier catagory, is the tarantula nebula sitting at about -70 degrees declination. This nebula is the largest star forming region in our local group, at nearly 2,000 light years across. While not as bright as the carina or orion nebulae, the tarantula nebula is by far the largest of the naked eye visible nebulae, quite the feat given it's around 160,000 light years away from us.
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4.3 Globular Clusters.
When it comes to globular clusters, the southern hemisphere has an advantage simply due to where the south pole points in the sky. Most globular clusters are found in the direction of our galaxy's central bulge, and because the southern sky faces this way, that is where most are seen (generally speaking).
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There are two globular clusters in the night sky that are a step above all others, these are omega centauri (3.9) and 47 tucanae (4.09). Containing ~10 million and ~1 million stars respectively, these two objects are easily visible to the naked eye from a dark site, and truly stunning through a telescope.
To the north, the brightest cluster is the great hercules cluster (5.8) aka messier 13, which is only the fifth brighest cluster in the sky. Still a beautiful sight but not comparable to the globulars in the south.
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4.4 Open Clusters.
As for open clusters, both hemispheres have their fair share, with many being visible reasonably well from both hemispheres. The brightest naked eye clusters of pleiades (1.6) and hyades (0.5) both sit slightly north of the equator, but these are visible basically everywhere other than Antarctica due to their relative proximity to the equator (+24 and +15 degrees north respectively). Through a telescope, the beehive and double cluster are readily seen from northern skies, while the wishing well, butterfly and jewel box clusters are seen from the south.
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In this category i'd put the north and south somewhat equal - with the north having a couple of brighter naked eye clusters, and the south having a few brighter telescope clusters. But again, neither hemisphere has a shortage.
image 12: the carina nebula
image 13: the orion nebula
image 14: omega centauri
5.0 Galaxies
There are two classes of galaxies worth talking about, naked eye visible galaxies, and telescope object galaxies. To the naked eye, there are four galaxies clearly visible in the night sky*. These are the milky way (our home galaxy covered in section 2.0), the large and small magellanic clouds (0.9 and 2.7 respectively) and the andromeda galaxy (3.4).
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Excluding the milky way, the largest and brightest galaxy in the night sky is the large magellanic cloud (lmc). The lmc is an irregular** galaxy sitting around 160,000 light years away from us, containing somewhere around 30 billion stars. The lmc covers a huge area of the night sky, stretching approximately twenty full moons across, however unfortunately it sits very deep into the southern hemisphere at -70 degrees declination. Because of this, it can only be seen with detail from deep in the southern hemisphere.
The small magellanic cloud (smc) is even further south at -72 degrees declination. This galaxy is more than 200,000 light years away, although estimated to have only a few billion stars. Regardless, it still stretches about eight full moons across the sky as a slightly dimmer patch next to the lmc.
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In the northern hemisphere, the andromeda galaxy is by far the most famous, and one that can be found on a dark night at most times of the year. Andromeda is a huge spiral galaxy sitting at around 2.5 million light years away, and slightly more than twice as massive as the milky way. However due to its enormous distance from us, it only appears as a fuzzy oval star in the sky. If you could see andromedas full spiral arms, it would stretch about six full moons across - almost as large as the smc in the south. But only andromedas core is visible to the naked eye which is approximately half a full moon across (~0.25 degrees). Andromeda is best viewed from the northern hemisphere, sitting at around 41 degrees declination. But because it's only in the mid north latitudes, it can be seen in the sky from as far south as even new zealand (depending on the time of year).
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As for telescope objects, both hemispheres have a nice selection to choose from. In the north, andromeda can also be considered a telescope object due to its relatively small size, and is the brightest galaxy that can fit inside a telescope (the lmc and smc are too large for this). The triangulum galaxy, whirlpool galaxy, pinwheel galaxy and bode's galaxy are some of the other brightest and most stunning galaxies available to the northern hemisphere. In the south, centaurus A, the sombrero galaxy and the southern pinwheel galaxy are the popular targets.
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I think in the category of galaxies, the southern sky beats the north for the naked eye - as the milky way and magellanic clouds are something quite special. But the northern sky beats the south for telescope objects with andromeda, the nearest galaxy larger than our own, taking center stage.
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* under perfect conditions, as many as eight galaxies can be seen without a telescope. However after the andromeda galaxy the next two brightest are the triangulum galaxy (5.7) and centaurus A (6.84), both significantly dimmer and near impossible to see for anyone other than a trained astronomer using averted vision.
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** the lmc galaxy is actually a magellanic spiral galaxy. This indicates the presence of a central bar, with a lopsided spiral arm (in the galaxy type SBm). The lmc was the prototype for this class of galaxy.
image 15: the large magellanic cloud
image 16: the small magellanic cloud
image 16: the triangulum galaxy