Sun|Trek Adventures Solar Surface Hot Solar Atmosphere Magnetic Sun Flowing from the Sun
Sun Earth Connection Solar Spacecraft Earth and Beyond The Sun as a Star
  Suntrek homepage
Green Loops & Sun
Down Arrow
Down Arrow

A - Factary




Accuracy – it is easy to confuse (or not bother about the difference between) accuracy and precision. In an experiment to measure the length of a piece of string for instance you might measure it several times to the nearest millimetre and take the average value. If all the readings give the same answer to within a millimetre or two, you might then say that you have measured the string length to an accuracy one millimetre or better. But what if the piece of string you were measuring was not quite straight when you were measuring it? You will have obtained a consistent answer, but it was wrong. What you did was obtain a precise but not accurate answer.


Another way of viewing the difference between precision and accuracy is to look at the results of throwing some darts. Assume two players aim three darts each at the dartboard’s bull’s eye. The first player lands all three darts within 5mm, but they are all around the treble-3 area. The second player’s darts are spread out more (within a circle of 15 mm diameter say) but that circle is centred on the bull’s eye. Which player is accurate and which player is precise?


Absorption Line


See also, Spectrum.


A sudden dip in 'brightness' at a particular wavelength (colour) in a spectrum. This means that either the radiation was not produced by the emitting object in the first place, or, more likely, something else has absorbed the radiation at that wavelength before it reached your eye or detector. In the case of the Sun and other stars, this absorption takes place near the surface and in the lower atmosphere.


This image shows the spectrum of the Sun. Notice the familiar 'rainbow' covered in dark absorption lines.




Image courtesy of University of Texas


The opposite effect (a sudden increase in brightness at a specific wavelength) is called an emission line.



WollastonWhy is it called a spectral "line"? History again.! The term 'line' was first used by William Wollaston in a paper he wrote in 1802. Wollaston, who was born in East Dereham, Norfolk, had been experimenting with a prism (triangular glass block) to split sunlight into its colours and described his experiment as follows:


"If a beam of daylight be admitted into a dark room by a crevice 1/20 inch broad and received by the eye at a distance of 10 or 12 feet, through a prism...the beam is seen to be separated into the four following colours only, red, yellowish green, blue and violet." As he was using a 'crevice' (a narrow slit) to let the light into his prism he would have seen a spectrum something like the one displayed above (although this one shows far more detail in absorption lines than Wollaston would have seen). The light is spread by the prism (giving the width of the spectrum) but the height is just a result of the length of the crevice."


However he noted that the spectrum was not smooth in brightness but apparently divided into different coloured regions by patches of darkness and since these were fairly sharp (left to right) and long (top to bottom) as in the picture above, he called them 'dark lines'. Ever since, scientists have been calling these breaks in the spectrum, spectral lines (absorption lines if they are dark and emission lines if they are bright).


BrainiacHowever Wollaston really blew it, as you might say, because he didn't think there was any special significance to these lines (WRONG!). He thought they were just nature's way of dividing the colours of the spectrum. If he had realised that he had just invented the very important science of astronomical spectroscopy he would have been a lot more famous than he is. That's life.


Active Region

A very hot area of the Sun where a lot of activity can be expected (flares or explosions for example). Active Regions are often associated with dark sunspots when viewed in visible light, but they show up as very bright patches on the Sun when viewed at ultraviolet or X-ray wavelengths. Active Regions are also associated with very strong magnetic fields. Since 1972 Active Regions have been catalogued and given numbers - we are currently just past number 11,000. Active regions can last for several rotations of the Sun before eventually fading away.


Here's a picture of the Sun taken by the EIT ultraviolet camera on SOHO on which the active regions have been numbered. At this part of the solar cycle (near maximum) there are usually quite a few active regions visible at any one time.


Here's an image taken at about the same time which shows the sunspots associated with those active regions.






Where and when did the numbering system for Active Regions start? The current numbering system was started in the USA in 1972 for support of the Skylab spacecraft. Numbers before that were provided by the High Altitude Observatory group in Boulder, Colorado, although actually each solar observatory around the world had its own numbering system for sunspot regions. The Royal Greenwich Observatory in England, for instance, had been numbering regions since around 1874, but that stopped in 1976 when their solar observing duties were closed down to save money.


Alpha particle (α)


The usual name for 'the bare nucleus of a helium atom' (well it's shorter for one thing!) consisting of 2 protons and 2 neutrons.


The Sun releases energy at its core by changing hydrogen into helium. It does this through a process called nuclear fusion where atoms of hydrogen are 'joined' together to make atoms of helium. What we are actually talking about here are the nuclei of hydrogen and helium atoms - there's far too much energy for any electrons to stay attached to the nucleus. In this process billions of alpha particles are created every second.



The name is historical (like X-rays). When scientists first started playing with particles (the bits atoms are made of) they discovered different kinds. The first they called 'alpha' and the second 'beta'. We now know that 'beta' particles are the same thing as electrons and that 'alpha' particles consist of 2 protons and 2 neutrons - exactly what is left if you strip off the 2 electrons from a normal Helium atom. Even when that was realised, the name 'alpha particle' still stuck. You'll notice a lot of these examples in science where once a name has been given, it's hard to change it even when our understanding of what the thing is improves.


Question: How many alpha and beta particles can you get from a single helium atom?

Answer: 1 alpha particle (the nucleus) and 2 beta particles - the electrons that normally surround the nucleus.


Ampere (A)


The unit of electrical current and one of the seven basic units in the SI system. It is named after the French Scientist Andre Ampere and usually abbreviated in everyday speech to ‘amp’.


What exactly is an amp?


"One ampere of current is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 (that's 0.0000002) newtons per metre of length."


Sorry you asked?! Wow, so the force created by one amp flowing through two parallel wires one metre apart would be enough to lift a one-hundred-thousandth of a gram.


BrainiacBeware, the definition of an ampere is not 1 coulomb / sec since the coulomb is derived from the definition of an ampere! If you're studying electricity try not to get confused.


Andre AmpereAmpere, Andre (1776-1836)


The French Scientist who gave his name to the unit of electrical current. He is famous for investigating the magnetic force between wires carrying electric current. In an article published in 1820, he discusses the difference between static and current electricity:


"But the differences which I have recalled are not the only ones which distinguish these two states of electricity. I have discovered some more remarkable ones still by arranging in parallel directions two straight parts of two conducting wires joined the ends of two voltaic piles..." (batteries) "...the one was fixed and the other, suspended on points and made very sensitive to motion by a counterweight, could approach the first or move from it while keeping parallel with it. I then observed that when I passed a current of electricity in both of these wires at once they attracted each other when the two currents were in the same sense and repelled each other when they were in opposite directions"




The maximum displacement (movement) of a medium when a wave passes through
it. For many waves, the energy of a wave is proportional to the square of the amplitude - so, double the wave amplitude and the energy carried will be four times as much.







An analemma is the pattern the Sun, or its shadow, makes if you mark its position at noon each day over a complete year. It forms a sort of figure of eight.


Here's a photograph of an analemma we made on our house wall. We marked where the shadow of the tip of the bracket was whenever we were around at 12 noon (and it was sunny). The numbers indicate the start of each month of the year.


Angstrom (Å)


A unit of length named after the Swedish scientist Anders Ångstrom which is equal to one ten-billionth of a metre or 0.000 000 000 1 metres (1 x 10-10 m).


The angstrom (usually shortened to Å in print) is a convenient unit for recording the wavelength of visible light. The wavelength of visible light is from about 4000 Å to 6000 Å.


A sheet of ordinary book paper is approximately 1,000,000 Å thick. A single angstrom is about the size of an atom.


10 Å are equal to one nanometre so that visible light has a wavelength of between 400 and 600 nanometres.


Ångstrom, Anders (1814-1874)Ångstrom, Anders (1814-1874)


Ångstrom was a Swedish astronomer and physicist. He was a pioneer in the study of spectra and in 1862 showed the existence of hydrogen in the Sun's atmosphere.


Angular resolution


This describes our ability to see or measure two or more things as separate items instead of being blurred into one feature. For example, the human eye has an angular resolution of somewhere between 0.5 and 1.0 arcminutes. That means if the angle between two items (dots or lines, say) measured from your eye is less than about 1 arcminute then you will probably be unable to say whether there is one, or more than one, dot or line actually present.


In general that means the naked eye cannot distinguish detail that is smaller than about one arcminute in angular size.


One of the uses of optical instruments such as microscopes, binoculars and telescopes is to increase the angular resolution achieved by the viewer. In other words to be able to see things both smaller and closer together.


Try this classroom project to determine the angular resolution of your eye.


Brainiacangular resolution of your eyeDraw two lines 1mm apart on a piece of paper - how far away must the paper be before you cannot see the two separate lines? In the diagram below we want the angle to be 1 arcminute (written as 1' ) and so imagine the lines are on a large circle - what radius ( r ) for the circle makes the correct angle when the distance between the lines is 1mm


1' = 1/(360x60) part of circle

circumference of circle = 2 x π x r (π = 3.1415927)
so 1 mm has to be 1 / (360 x 60) of (2 π r)
so r = (360 x 60) / (2 π) = 3438 mm = 3.5m


Try it! But do it with a friend and draw several lines (don't tell anyone how many) 1 mm apart on the paper and stand 6 metres away. Ask your friend how many they can see - if you just draw 2 lines and say - can you see two lines - they will nearly always say yes! Now move closer by 1 metre each time and repeat the experiment until they get the right answer.


The angular resolution of your friend's eye can be calculated as:


ang res = (arc mins in a circle) / 2 x π x 1000 x r)


where r is the distance in metres at which they can 'see the right answer'




ang res (in arc min) = 21600 / (6283 x r)

so if your friend gets the right answer at 5 metres distance their eye's angular resolution = 21600 /(6283 x 5) = 0.7 arc mins - exceptionally good eyesight.


Archimedes of Syracuse (287-212 BCE)


Archimedes of Syracuse (287-212 BCE)Archimedes was born in Syracuse in Sicily. His father was Phidias, a wealthy astronomer. Archimedes was sent to study in Alexandria in Egypt when he was young. He studied with the best minds in ancient Greek, the 'disciples of Euclid'. After completing his studies Archimedes returned to Syracuse to become a mathematician and inventor.


Much of his time was devoted to the development of weapons. Archimedes had helped try to save Syracuse from the Romans, by inventing things that could be used to defeat the Romans. He designed the catapult, which was used to throw large objects long distances. He is also said to have invented a mirror system that reflected the Sun's rays to set the enemy's ships and boats on fire (there is little evidence to suggest that this ever existed or would have ever worked - it would have taken huge mirrors!). However his most famous discovery was the 'Archimedes principle' which he is said to have discovered while having a bath.


Archimedes principle states that 'a body immersed in fluid loses weight equal to the weight of the amount of fluid it displaces'.


Archimedes invented many things but is most famous for his ‘principle’ and his work with mathematics (he came up with the equation for calculating the volume of a sphere, you know!). According to legend, Archimedes died while drawing some of his maths problems in the sand. A soldier walked up next to him and said, "Follow me!" Archimedes refused and said, "Do not disturb my drawings." The soldier killed him on the spot. He died at the age 75 and to honour his memory friends and family erected a tombstone showing a difficult mathematical problem on it.



Part of Archimedes’ fame is said to have come from the fact that he was able to solve the problem of whether King Hiero II's crown was fake (made of a mixture of gold and silver instead of pure gold). However, we doubt he could have done so accurately enough with the equipment available to him at that time, at least in the way his experiment is usually described.


A more imaginative and practical method of determining whether or not the crown was fake would have been to make use of Archimedes principle of buoyancy and his law of the lever. He could have suspended the crown from one end of a pair of scales and balanced it with an equal mass of gold suspended from the other end. Then he could have placed the crown and gold in a tank of water. If the scales remained balanced then the crown and gold would have had the same volume and so they would have been made from the same substance. They would have had the same density (mass / volume). But, if, under water, the scales tilted in the direction of the gold, then the crown would have had a greater volume and it could have been exposed as a fake!


How to get rich:

Weigh out six, 5g lumps of modeling clay. Mould each one into the shape of your favourite animal. Cut open five of the clay animals and take out 1g of clay from inside them.


Replace the clay you have taken out with equal weights of other substances such as aluminium, foil, bubble wrap, paper, etc. Now for the good bit. If you want to make some easy money bet your friends that they can't tell the fake animals from the one made of 5g of pure modelling clay.


Tell them that they may use a pair of scales, a tank of water and a 5g lump of modelling clay. Bet they won't be able to figure it out. Follow the instructions above if you want to be as clever as Archimedes!


Arc Degree ( o )


A unit of angular measurement. There are 360 arc degrees in a full circle.


A British pound coin covers an angle of one arc degree when it is 1.25 metres from your eye. The Sun and Moon are about the same size in the sky, both of which cover about half an arc degree as observed from the surface of the Earth (you can cover the full moon with your thumb at an arms length, but don’t do this with the Sun, as looking at the Sun without protection can damage your eyes!).



Why are there 360 degrees in a circle? Why not 300 or 400? An interesting question! The Greek astronomer Hipparchus was the first to suggest the division of a full circle into 360 degrees. He probably derived that number from a division of the 12 signs of the Zodiac (which was a much more ancient idea) into 30 parts each and that related pretty well to about 30 days in a month or about 360 days in a year.


Also don't forget that when counting and arithmetic were pretty cool things to be able to do (without a calculator) it was important to make it easy. So using 12 as a unit was maybe easier than using 10 as a unit because with 12, figuring out halves, quarters, thirds and sixths is easy. Why? Because 2,3,4,6 all divide into 12 but only 2 and 5 divide into 10. We say that 2,3,4,6 are FACTORS of 12. Looking at it that way you can also see why 360 was a good number rather than 300 or 400.


Question: How many factors does 300 have? Answer - 16
Question: How many factors does 400 have? Answer - 13
Question: How many factors does 360 have? Answer - 22!


BrainiacSee the Factary entry for Celsius to see how such thinking might also have influenced other number scales.



Arc Minute ( ' )


See also, Arc Degree.


A unit of angular measure in which there are 60 arc minutes in 1 arc degree - remind you of anything?! In words it is usually abbreviated to arcmin and a single apostrophe is used as the mathematical symbol for an arcminute ('). For example 3' means 3 arcmin.


A British pound coin covers an angle of 1 arcmin when it's at a distance of about 75 metres from your eye.


Arc Second ( " )


See also, Arc Degree and Arc Minute.


Arc Second SunA unit of angular measure in which there are 60 arc seconds in 1 arc minute and therefore 3600 arc seconds in 1 arc degree. In words, abbreviated to arcsec, and a double quotation mark is used as the mathematical symbol for an arcsec ("). For example 3" means 3 arcsecs.


1 arcsec is the angle covered by a one pound coin at a distance of 4.5 km - or by the thickness of a sheet of paper at a distance of 25m (make up your own examples)! Which ever way you describe it, an angle of 1" is very small!


When instruments observe the Sun from Earth, one arc second is equal to about 725 km on the Sun's surface. Here is an image from SOHO showing how big and area 240" x 240" is on the Sun (from SOHO).


Since the Earth's orbit around the Sun is elliptical and not circular, the distance between the Sun and Earth changes throughout the year. At their closest, around January 4th, an angle of 1" translates to 713 km on the Sun. Around July 5th when the Sun and Earth are at their furthest apart, 1" corresponds to 737 km on the Sun.


BrainiacHere’s a project you might like to try in class or at home:

In the Northern Hemisphere in winter the Sun appears just over 3% larger than it does in summer. On 4th January its angular diameter is 32' 32" but at the beginning of July it is just 31" 28"

Design an instrument which projects the Sun’s image and from which you can measure the angular diameter of the Sun. Remember you must never look at the Sun directly. Try and measure the angular diameter of the Sun as regularly as possible for a year. How does it change? Can you detect the very small change? If the diameter of the Sun’s image is 100 mm in summer it should be about 103 mm in winter. Before starting, decide whether your measurements will be accurate and precise enough to show the difference.


BrainiacIncidentally (and we can't emphasis this enough!), notice that the Sun is closest to Earth in January so it's not the varying distance between the Sun and Earth that causes our seasons!




Image of an ArcadeArcade


The name given to a series of magnetic loops in the solar corona when they appear side by side, as in this image taken with the TRACE spacecraft. A bit like a shopping arcade!






The belief that the position and movement of the planets in the solar system can influence people's behaviour. An idea not supported by most astronomers, but who knows, astronomers have been wrong in the past.


Many people look at their horoscopes in magazines, to see how rich they’ll become next month. In ancient times, and even today in some cultures, marriages are made in heaven by matching your birthdays and even the hour you were born.


Astronomical Unit (au)


Often defined as the average distance between the Earth and Sun, which is about 150 million kilometres. Because the Earth’s orbit is elliptical and not circular, the Earth is about 3.4% closer to the Sun in January than it is in July.


The Earth is closest to the Sun in early January (and furthest in early July). This change in distance has only a small effect on the intensity or strength of the radiation received from the Sun, and is not the major cause of the seasons. In the southern hemisphere the effect adds to the seasonal summer effect but detracts from the northern hemisphere summer. On average therefore southern hemisphere summers ought to be warmer than the corresponding ones in the north, though all kinds of geographical effects must be considered too.



If you really want to impress (or bore) people, insist on the correct definition of the Astronomical Unit, which is that it is the radius of a circular orbit in which an object of negligible mass and free of other disturbances, would revolve around the Sun in 365.2568983 days! This is 149,597,871 km - no where near the ‘average distance from the Sun’ which is 149,598,023 km. The average distance is therefore actually 1.0000010178 au.




Now you can see why people cheat and use the first "definition"!



Atmospheric tricks (sunrise and sunset)


Why does the Sun often appear to go red as it begins to set? The answer starts with explaining why the sky is blue!


Light from the Sun is composed of all the wavelengths of visible light from violet to red. However when sunlight hits the atmosphere it is affected differently depending on its wavelength. Short wavelengths (violet / blue) are batted all over the place (the technical word is 'scattered') whereas the longer wavelengths (red) are relatively unaffected. Blue light from the Sun that was originally NOT heading for our eyes can be scattered at random so some of it does end up in our eyes - blue light comes from all directions and everywhere in the sky appears blue.


When the Sun sets and gets lower in the sky the sunlight has to travel through a much thicker layer of atmosphere than at midday. This gives more chance of the blue light being scattered out of the line of sight. What we see is what is left - that is a mainly red Sun.




The basic building block of all matter. All atoms consist of a nucleus containing protons and neutrons surrounded by different numbers of electrons. (Hydrogen is the only exception since it normally only has one proton and no neutrons in its nucleus). The different number of protons in the nucleus is what make the atoms appear as different elements (for example, Helium has 2 protons and 2 neutrons, Oxygen, has 8 protons and 8 neutrons, Carbon has 12 protons and 12 neutrons).


Image of an Atom


However, this picture is only partially true, since it shows that the electrons live around the nucleus but it is also misleading since it gives you the idea that the electrons move in circular paths. In fact, this is not the case. An electron should be thought of more as a fuzzy "cloud" of energy that is able to fill some of the space around a nucleus.




A word which can be attached to another which means 0.000 000 000 000 000 001 of something (1x10-18).


About 1 million hydrogen atoms would have a mass of about 1 attogram or it takes light about 1 attosecond to travel the width of an atom!




AuroraA display of coloured light in a planet's atmosphere, usually near the magnetic poles. This light is emitted when charged particles trapped in a planet's magnetic fields interact with atoms of atmospheric gases. Aurorae are visible on Earth as the Aurora Borealis or northern lights and the Aurora Australis or southern lights.


The word 'aurora' is the Latin word for "dawn".


Here's an example of some colourful Aurorae photographed by Jan Curtis in Alaska.





Aurorae (OK, auroras if you want to be lazy) come in all shapes and sizes and colours. The colour depends on which atoms in the Earth's atmosphere are emitting the light. Oxygen atoms give red and green light, nitrogen gives blue light.


Find out more about aurorae


back to top


Sun|trek homepage | Sun|trek Adventures | Solar Surface & Below | Hot Solar Atmosphere | Magnetic Sun | Flowing From The Sun

Sun/Earth Connection | Solar Spacecraft | Earth & Beyond | The Sun our Star | Factary | Gallery | Hot News | Contact Us