Posts Tagged astronomy

The earth’s magnetic field: much more complicated than you might think Marcus Wilson Nov 13


At the recent NZ Institute of Physics conference, we were treated to a wonderful description of the earth's magnetic proceses, by Gillian Turner.  What makes up the earth's magnetic field? What effect does it have? How is it changing?

At first glance the magnetic field of the earth is pretty straightforward. There's a magnetic north pole and a magnetic south pole. In fact, the earth's magnetic field looks a lot like what you get from a simple bar magnet. 

But look only a little more closely, and it's clear there's a lot more to it than that. For example, the magnetic south pole is not diametrically opposite to the magnetic north pole. In fact,  both are moving about quite substantially – like a lost polar bear / penguin, depending upon your hemisphere. At sixty-something degrees south, the magnetic south pole currently isn't all that far south at all!

Then there are the reversals in magnetic field, that occur from time to time. We're talking hundreds of thousands of years. But they're not regular, suggesting a great deal of randomness is going on. 

It helps in interpreting what we see to remember that we are observing the field at the surface of the earth. It originates deeper within the earth – in the inner and outer cores. The distance from the centre of the earth is rather important. Why? Because as we move away from the centre, the smaller-scale variations get 'ironed-out' more quickly than the larger-scale variations. The effect is that the large-scale behaviour (i.e. the bar-magnet-like shape of the field) is emphasized at the earth's surface, whereas deeper down it is much less like a bar magnet. 

For those more mathematically inclined, one can see this with multipole expansions. This is a mathematical way of breaking up the description of a shape of an object into different components. One starts with a monopole – how much like a sphere an object is. Then we move on to a dipole moment – this is describing how separation of material along an axis there is.  Next are quadrupoles, then octupoles – each describing finer variations in the shape. So, as an example, a perfect sphere has a monopole moment of 1, and no multipoles of any other type. A rugby ball is quite like a sphere, so it has a high monopole moment. While it's got a preferred direction (its axis) both ends of the axis are the same and so there is no dipole moment. The next term, the quadrupole moment, however isn't zero – it's this moment that describes the bulk of the distortion from a sphere.  The link above gives a nice example of a skittle – it has monopole, dipole and quadrupole moments. 

Now, with the earth's magnetic field, there is exactly no monopole moment. Magnetic field isn't like electric charge – while one can have an isolated 'positive' charge, one can't have an isolated 'north' pole. The leading term for the earth's field is the dipole moment – there's an axis and a distinct split of field on the axis – at one end it points away from the centre, at the other towards the centre.  Now, the interesting thing is how the impact of the moments changes with distance away. The n-th order multipole has a strength that varies as the inverse of distance to the power n plus one. So the field due to a monopole varies as 1/r^2 (where r is distance away), the field due to a dipole varies as 1/r^3, that of a quadrupole as 1/r^4 and so on. As r gets large, the effect of the higher-order (higher n) moments diminishes quickly. Consequently, it doesn't matter what mish-mash of magnetic behaviour one has, at large enough distances away, the field from it  will look like that of a dipole. 

At the boundary between the outer core and the mantle, there is such a mish-mash of magnetic behaviour. A picture from an impressive computer simulation of the field by Glatzmaier and others  is here. At the earth's surface, however, it is much smoother and we see it as approximately a dipole – with a clear north pole and south pole, (very) approximately diametrically opposite. 

But the mish-mash of the field in the liquid outer core isn't the whole story. It's tempered by the solid inner core, which isn't going to change its magnetism so easily. It provides a large inertia against any changes, meaning that flipping the field of the inner core required some extreme behaviour in the outer core. It gets extreme enough just occasionally, and indeed the inner core can then be flipped, but it's not often. Our compasses are still likely to work tomorrow.

Glatzmaier, Gary A.; Roberts, Paul H. (1995). "A three-dimensional self-consistent computer simulation of a geomagnetic field reversal". Nature 377 (6546): 203–209.Bibcode:1995Natur.377..203Gdoi:10.1038/377203a0


Precision Cosmology – Yeah, Right! Marcus Wilson Sep 27

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We've just had our first session at the NZ Institute of Physics Conference. The focus was on astrophysics, and we heard from Richard Easther about 'Precision Cosmology' – measuring things about the universe accurately enough to test theories and models of the universe. We ablso heard about binary stars and supernovae, and evidence for the existence of dark matter from observing high energy gamma rays.

Perhaps the most telling insight into cosmology was given in an off-the-cuff comment from one of our speakers, David Wiltshire. It went something like this. “In cosmology, if you have a model that fits all the experimental data then your model will be wrong, because you can guarantee that some of the data will be wrong.”

Testing models against experimental observation is a necessary step in their development. We call it validation. Take known experimental results for a situation and ask the model to reproduce them. If it can't (or can't get close enough) then the model is either wrong or it's missing some important factor.(s). Of course, this relies on your experimental observations being correct. And, if they're not, you're going to struggle to develop good models an good understanding about a situation.

The problem with astrophysics and cosmology is that experimental data is usually difficult and expensive to collect. There's not a lot of it – you don't tend to have twenty experiments sitting in orbit all measuring the same thing to offer you cross-checks of results – so if something goes wrong it might not be immediately apparent. And if you can't cross-check, you can't be terribly sure that your results are correct. It's a very standard idea across all of science – don't measure something just once, or just twice, (like so many of my students want to do), keep going until you are certain that you have agreement.

Little wonder why people have only very recently taken the words 'precision cosmology' at all seriously.

Don’t miss the eclipse (hee hee) Marcus Wilson May 08

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Friday is the last opportunity to view a solar eclipse in New Zealand for a long time (till 2021 – or 2025 if you don’t count anything of a few percent or lower). I say ‘view’, but the reality is that such a smidgen of sun is going to be covered that you’re going to have to look carefully at the right time. And that’s only for us northerners – most  in the South Island are going to miss out. (Details for this eclipse are here). 

For Hamilton, the eclipse hits its maximum coverage (a mere 5%) at 11:49 am. 

But it’s not all bad news – an eclipse famine is followed by an eclipse bonanza – three total and three annular eclipses visible from New Zealand between 2028 and 2045. Worth looking forward to. I’ll be into my seventies for the last one of these. Ouch. 


11:51am, Friday 10th May. Just caught a glimpse of the sun in a clear patch between the clouds. Can I detect any ‘nibble’ out of it. Nope. I thought 5% was a bit of an unlikely viewing situation.

Pinhole cameras and eclipses Marcus Wilson Nov 15

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Well, the eclipse yesterday was fun. There were enough patches of sky between the clouds to get some good views. I was pleased that the pinhole cameras I made out of miscellaneous cardboard tubes, tins, paper and tinfoil worked really well. Also, the trees around the front of the sciences building gave some nice natural pinholes as the sunlight worked it’s way through the gaps between the foliage – we could see lots of crescents projected onto the wall of the building. Not something you see everyday.

The trick with the pinhole camera is to get the combination of length between pinhole and screen and size of pinhole correct. (Basically – the f-number in photography-speak) A long length means a larger image – but also a fainter one. To increase the brightness, we need to let more light through (a bigger pinhole) but the drawback of this is that it blurs the image. It takes a bit of experimenting – best done well before the eclipse that you want to see.

On the subject of which…if you live in New Zealand…you don’t have a lot of opportunity for a while. We northerners get an iddy-biddy eclipse next May (10th) – sorry Mainlanders – you miss out – and then it’s nothing for ages before we get a few more feeble partials in the 2020s. BUT, as I said earlier, it’s then non-stop eclipse mayhem from 2028, with THREE total and THREE annular eclipses before 2045, for those of us who are still alive to see them. Details are all here courtesy of RASNZ.

There are a few videos up already from the Cairns region – here’s one. However, video does not do an eclipse justice, partly because of the difficulty in video capturing parts of the corona at different luminances simultaneously. If you want to see the fainter, whispy stuff at the far edge of the corona, you end up well overexposing the brighter area nearer the moon.  The naked eye does a far better job of capturing the totality phase than a camera.

I note a fair amount of pink on the video – this is the chromosphere – a thin, cooler area of the sun, between the photosphere (the bright yellow bit that we normally see) and the corona.





Look out for the eclipse, 14 November Marcus Wilson Oct 30

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There’s a great event coming to our neck of the woods soon (by neck-of-the-woods I mean Australasia and South Pacific) – a total solar eclipse, on 14 November (for those like NZ on the west of the international date line) or 13 November (for those on the eastern side – which won’t be many – save the odd ship). The NASA website above gives details in Universal Time (Greenwich Mean Time) and so reports it as 13 November – don’t get confused.

For those lucky enough to be in Cairns, there’s the full spectacle of a total eclipse. For us lesser mortals in NZ, it’s a pretty sizable partial eclipse, especially for those in the north of the North Island. Hamilton gets about 85% coverage at the maximum. (Note that anywhere in NZ will do – even Scott Base in Antarctica, I think, gets a few percent coverage, if you count that as NZ)

For Hamilton, the eclipse starts at about 9:20am, reaches its maximum at 10:30am, then is all done and dusted by 11:45am. Times for the rest of NZ are similar.

I thought hard about travelling over to Cairns for the event. The reason is simple – a large partial eclipse is nothing compared to the experience of a total eclipse. I was fortunate to be able to see the 1999 eclipse in Europe, from a small village in northern Bulgaria,  and, having experienced that, partial eclipses don’t have much interest. But, travel doesn’t come cheaply, and there’s a baby at home, so this time  I’m staying put. While it would be great to see another, I’m happy with one in a lifetime.

So what does a total eclipse give you that a partial one doesn’t. Here’s a list, that’s not at all exhaustive.

1. You get to look at the sun with your naked eye, quite safely.  DON’T do this at any other time.

2. The wispy corona comes into view.

3. If you’re lucky, so does the pink chromosphere (this was particularly prominent in the 1999 eclipse).

4. You get to experience the birds coming down to roost, and then taking off again.

5. If you’re lucky, ‘Baily’s Beads’.

6. Shadow Bands

7. Stars out during the day. Possibly a good view of Mercury, which is hard, though not impossible, to observe well otherwise, because it is so close to the sun.

8. The diamond-ring, as the bright photosphere bursts back into view.

And so forth. One of the things I remember from Bulgaria is just how quickly things went black in the final few seconds before totality. It was like standing in a well lit room and someone turning off a dimmer switch.

So, what do we get for 85% then? Well, not much, actually. You might not even notice that things have gone dim. The human eye is really good at adjusting to different light levels, and it’s really only when only a few percent of the sun remains that you’ll notice any obvious change in illumination. It’s fun to observe the crescent shape of the sun – but do so SAFELY – with decent eclipse glasses or solar projection. A fun thing to do is pinhole projection – put a tiny pinhole in a piece of card and project the sun’s image onto the ground or a sheet of paper.  In Bulgaria we had pinholes provided by way of the old tin roof on the cafe which our group occupied for the event – it was loaded with little tiny holes (not much good in the rain then) which gave some wonderful projections of the sun onto the tables below.

 So, when’s the next total eclipse to hit NZ? There are actually a few coming ‘soon’ – ‘soon’ being used in an astronomical sense. 22 July 2028 sees most of Otago including Dunedin eclipsed totally. But it won’t be an easy eclipse to view, coming near sunset with the sun just 8 degrees above the horizon. The same eclipse, however, tracks right over Sydney (once again the Aussies get it – though there is far more of Australia for an eclipse to hit) so one might be better off heading westward.

But then, like buses, there’s a positive flurry of them. 10 March 2035 sees NZ get an annular eclipse (the moon doesn’t quite cover the whole sun – not as impressive but pretty spooky) – then 13 July 2037 and a total eclipse tracks over the central North Island, including Napier (Hamilton lies just to the north) and then 26 December 2038 we get another chance – this one over Golden Bay, Manawatu (including Palmerston North) and Wairarapa. (Wellington is just off to the south). That will add interest to the Boxing Day barbie on the beach. The really freaky thing is that there is a small slice of land near Waipukurau that will get a total eclipse in both 2037 and 2038.



Distant galaxies and hobbits Marcus Wilson Oct 01

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I haven’t read ALL of Tolkien’s work, but I suspect space-travelling hobbits don’t feature anywhere. However, what do feature are hole-dwelling hobbits, and I had the fun of seeing their holes in the countryside near Matamata yesterday. The original set for Lord of the Rings was mostly removed after filming, and rebuilt for the filming of the Hobbit trilogy.  (Trilogy? Since when was The Hobbit a trilogy? This is just milking money out of Tolkien fans, isn’t it?) But this time the set will remain, for all to see, for an appropriate fee of course. It certainly was fun to have a look around – what made it was the commentary provided by our excellent guide.

One of the fascinating things pointed out was the perspective tricks that were used. For The Hobbit, there are three different versions of some of the holes.  One, a ‘large’ version, appropriate for a normal-sized actor, dressed as a hobbit, to walk through. One, a smaller version, to make the dwarfs look bigger than the hobbits. And another, an even smaller version, to make Gandalf look bigger than the dwarfs. And the three had to be identical.

And then there are the perspective tricks. To make the view look like it is over a longer distance, the more distant holes are of smaller size than the nearer ones. On a 2d movie it works – your mind interprets what you see as being of equal-sized holes spread over a larger distance. But being there in 3d you see it more as it is.  

That’s the problem that’s faced when determining the distance to distant stars and galaxies. Just how far are they away?  The moon, and anything further away, we perceive as 2 dimensional. We can’t get any 3-dimensional cues and so we have no idea, just by looking, of how far away they are.  So how can we measure distance to the stars? 

One way, which works for the nearest stars, is parallax. The earth orbits the sun, and six months from now it will be about 300 million km away from where it is now. That gives a different viewpoint. The nearest stars, therefore, appear to move against the background of stars that are further away. We can therefore use a bit of simple trigonometry to work out the distance to the star. Indeed, one of the units of distance in astronomy is the parsec – one parsec being the distance over which the diameter of the earth’s orbit subtends a parallax angle of one arc-second.  Essentially, using parallax in this manner is like viewing the situation with two eyes – 300 million km apart.

Parallax, however, only works for our nearest stars, since the distances to our neighbours are so huge. To work out distances further away, there are other methods – such as looking at the intensity of Cephid Variable stars, and, for really long distances, the famous redshift. However, somewhat disappointingly, neither of these are exemplified by the Hobbiton movie set.

Transit of Venus Marcus Wilson Jun 06

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That was different! Yesterday no-one expected the sky to be clear enough to see the transit, but see it we did. We had an early start – herded onto buses and shipped up to Uawa/Tolaga Bay – a rather poignant place to see the transit, given that’s where Captain Cook arrived in 1759 after viewing the transit in Tahiti.

We squashed onto the marae for a powhiri before undertaking some rather unusual events not usually part of a physicist’s schedule – watching the rededication of the wharf, planting trees, watching the internment of a time capsule, etc, while mingling with the rich and famous. (Well, the famous anyway – my colleague Jo from Chemistry managed to sit next to leader of the opposition, David Shearer, for lunch!) And of course there was plenty of opportunity to view the transit, which has now finished. If you missed it, then you’ll need to wait until 2117 for the next one.



One thing I learned which is vaguely physics, is that my eyes aren’t what they used to be. We were all issued with proper solar glasses to cut out the ultraviolet from the sun, making it safe to view. Unfortunately, I really couldn’t see this dot that most others were talking about. I just saw a dim orange disc. But there were plenty of telescopes around projecting the sun for everyone to have a closer look, and that showed it clearly (plus just a few sunspots). I’ve attached a photo – you should be able to make out Venus in the bottom right of the sun’s disc, close to the edge.


How to win a Nobel Prize in Physics Marcus Wilson Oct 10

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Well, if I knew that I would be busy doing it. Perhaps you’d be better off asking Perlmutter, Schmidt and Riess who have just won the 2011 prize for their discovery of the ever accelerating expansion of the universe. I love the story I’ve heard (whether it is true or not I don’t know) that, on receiving the phone call from a woman with a Swedish accent, Adam Schmidt at the Australian National University assumed it was a prank from some of his graduate students.

It’s worth a note that while I was an undergraduate, there was still a lot of debate in cosmology as to whether the universe would expand forever or start contracting again – the data available at the time suggested it was close to the critical point between those two options. Not any more.

There’s been a bit of a theme (or two themes) to the Nobel Prizes for physics in the last few years (by which I mean since about 2000). You can split them into two roughly equal groups: (i) Materials (e.g. graphene, giant magneto-resistance, superconductors…) and (ii) Particle physics and Astrophysics (expanding universe, symmetry breaking, microwave background…). Maybe that’s three groups. If you want your prize, those two/three areas seem to be the places to be in. No hope for me then.

The one that stands out as not fitting in with the themes is Glauber / Hall & Haensch’s prize in 2005 for optics, including in Hall and Haensch’s case the development of the optical  ‘frequency comb’ for precision spectroscopy.

An interesting point is that the prizes in recent times are roughly evenly split between those in extremely practical things (often that have changed modern technology and made a lot of money, such as integrated circuits) and those in ‘blue-sky’ things, that have less obvious application. Physics covers a huge spectrum (if I can use that optical word) of research, and the practical stuff and blue-sky stuff both form a part of it. It’s nice to see that both are being recognized in this way. Governments take note.






Teh most bestest fizx lolcat eva? Marcus Wilson Sep 30

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This one’s a bit old, but it’s quite topical. Love the colour. From, of course.


Where will a PhD take me? Marcus Wilson Sep 29

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It’s one of those busy weeks – blogging’s been pushed to one side a bit, and I’m writing this at home with a cat on my lap who wants to walk all over the keyboard. So any bizarre sllepingh msitkaes or random characters *&fh$f{ are probably not my doing.

I was talking with a student earlier this week about his choice of PhD topic. He wants to disappear off to Australia to research dark matter, and was asking about what would happen to his career opportunities if dark matter met with a sudden demise.  Would he be better off researching something more mainstream?

My view is it is unlikely to make any difference – at least, not long-term.  While it is true that some people use their PhD as a gateway into a particular area of work and make their entire career out of it, I suspect it’s more common for someone’s area of work to change, maybe several times, after their PhD. That’s certainly the case with me. It would be interesting to find out what percentage of people with, say, a PhD acquired 25 years or more ago, are still working in the area that their PhD was in. I suspect it’s quite low. For one thing, science research changes a lot in 25 years – new things come up and old areas fall away.

Having a PhD is a statement in itself, regardless of what topic it is in. What it says is that you are able to carry out quality independent research – and that’s what employers are going to take note of. So, if your PhD doesn’t open up career doors in your particular topic area, don’t worry; it will certainly open doors in other areas.

In my opinion a student should go with what they are interested in -and enjoy the experience. So Dark Mattter will be a great topic to research for a PhD, even if it turns out to have serious problems.



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