Two moons, circling

By Phil Plait | August 20, 2009 11:00 am

All the planets in the solar system orbit the Sun roughly in the same plane. That means that from the side, the orbits of the planets would all be very close to falling on the same line. Since we orbit the Sun on the Earth, we’re on that line, too.

As it happens, Jupiter’s poles are almost perfectly perpendicular to the line. In other words, its equator lines up with the plane of the solar system… and its big moons also orbit the planet right above the equator.

All this together means that when we look at Jupiter, the moons appear to orbit the giant planet on a line, too. They swing back and forth, moving "left to right" and "right to left" over time. In fact, if you watch Jupiter for long enough you’ll certainly see a moon pass directly over the planet’s face, and sometimes you can see the moon’s shadow as well. It’s very cool.

animation of transit of Io and Ganymede

It’s much less common to catch the shadow of one moon falling on another; the moons are small and it’s a rare thing to see such an event. But amateur astronomer Christopher Go caught exactly that on August 16, 2009: the shadow of the moon Io going right over the moon Ganymede.

That. Is. So. Cool.

You can see the shadow of Io (which is roughly the size of our own Moon) pass over Ganymede (as big as Mercury!), then Io itself pass directly between us and the giant moon. Not only that, but take a closer look at Ganymede: you can see surface features! That’s truly astonishing. I can almost swear I see similar features on Io, but the distortions due to Earth’s atmosphere on the image make that a very difficult thing to be certain about.

I find it amazing that we can see things like this. Imagine! Christopher had software that was so accurate in its mathematical and physical modeling of the solar system that it could predict this transit. He had a telescope and a digital camera powerful enough to record it. And, of course, we have the intertoobz which allows you to see it.

In my line of work (y’know, truth promotion) I hear from people who think science is all guesswork. "Yeah, but how do you know?" they ask. The answer is: math. And physics. And chemistry and optics and engineering and Kepler and Newton and Einstein. We know because we test our assumptions, and if they don’t hold up they’re gone. We keep the good stuff, the stuff that’s proven itself. And eventually we get models that are so good they can predict when and where two objects hundreds of millions of kilometers away pass in front of each other.

Yeah. That’s how we know.

Man. I love this stuff.

CATEGORIZED UNDER: Antiscience, Cool stuff

Comments (64)

  1. That is incredibly cool. I once heard someone say that astronomy is the last of the sciences where an amateur can still do some meaningful work.

  2. What? No hidden ST:TNG references within the post? (Or were they so hidden that I missed them?)

    Edit: Darn, that was “one moon circles”. :-(

  3. Kevin

    While I was waiting to photograph the moon occulting the Pleiades last week, I was observing Jupiter through my scope, and watched Io pass in front of Europa.

    Just like Christopher’s video, it was major coolness on an astronomical scale! :)

  4. What determines the exact 0-degree baseline of the solar system’s plane? Is it the rotation of the Sun, or the plane of Earth’s orbit?

    Also, do we know why Jupiter is so perpendicularly aligned while Earth and its moon are so tilted? I’m curious…

  5. drow

    that is spectacularly awesome.

    toasterhead; the solar system coalesced out of a spinning disk of dust and material, which results in nearly everything rotating and revolving in close to a single plane. larger deviations are probably the result of collisions between objects. something can be very close to the plane of the solar system as a whole, but still well off-center compared to something the size of another planet. jupiter would be close to the average because of its size. it accumulated more of the disk, and it has never been struck by anything comparable in size.

  6. “Not only that, but take a closer look at Ganymede: you can see surface features!”

    Canali! ;-)

  7. Ken B (#2): That was the first thing I thought of. “Eyes in the dark. One moon circles.”

  8. IBY

    I love science, it is the closest thing to predicting the future we have. And it is based on actual fact! Much more accurate than astrology, I should say, plus scientists don’t claim to do stuff they can’t do. ^_^

  9. IBY:

    plus scientists don’t claim to do stuff they can’t do

    Well, some people (not me, of course), would say that scientists claim they can “see” things from billions of years ago, which is “impossible” since the Universe is “only 6,000 years old”. :-)

  10. Gadfly

    I had wondered where those special effects guys went after they faked the moon landing…

  11. Lawyer

    Phil, great post. And predictability has always been the linchpin of the scientific method, among the coterie of scientists you mentioned, but specifically Newton had several rules he laid out in his Principia Mathematica (translation available at B & N I believe).
    He lays out Rules of Philosophy which basically show his true genius knowing that in the physics of the large (gravity at galaxy level – although he could not have known about dark matter then and bizarre constant angular speed) and the physics of the small (quantum) other rules may apply, but only predictable results would be published.
    So, when you said if our assumptions don’t hold up, we toss them out… Well, I all up for shout outs to the scientific method and reason! Holla. And you could not have chosen a better group of scientists to illustrate.

  12. Ken B: I also though of ST:TNG, but my thought process was:

    “Two moons, circling? Temba, his arms wide.”

  13. Ganymede, eh? Ironic/coincidence I just finished (re)reading Farmer In The Sky (Robert A. Heinlein) which is set on a ‘terraformed’ Ganymede… and the catastrophe in the story involved the alignments of all the moons.

    as for math:
    We all use math every day; to predict weather, to tell time, to handle money. Math is more than formulas or equations; it’s logic, it’s rationality, it’s using your mind to solve the biggest mysteries we know.

    J/P=?

  14. cletus

    And he did it with a modest little 11-inch SCT! That’s stunning. Simply stunning.

  15. Mchl

    I had been watching the animation, reading the post and expressing my excitement aloud. That got my sister somewhat interested, so I’ve shown all this to her. I had to explain what and where Io and Ganymede were. Explained why this is so awesome. Expressed some more excitement. Then she turned back to what she was doing before…

    Some people…

    [edit]

    Oh yeah. And xkcd #54 !

  16. Totem

    Very cool stuff!

    Any idea how far the two moons were from each other during the transit?

  17. Matthew Ota

    That animated GIF is a keeper. Some of the best planetary imaging is coming from astronomers locaed in the Phillipines, Hong Kong and oterh places in the lower latitudes. They view and image the planets higher in the sky than us Nortern Latitude types can.

    BTW Phil, please jump on debunking the Hellion-1957 scare that is circulating all over the web at this time. One claim is that the Sun will be hit and damaged this weekend.

  18. Marc Geerlings

    I love it.

    I immediately had to try it in Celestia (http://www.shatters.net/celestia) the open-source space simulator and when I saw the distance and how small Ganymede looked from the orbit of Io I doubted it would show, but I went to the time and played it and it showed! A nice shadow of Io on Ganymede.

    Got to love the time people spent on perfecting a free application to simulate in such detail our Solor System.

    Here is the link to a snapshot of the simulated pass in Celestia: http://imgur.com/AyyFd

  19. Merijn Vogel

    This one got to me through spaceweather.com, and is really is an very very very cool animation. The fact that you discover a transition like this in a piece of modeling software. In stellarium while replaying the situation, I could see an optically very close passing of the two bodies. Today Ganymede and Europa may do a similar thing. But Stellarium did not predict the shadow-casting. That may have been a little bit of luck on the side of the photographer, but it is stellar nonetheless!

  20. Merijn Vogel

    @19: Matthew, well, that will have debunked itself next sunday won’t it?

    Or that must be the day that the sun respots itself from it’s long sunspot-rest, and Hellion-1957 conspirers will blame this ‘damage’ from that :)

  21. Chris

    I have always wanted to know why moons orbit right near their host planet’s equator. Why not circumpolar? Uranus is on it’s side but sure enough, the rings and moons are still orbiting above the equator.

  22. Caleb Jones

    I’m looking to buy a telescope since my kids have expressed interest in astronomy (partly my doing), but the light pollution is so bad where I live that you can barely see any stars. How much does the ambient light pollution affect low to mid range amateur telescopes?

    On a somewhat unrelated note, my kids love the “Ask an Astronomer” (http://www.youtube.com/view_play_list?p=1159438A60EB0E4E) clips on YouTube. My 4 year old son recently told me that when a star dies it gets really small and becomes a black hole. I think there should be more “outreach” efforts like that to bring the sciences to the masses.

  23. Matthew Ota – Don’t forget about Australia! Anthony Wesley (who discovered the impact scar last month) and Mike Salway are both from down under and do great planetary stuff. I have to agree that some of the best planetary stuff is being done by amateurs. The price for entry is quite low and I am kicking myself for not getting into this stuff earlier.

    As for the distance between the two planets….I fired up my Starry Night software and it was saying the two were ~700,000 km away from each other (not sure how accurate that number is though).

    For more animation fun, I was out capturing an Io transit of Jupiter earlier this month. Here is a link to the animation:

    http://www.flickr.com/photos/zamb0ni/3795514101/sizes/o/

    I wish more people would know that the equipment needed to capture these type of images isn’t *that* expensive, and most of the software for processing is available for free.

  24. Again, you bring us the awesomeness.

  25. Chip

    Also the Io shadow across Ganymede is really big. I wonder how large Io is in the sky as seen from Ganymede.

    (Yes – not a LARGE as Jupiter would look.)

  26. Torbjörn Larsson, OM

    I love science, it is the closest thing to predicting the future we have.

    Oh, it does predict the future.

    Of course the risk that, say, the sun won’t rotate into view in the next 24 hours is small. At less than 1 % risk for “beyond reasonable doubt” you need to see on the order of 100 days pass to have made a successful prediction “of the future”.

    Of the system outcome really: past, present, future. No measurable difference that I know of. Actually, and nothing personal, but this much fabled “future” feels IMHO like a non sequitur on the order of “gods”. (o.O)

    [I prefer Einstein’s fabled way of looking at these things:

    “The only reason for time is so that everything doesn’t happen at once.” – Attributed to Albert Einstein. Corollary: The only reason for space is so that everything doesn’t happen to me (“here”).

    I’m sure the use of the anthropic principle is just a mere coincidence. :-o Corollary: The only reason for the anthropic principle is so that everything bad doesn’t happen here.]

  27. Crudely Wrott

    Take heart, Matthew Ota! Inasmuch as an amateur astronomer with off the shelf equipment can capture a transit of two Jovian moons, one might safely assume that any object on a collision course with the sun that is sufficiently large to cause alarm would have been detected long ago*. It’s not as though nobody is looking.

    *unless it emits no radiation at all and thus masks it presence. not likely.

  28. gopher65

    That is really cool. The quality of the images that can be taken by “amateur” astronomers with relatively reasonably priced equipment (<$25000) and (in some cases that I've seen) only a few years experience is incredible. They do some awesome work.

  29. Rob Jase

    Vastly cool!

    Years ago I caught the eclipse of Epsilon Gemminarum (sp?) by Mars, I miss those days when stars were actually visible.

  30. Paul M.

    That is without a doubt the coolest thing I’ve seen in ages.
    Yay for math!

  31. t-storm

    Yay science.

    full of conversations like
    Can we go to the moon? I don’t know, let’s try.
    Can we transplant a monkey heart into a person? I don’t know, let’s try.
    Can we fly faster than sound? I don’t know, let’s try.
    Can we convince people that vaccines are very helpful? I don’t know, let’s try.

  32. MadScientist

    @Captain Mike #1: Whoever said that is dead wrong. Astronomy is one of the few sciences where amateurs can still make many and meaningful contributions. For example, the biggest names in spotting comets are amateurs, and contrary to popular belief they typically use binoculars and not telescopes. Although the amateurs generally don’t work on new theories they do help find objects which are of interest to the professionals and sometimes there are opportunities to help with observations of not-so-repetitive phenomena such as transits.

  33. Spectroscope

    @ 31 Rob Jase :

    Epsilon Gemminarum (sp?)

    I presume that would be Epsilon Geminorum also known as Mebsuta, which is in line with Castor in the “twins'” figure on the Taurus -&-Auriga facing side of Gemini and opposite the Cepheid variable star Mekbuda or Zeta Geminorum.

    The ‘Collins Guide to Stars & Planets’ (Ridpath & Tirion, Collins, 2007, Page 152-3) lists Epsilon Geminorum (Mebsuta) as a yellow supergiant star located about 900 light years away . “Binoculars or a small telescope reveal a wide companion of mag. 9.2″

    For more info on this star see James Kaler’s superb website & specifically :

    http://www.astro.illinois.edu/~jkaler/sow/mebsuta.html

    Incidentally, for those who don’t know and may be interested, the constellation of Gemini (as I see / describe it) consists of two parallel rows of stars :

    * On the Auriga-Taurus facing side and in line with Castor are :

    Castor (Alpha Geminorum),Mebsuta (Epsilon Gem.), Upsilon Geminorum & then Propus
    (Eta Gem. ) – with open cluster M35 in line with Mebsuta and a bit Taurus-wards of Propus.

    &

    * On the Cancer-Canis Minor-Orion facing side and in line with Pollux are :

    Pollux (Beta Geminorum),Wasat (Delta Gem.), Mekbuda (Zeta) & Alhena (Gamma).

    Hope this is helpful / interesting for folks.

    I miss those days when stars were actually visible.

    They’re still out there & visible but you may just have to travel a lot further from the cities to find a distant location with nice dark skies to see them.

    Light pollution sure is a real pain and growing problem. I fear we are losing the stars and starlore, generation by generation as the sky glow swamps the crystal black dome above us. Astronomers do need to fight light pollution & I’d like to see the BA do more here. I guess he can’t fight everything & already has his hands full but BA, please consider this.

    *******

    BTW. Awesome video -thanks BA & Christopher Go! 8)

  34. StevoR

    @ 34. MadScientist Says:

    @Captain Mike #1: Whoever said that is dead wrong. Astronomy is one of the few sciences where amateurs can still make many and meaningful contributions. For example, the biggest names in spotting comets are amateurs, and contrary to popular belief they typically use binoculars and not telescopes. Although the amateurs generally don’t work on new theories they do help find objects which are of interest to the professionals and sometimes there are opportunities to help with observations of not-so-repetitive phenomena such as transits.

    MadScientist, you are so absolutely spot on correct there! Well said. :-)

    Studying Variable stars and recording their light-curves and outbursts, conducting searches for comets, nova & supernova, ocultations esp. asteroid occultations, finding asteroids & even Trans-Neptunean Objects, astrophotography and a lot more is where amateur astronomers make a huge positive contribution to the science and art that is astronomy.

    I’ve heard it said that many amateur astronomers actually know the sky better than most professional astronomers because the amateurs spend all their time observing and get to know the sky intimately while the professionals (theorists esp.) spend too much time doing maths and computer simulations indoors! ;-)

    (Not to knock the pros in any way, mind you, they do different stuff that illuminate and enlighten us in different ways and I’m sure there’s plenty who do both too!)

    I would strongly recomend reading Tim Ferris’ es marvellous book ‘‘Seeing in the Dark : How Backyard Stargazers are Probing Deep Space and Guarding Earth from Interplanetary Peril.” (Publisher – Simon & Schuster, 2002.) for a lot more detailed information and personal insights on how amateur astronomers are making a real fantastic difference.

    ***

    “Yet here we are with our eyes and our minds and our curiosity, six billion passengers aboard a tiny blue boat, bobbing and wheeling our way around one vast Catherine wheel among many.”
    – P.246, Ferris, ‘Seeing in the Dark’, Simon & Schuster, 2002.

  35. Plutonium being from Pluto

    @ 2. Ken B Says:

    What? No hidden ST:TNG references within the post? (Or were they so hidden that I missed them?) Edit: Darn, that was “one moon circles”.

    Afraid you’ve got me puzzled there – what’s the Star Trek : NextGen connection here?

    BTW. I loved that show (the best of all the Trek series, IMHON) & had quite a crush on Deanna Troi as a teen so I do have some knowledge of it; just wracking my brain to find the linkage here somewhere? ;-)

    PS. Yes, of course, they showed Star Trek on Pluto – TV signals do travel you know – even as far as Omicron Persei! ;-)

    PPS. That video of Ganymede & Io? Just WOW! That. Is. Just. Astounding! 8)

  36. That gif is so cool!

    How close do you think those two moons are?

    Do you think there’s an exchange of orbital or atmospheric material?

  37. @ Caleb Jones:

    How much does the ambient light pollution affect low to mid range amateur telescopes?

    That really depends on a couple of other things. First, what are you looking at? Moderate light pollution won’t really affect your enjoyment of the planets and bright stars at all. Jupiter and Saturn can be magnificent sights even in seriously light city skies. The moon, too, is an obvious target, and great for kids. But nebulae and galaxies? Meh, probably not so. Orion is always good, and the ring nebula is bright enough to punch through (barely), but most deep sky stuff will be disappointing.

    Also, depending on what sort of light is messing up your pretty black skies, you might have luck adding a filter to the eyepiece. Adverts are all over the pages of sky & telescope magazine. Of course, as the old joke goes, the best filter is gasoline…in your car, taking your and your telescope to a dark site.

    Hope the kids get into it.

  38. Buzz Parsec

    MadScientist and StevoR –

    I think you misunderstood Mike’s quote. He wasn’t saying that amateurs don’t make valuable contributions to astronomy. He was saying astronomy is the only science where they still make valuable contributions. That said, I’m not sure if its true. Botany and ornithology also spring to mind. In general, observational rather than theoretical or laboratory sciences …

  39. cory

    @ caleb jones—time to sit down with the 8yr old and watch youtube! cool!

  40. Michelle

    That’s SO COOL. I love that christopher go guy. He always ends up taking the coolest amateur pics.

  41. Plutonium being from Pluto

    @ 40. Buzz Parsec Says:

    MadScientist and StevoR –

    I think you misunderstood Mike’s quote. He wasn’t saying that amateurs don’t make valuable contributions to astronomy. He was saying astronomy is the only science where they still make valuable contributions. That said, I’m not sure if its true. Botany and ornithology also spring to mind. In general, observational rather than theoretical or laboratory sciences

    Fair enough. Looking back at the first comment again I have to agree. I did misunderstand.
    Not for the first or last time I guess …

    – StevoR (aka Plutonium being from Pluto)

  42. Jar Jar Binks Killer

    Looks like an animation or GCI to me. It is really real isn’t it? ;-)

  43. Asimov Fan

    Awesome. :-)

    I’m just imagining this same view from closer to the scene of the action – like the outermost Jovian satellites but seen from the same angle. Perhaps one day humans will see this from that perspective. Maybe in a few centuries time if Humanity does well and follows the path of science not self-destruction – who knows? Possibly there’ll be people seeing that outside their windows live with their own unaided eyes. I hope so. :-)

  44. Asimov Fan

    @ 39 Kuhnigget :

    … First, what are you looking at? Moderate light pollution won’t really affect your enjoyment of the planets and bright stars at all. Jupiter and Saturn can be magnificent sights even in seriously light city skies. The moon, too, is an obvious target, and great for kids. But nebulae and galaxies? Meh, probably not so. Orion is always good, and the ring nebula is bright enough to punch through (barely), but most deep sky stuff will be disappointing. …

    What about :

    1. The Andromeda Galaxy (M 31)
    2. the Triangulum Galaxy (M 33)
    3. Hercules Great Globular Cluster (M 13)
    4. Omega Centauri globular cluster
    &
    5. The Coalsack nebula near the Southern Cross

    Do they have enough luminosity to punch through light pollution?

    Yeah, I know the last two are Southern hemisphere objects but still..

  45. @ Asimov fan:

    The Andromeda galaxy can easily be seen with a pair of good binoculars, if the sky is dark enough. At higher magnifications it actually tends to be harder to see when conditions aren’t great. Best bet is a wide-field lens on any telescope over 1.5 refractor/3 inch reflector. Avoid high magnification when the sky isn’t dark.

    Triangulum is tough. Probably wouldn’t see it from light polluted skies. Or maybe you’d see the bright center as a faint fuzzy star, but you probably wouldn’t recognize it as a galaxy.

    M13 is easily visible, though you might not resolve many stars. (Actually, I should have made an exception for star clusters of any kind. Open clusters can punch through haze, as long as they’re big enough and you’ve got a wide field lens. The brightest globulars will be okay, but probably not much more than a round fuzzy haze.)

    Omega Centauri…sigh…never spotted it. Too low on the horizon. As the biggest of the globulars, tho, it’s probably pretty cool.

    Coalsack: by definition, isn’t “bright” at all. It’s a blob of dust blocking off a chunk of the background milky way. If your sky is dark enough to allow you to see the milky way arcing across the sky, the coal sack should be visible…assuming you’re one of those antipodean people hanging upside down from the bottom of the planet.

    Southern cross is an asterism (constellation? can’t remember) spread across a fairly large chunk of sky, relatively speaking. You won’t be using a telescope to see it.

  46. @ spectroscope:

    Regarding light pollution and the “loss” of the stars, here’s a true story:

    A friend of mine (we live in the Los Angeles area) was miffed one night when his daughters couldn’t name a single constellation. Then he realized, they had never seen a sky with more than a handful of bright stars. So he packed them up in the car, drove about 100 miles out to the Anza Borega desert, and dumped the kids out in the middle of the night. He told me his oldest daughter started crying – not because she thought her dad was a psycho, but because of what was over her head: stars. Bajillions of them. And a sky so black and deep she thought she might “sink up” into it.

    We tend to forget how powerful a truly dark sky can be. There are reasons why ancient people populated the heavens with gods…

  47. bkallee

    Someone remind me to close my mouth later… Jawdropping

  48. Elwood Herring

    I checked it on Celestia too, and sure enough it’s there. Here’s how I did it:

    Select and go to Ganymede (from the Navigation/Solar System browser menus), then press H then C to look back at the sun, then use the J, K and L keys to adjust the time/date, you will see Io fly right across the sun in the centre of the image. Io eclipses the Sun at August 16, 16:22, then stops and reverses direction (relative to Ganymede of course) and 4 hours later you get another partial eclipse!

    Then you can click on Io, press G to fly to it, then press Return and type in “Ganymede”, select it then C to centre it, then using the J, K and L keys again you can watch both eclipses move across Ganymede’s surface. (Press the comma key to zoom in)

    What do you mean, you haven’t got Celestia? It’s free. Go get it NOW!

  49. Wendy

    That is so awesome. What a great post!

  50. bela okmyx

    @37. Plutonium being from Pluto:

    “Afraid you’ve got me puzzled there – what’s the Star Trek : NextGen connection here? ”

    TNG Season 4 episode “Night Terrors” – http://www.startrek.com/startrek/view/series/TNG/episode/68488.html

    “One moon circles” is a reference to a hydrogen atom (hydrogen gas being necessary to escape from the trap the Enterprise is caught in)

  51. Crudely Wrott

    “Two Moons” is the name of an old buckskinner friend of mine. He was a man of, er, ample stature. At some rendezvous in Wyoming he received his name when his loin cloth suffered a wardrobe failure. When he bent over the fetch it back, Two Moons!

    As astronomically galactic as that moment was, the video of the eclipsing moons is mesmerizing. Of course, the moment was anticipated. Thank you, science!

    And thanks to Phil for this most enjoyable and informative site.

  52. Bill Nettles

    Since Ganymede has a larger orbital radius than Io, this pic had to be taken when Ganymede was on the back “half” of its orbit. Does anyone know whether Io was on the front side or the backside of its orbit? If it was frontside, that makes the sequence even more impressive because the two moons would have been passing in “opposite” directions rather than Io playing catchup (at 4X angular speed).

  53. Vishnu

    I think this crossing would be repetitive as the Orbital periods of Io and Ganymede are in 1:4 resonance. They should have been crossing at the same point with respect to a reference frame fixed on Jupiter’s center. But yes, the transiting would be indeed a very rare event, as Io, Ganymede lining up with Earth is a different ball game altogether!

  54. Vishnu

    If Jupiter spins faster than Io, Ganymede or Europa can rotate, then Jupiter must be dragging them increasing their speeds. Then, why don’t their orbit gradually increase in size, as it happens with Earth’s Moon – as Moon moves away from Earth at a rate of a few centimeters every year. Or wait, does it happen them too? Or in general does it happen to any satellite whose time period around its parent planet is less than the time period of rotation of the planet itself.

  55. Why does Ganymede appear to darken as Io passes between us and it? Is it just an optical illusion, or is there a secondary shadow (Io’s penumbra, perhaps?) after the main shadow passes?

  56. 50. bkallee Says:

    Someone remind me to close my mouth later… Jawdropping

    Remember to close your mouth

    (just feeling helpful today)

    J/P=?

  57. Does anybody have the vaguest idea how far away Io passed Ganymede, damn it?

  58. Joe Meils

    Bless you, Phil… I was looking for a new astronomical subject to paint… :)

  59. Plutonium being from Pluto

    @ 53. bela okmyx :

    Thanks for that! :-)

  60. Mark Hansen

    @kuhnigget (48),
    Sorry to be posting late but;
    The coalsack nebula was one thing I did enjoy looking out for when I drove freight trains from Sydney to Goulburn (NSW, Aust.). It wouldn’t be visible all the way out to Picton but as soon as you got to Tahmoor, there it was, a black patch that was blacker than the rest of the sky.
    The Southern Cross is definitely a constellation (we don’t put any mere asterisms* on our flag, thank you :) ) and readily visible even in capital cities here. Epsilon Crucis is usually not visible, however. Where I live (near Campbelltown, NSW) the skies are usually good enough to not only see E Crucis but right on the edge of visibility is NGC 4755 (the Jewelbox). The Great Nebula is also clearly a fuzzy blue cloud.

    I’m surprised at you not recommending the Pleiades to Asimov Fan. There is an open cluster that lets you test how light polluted your skies are (and how good your eyes).

    *For the pedants: I do realise that a constellation is also an asterism. What I mean is that a constellation is recognised as such, whereas an asterism is any image or pattern you care to trace out using whichever stars take your fancy. If you are still not happy with this, please take a number and wait for me to care.

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