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    Measure The Sun Yourself
    By Alex "Sandy" Antunes | August 26th 2011 01:49 PM | 7 comments | Print | E-mail | Track Comments
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    Do you know what a creepmeter measures?  Measurement is the heart of science.  What distinguishes science from opinion or philosophy is measureables.  The root of science is facts that are determined by actual observation, compared, then extended into predictions.

    Any good measurement has three parts: the number value, the units you're using, and the error.  If I say I am 6 feet tall, that's a number (6) and a unit (feet), with a presumed error of 'within an inch or two'.  All three parts are crucial.

    If you want to know how fast your car is going, a number can be a useless answer.  My car's speedometer shows both miles per hour (MPH) and kilometers per hour (KPH).  I trust it's accurate to within a few MPH (or a few KPH).  If I just read a number, though, I could be in trouble.

    A speed of "90" may or may not be lawful if it's a 65MPH highway.  Is that 90MPH, or 90KPH?  90MPH means I get a $200 ticket, but 90KPH means I'm the slowest car out there.  Units matter.

    the sun, photo source unknownOne question I ask my physics students is what can you measure on the Sun?  We often think of the sun as a big glowing yellow ball in the sky, but the Sun is a complicated entity.  Think about everything you might be able to measure.  We'll get back to it in a moment.

    While you're thinking, look at the sun using the NASA's 3D Sun iPhone app.  Playing with a digital sun can be as interesting as a game.  Twirl the sun around, look at different layers, and let it sink in that it's all real.  Everything you're looking at is real data from the STEREO spacecraft.

    3D Sun iPhone app

    Still thinking about measurements of the Sun?  Your opinion might change if you also note (as these SOHO images show) that the Sun is both active and rotating.  It's more than a big yellow ball in the sky.
    SOHO views of the Sun, stitched together as an anim gif

    While you're making your list of everything you can measure about the Sun, here's a look at things-- solar and not-- that engineers use to measure things.  A peek at the Wikipedia list of measuring devices reveals the insane number of existing gadgets for measuring things.  Here's just the list from A to C:

    Device                 Quantity measured
    accelerometer     acceleration
    actinometer        heating power of sunlight
    alcoholometer     alcoholic strength of liquids
    altimeter             altitude
    ammeter             electric current
    anemometer      windspeed
    atmometer         rate of evaporation
    audiometer        hearing
    barkometer       tanning liquors used in tanning leather
    barometer         air pressure
    bettsometer      integrity of fabric coverings on aircraft
    bevameter        mechanical properties of soil
    bolometer         electromagnetic radiation
    calorimeter       heat of chemical reactions
    cathetometer    vertical distances
    ceilometer         height of a cloud base
    chronometer     time
    clap-o-meter     volume of applause
    colorimeter       colour
    creepmeter      slow surface displacement of an active geologic fault in the earth
    [etc etc]

    There may be 'more things on Heaven and Earth' than we can measure, but there's no shortage of devices to measure by.  Finish your list of sun measureables yet?  Just off the top of my head, I hope you included size-- aka volume, distance to it, mass, and temperature.  Density and surface area are worth noting.  Also, it's rotating, so measure that.  Also light output, perhaps broken out by spectrum (amount for each 'color') all the way from radio through visible light and up to X-rays and gamma rays.  Particle emission, too.  Temperatures, obviously.  Chemical composition-- what elements exist, including its metallacity (defined as 'everything other than Hydrogen, H, and Helium, He).

    The sun has strong magnetic activity, so you'll want to measure the magnetic field and the electric field.  You'll want to measure the sound waves that go through it, perhaps add some helioseismology.  Opacity (how transparent parts of it are)?  Rate of fusion (conversion of H to He + heat), measure that.  Particle emissions, from high energy stuff to neutrinos, are measureable.

    You can derive values such as its gravity (from mass) and its age (from a variety of things).  The sun is moving through the galaxy so you can measure that.  The sun is joyfully complex.  Given the sun has layers, we'll want to measure all these properties at different layers.  The rotation is at different speeds at different distances from the equator, and in fact most of the things you measure change with time as well as where it occurs in the sun.  Add in transient activity-- loops, flares, CMEs, and other brief yet potent events.  The wonders never cease!

    If you want to be boring, though, you can always fall back on the most basic of measurements: size, mass, and whether it'll set you on fire or not.  Just ask Brewster Rocket!

    Brewster Rocket on the Sun
    Until next week,
    Tuesdays at The Satellite Diaries and Friday at The Daytime Astronomer (twitter @skyday)


    The Stand-Up Physicist
    I think we could get by with just numbers and errors. It would be scary to do, but here are two examples: The mass of the sun and its diameter. Just ask Wolframalpha.com for the mass of the Sun over the Planck mass, or the diameter over the Planck length.
    9.91 x 1039
    8.61 x 1043

    So the dimensionless mass is a small number compared to its dimensionless size. The dimensionless size is a bit over 4.5 seconds:
    8.35 x 1043
    The age of the Universe is almost 8x1060 so the Sun is under 2 times ten to the sixty. I feel certain Nature does not measure time in seconds or years, so the only other choice is being dimensionless consistently.

    As you know, velocities can all be measured relative to light. 
    1.34 x 10-7
    8.34 x 10-8
    People cannot count in scientific notation. 10-7 turns out to be 67 mph. If I had to train myself to be dimensionless, I would use that as a guide. A fast walker would be 10-8,, a slow pace would be 5x10-9. Good things to know :-)

    There are so many units, a move to go dimensionless would be far weaker than the drive to get the metric system in the US.
    Interesting concept, but I will argue that ratios are not dimensionless.  They are using implied units, relative to the scale of the universe.  A velocity of 8.35x10^-8 is not dimensionless but in units of 'lightspeed'.  A mass of the sun over the Planck mass is measuring the mass of the Sun in units of Planck masses.

    The units may be inherently physical quantities, rather than arbitrary standards like 'meter' or 'second', but they are still units.

    Astronomy uses relative units all the time.  The period of an orbit relative to its semimajor axis (the equivalent of 'radius' for an ellipse) is P^2 = A^3.  No units needed-- if you are working in Astronomical Units (AU, where 1 AU = the distance from the Sun to the Earth) and in units of years.  Thus the period of Earth's orbit, given Earth is 1AU away, is.... 1 yr.

    So I'd call the units you propose 'natural units', but they are still, alas, units.

    The Stand-Up Physicist
    These are Planck ratios units. Galileo did all his work as ratios. Using ratios for everything has an important effect on the math one can use. Say one measures a distance and writes that in terms of meters, call it R. What can you do with R? Not much because of the constraints of units. If the distance R was taken as a ratio with respect to the Planck length, the number would get stupid big. From a math point of view, you could take the sine of R. The Taylor series goes like:http://www.wolframalpha.com/input/?i=Taylor+series+of+sine+R

    It makes no sense to add a meter to a cubic meter to a meter5. The sine function is all about ratios. There are so many systems that are harmonic. If all the numbers going in are ratios, then it makes sense to see so many harmonic systems. This number is huge relative to that number, so we get is simple harmonic motion.

    If I say I made a measurement, and using these Planck ratio units, the value of the measurement was 8.35x10-8, that statement reveals only that the number is small relative to unity. If this was a time in seconds, the units give away information about the measurement itself. Nature is the secretive sort, so that is a reason to go with the Planck ratio units.

    It is essential that the ratios not be made with something that has an arbitrary human element. The Astronomical Unit depends on the path of the third rock from the Sun, useful to astronomers, but not hydrogen atoms.

    My proposal is not what is known as natural units. That label is already taken by theory people who want to set c=G=hbar=KB=1 so they can write down fewer things. Such a system is worse in my opinion because the constants reveal information about relationships. I enjoy writing all my c's, G's, and hbar's, an exercise in dotting all the i's and crossing all the t's.
    When you’ve had your fill of measuring, try modelling.  The following picture is from this page by Michael McIntyre at DAMTP, Cambridge.

     from Gough and McIntyre 1998,'Inevitability of a magnetic field in the Sun's radiative interior'(Nature, 394, 755--757.)
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    We often think of the sun as a big glowing yellow ball in the sky ...

    Not me - I think of it as a big glowing blue ball which appears yellow due to selective absorption in our atmosphere.

    By a strange coincidence I am currently writing an article not entirely unconnected with astronomy in which the true color of the Sun gets a mention.   Teaser: I also describe an astronomer's measuring instrument which is not in Wikipedia's list.   :-)

    btw, thanks for yet another very informative article.

    Do you remember this?

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Robert: yes, thanks for the reminder.  Those articles show what can be done with some home-brew kit - kudos to the author.  My reference was, however, to the original scientific discovery through spectroscopy, in the 19th century, that the Sun is neither white nor yellow.  That discovery is one of the foundations of modern climate science.  My (very long) science history article is almost ready - should be up by Wednesday.