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S - Factary




Seasons are the changes in surface and atmospheric conditions that occur on a planet during its orbit around the Sun. Significantly different seasons only occur if the planet's axis is tilted relative to the plane of its orbit.

Most places on the Earth have four seasons: Spring, Summer, Autumn and Winter.
These are mainly caused by the 23° tilt of the Earth's axis because the northern and southern hemispheres of the Earth are alternately pointed 'towards' or 'away' from the Sun at six-monthly intervals.

See our ‘Earth and Beyond’ section for more details.


Image of Samuel Heinrich SchwabeSchwabe, Samuel Heinrich (1798-1875)


Observer of the Sun who discovered the Solar cycle, that is the 11-year variation of solar activity as shown by the varying number of sunspots. See our 'solar cycle' factary entry for more details.




Abbreviation for Systeme International d'Unites. This is the international system of units used, by international agreement, for physical quantities. It is based on the former MKS system which derived its name from three of the units used (Metre, Kilogram, Second).


There are only 7 fundamental (base) SI units - they are listed below.

















electric current






amount of substance



luminous intensity




These seven units are independent of one another and the amazing thing is that all other units can be described by combinations of these seven - even though in many cases we give the new 'derived' unit its own special name. For instance:


velocity or speed is measured in: m/s


density is measured in: kg/m3


For units concerned with electricity, it often takes four base units to define the new unit. For example electrical resistance is measured in:




Thank goodness it's also called an Ohm for simplicity (look up the entry for resistance for more on this).


Another one to test your friends and teachers with - what is measured in

m-1·kg·s-2? That unit is better known as a pascal - the unit of pressure!


sievert (Sv)


A secondary, or derived, SI unit of radiation dose equivalent, symbol Sv. This takes account of the type of radiation as well as the amount. For example, alpha radiation is 20 times more harmful than beta radiation. A dose of 0.25 Sv will not normally cause any ill-effects. 1 Sv normally leads to radiation sickness while doses of 8 Sv or more will almost certainly cause death.


Image of Rolph Maximilian SievertSievert, Rolph Maximilian (1898-1966)


Swedish physicist who carried out a great deal of research into the effects of radiation on the human body. The unit of radiation dose, the sievert (Sv), is named after him.




Image of Skylab in orbit around the EarthA space station which orbited Earth in the 1970s and performed many important observations of the Sun. Outside the scientific community, it is best remembered for its fiery end. On July 11th 1979 it crashed into the Indian Ocean and sparsely populated regions of Western Australia.


This photograph of Skylab was taken by an astronaut after they had finished working in it and were on their way back to Earth.


The bit that you can see with the four solar panels shaped as a cross was called the Apollo Telescope Mount (or ATM for short) and contained several instruments that studied the Sun. Data from the ATM is still used today, over 25 years later.




Image of SOHO SpacecraftThe Solar and Heliospheric Observatory. The largest and most comprehensive solar observatory ever put into space. It was launched in December 1995. Much of the data shown on this site and much of the research described are based on SOHO data.


Find out more about SOHO and other solar satellites.


Solar atmosphere


In general terms, an atmosphere is the outermost gaseous layer of a planet, natural satellite, or star. Virtually all of the matter in the Sun exists not as solid, liquid or gas but as plasma. Even so, the word atmosphere is used to describe the outer layers of the Sun because, like the Earth’s atmosphere, they are transparent at visible wavelengths.


The solar atmosphere includes the photosphere, chromosphere, transition region and corona.


Solar cycle


Charts showing the annual sunspot numbersThe 11-year variation in the Sun's magnetic field and all the features caused by it such as the number of sunspots, coronal mass ejections and solar flares. The variation is seen as an increase and decrease of solar activity on a regular basis, lasting approximately 11 years from beginning of end.


Here's a plot showing how the number of spots on the Sun has varied over the years.



Many things on Earth have been linked to the solar cycle, from weather to the quality of wine and the value of stock markets.


Below is a plot that shows the relationship between the length of the solar cycle (it is never exactly 11 years long) and the average temperature in the Earth's northern hemisphere.

Graph showing the relationship between the length of the solar cycle and the average temperature in the Earths northern hemisphere


The data were published by Friis-Christensen & Lassen in Science, 254, 698, 1991.


Although the link looks very good for the years studied, there is still the chance it's not true for all times. However if the temperature does change with the solar cycle length maybe that could be a reason for people finding links of the solar cycle with weather, quality of wine, length of skirt fashions or whatever.



The solar cycle was discovered by Samuel Heinrich Schwabe (1798-1875) - though it was an accident.


His original aim had been to try and discover any planets nearer the Sun than Mercury. Because he hoped to see them passing in front of the Sun (and appearing as small black dots on the Sun's disk) he kept records of any dark spots he could see on the Sun every day the weather allowed for 42 years.


An example of what he was looking for is shown in this image:


The circular black spot is Mercury passing in front of the Sun - the other black patches are sunspots.

Image showing Mercury passing in front of the Sun

In the end, Schwabe's work meant that, rather than collecting drawings of planets crossing in front of the Sun, he ended up collecting thousands of detailed sunspot drawings. But there was a happy ending! After 17 years of careful observation, he noted a roughly 10-year cycle in the number of sunspots. The numbers of sunspots increased and decreased every 10 years or so. This was an amazing discovery and all the data he continued to collect supported this conclusion.


Unfortunately, as is often the case with scientific discoveries, no one took much notice at first. However, in 1851, another German, Alexander von Humboldt included Scwabe's results in some of his work and that led to Schwabe's contribution being widely recognised.


Image showing Jupiters great red spot



Question: Schwabe discovered the sunspot cycle. Which other spot did he discover?


Answer: Jupiter's Great Red Spot which he was the first to draw in 1831.


These are pictures of Jupiter's Great Red Spot, which has now been around for at least 170 years. The pictures were taken with the Hubble Space Telescope. And, yes, we agree it doesn't look very red.


Solar limb


Image showing the sharp edged solar limb of the SunWhen viewed from a large distance away, the disk of the Sun appears to have a sharp edge. This edge is called the solar limb.


In this picture the Sun appears to have a sharp edge.


But in a close-up picture we see it isn't quite so sharp as we might think.



Close up image of the Suns surface

In fact the edge really is not sharp. Powerful telescopes that give a close-up view show all kinds of spikey features - as if the Sun is covered in grass. Also, throughout these pages you will see images of the Sun taken at different wavelengths. In some of those you can see that the solar limb is actually very fuzzy. So how sharp you think the edge of the Sun is depends on how detailed the picture is you're looking at and at what wavelength the picture was taken.


Solar maximum


The month(s) during the solar cycle when the number of sunspots reaches a maximum. Solar maximum occurred in 2001 and will occur again sometime around 2012.



What do you mean “sometime around 2012”? The length of the solar cycle (the time interval between one maximum and the next) varies unpredictably. Over the last 150 years, the cycle length has varied from 10 years to over 11.5 years. Having seen that the last two maxima were in 1990 and 2001, we can expect the next one sometime between 2011 and 2013. We will just have to wait and see!


Solar Maximum Mission (SMM)


SMM RepairSMM Repair


A satellite dedicated to observing the Sun, especially solar flares, which occur most frequently during solar maximum. It was in orbit throughout the 1980s - longer than had originally been planned. The mission was extended following a successful repair carried out by the crew of the shuttle Challenger in 1984. SMM re-entered the Earth's atmosphere and burned-up on December 2, 1989.


Solar minimum


The month(s) during the solar cycle when the number of sunspots is lowest. The last minimum occured in 2006.


Solar structure


The Sun is not just a uniform 'ball of gas'. It has many different regions that have different characteristics. This picture illustrates the main structures inside the Sun and the features we see on and above the surface.

Image showing the structure of the Sun

Image courtesy: Dr. S. Hill, SOHO Project


Solar wind


A stream of particles, mostly electrons and protons, flowing out from the Sun at speeds of 5-10 km/s. As it flows out, the solar wind accelerates and can reach speeds as high as 900 km/s. The solar wind is the result of the hot solar corona expanding into space.


Sound speed


The speed of sound in a gas or plasma (like the Earth or Sun's atmospheres) can be calculated if you know what particles, atoms or molecules the gas is made of and their temperature.



The formula is
speed = constant x (temperature)1/2
where the constant number depends on the types of atoms and molecules in the gas.



For any gas a useful rule is that the speed of sound is just over one half the average kinetic speed of the atoms and molecules.


The speed of sound in the Earth's atmosphere is approximately 330 m/s, but sound travels faster in liquids and solids. In water it can be around 1500 m/s and in something like steel more than 5000 m/s.


South Atlantic Anomaly


A region over the South Atlantic Ocean where the shielding from energetic particles (electrons, protons) by the Earth's magnetic field is not very efficient at the altitude that many spacecraft orbit the Earth. It is a 'nuisance' because many satellite orbits pass through this region and so satellites in these orbits are bombarded by showers of charged particles. These can upset sensitive instruments.


Below is a picture made from data taken by a NASA spacecraft. It shows where the spacecraft's instruments were affected by charged particles during certain orbits. The SAA is seen to be centred in the Atlantic just off the coast of Brazil.

Image showing the SAA region over Brazil

Data: NASA/GSFC/JPL, MISR Science Team



The energy of these particles presents a real problem for satellites in orbit around the Earth as the particles can affect the satellite's electronics.


Astronauts have even been known to see flashes in their eyes when their spacecraft travelled through this region as a result of the charged particles interacting with their eye.


Space weather


In much the same way that we experience changes in Earth weather (wind, rain, temperature), conditions in space around the Earth also change. The speed and density of the solar wind, the intensity of electromagnetic radiation from the Sun and the density of cosmic rays for example all vary and have different effects upon the Earth and orbiting satellites. The study of these conditions is called the study of space weather.




Long thin pieces of pasta. Typical diameter 0.6 mm, typical length 0.5 m. Chaotic forces operate such that accurate predictions of the pattern taken by more than, say 0.025 kg of the substance is impossible. Typical half life when exposed to omnivores, around 300 seconds.


Spectral line


A wavelength in a spectrum where there is a sudden increase or decrease in the intensity of electromagnetic radiation compared with the surrounding wavelengths.

So what you might ask! Amazingly, every substance in the universe produces a unique pattern of spectral lines. There are different patterns for hydrogen, oxygen, copper, iron - you name it.

By looking for these patterns of lines in the spectrum of their radiation we can tell what stars and other objects, even though they might be millions of light years away, are made of.




This is the ultimate 'analyser' tool. Imagine you have a substance and you have no idea what it is - it's just a lump or a blob of 'stuff'. Shine a strong light through it and into a spectrometer (you may need to make a very thin slice of a solid). Inside the spectrometer, the light is split up into a spectrum and the spectral lines are analysed by a computer. Since every chemical has a unique spectral line 'signature', hey presto, it tells you what chemicals are present in the sample.


BrainiacThe same tool is used to analyse the light coming from stars, planets and other astronomical objects.




Artists impression of the surface of the Saturn moon, TitanThis is how we get a lot of information about not only other stars but about other objects in the solar system. Titan, for instance, is a moon of Saturn. We know, by observing its spectrum, that its atmosphere is largely made of methane and nitrogen - the same sort of atmosphere we believe existed on Earth billions of years ago. So might there by life on Titan? It's unlikely, but thanks to analysis of the light from Titan, we know that it's worth taking a look. Even if we don't find life there, we might find out a lot about how life might have begun on Earth. That's why a European Space Agency probe called Huygens flew through Titan's atmosphere in January 2005. It was part of the Cassini mission to Saturn, launched on October 15, 1997 from Cape Canaveral.


The image above is an artist's impression of what the Huygens probe parachuting down onto Titan's surface might look like




In general this word is used to indicate a range of a particular thing. For example we talk of a spectrum of sounds, a spectrum of ideas. In our case we use the word to refer to the range of wavelengths of electromagnetic radiation. A rainbow for instance is a display of the natural spectrum of visible light from the Sun.


Spectra often contain emission or absorption lines, which can be examined to reveal the composition and motion of the radiating source.


Image of Spicules



A small, spikey feature in the Sun's atmosphere. It is best seen on the limb of the Sun where it makes it look as if the Sun has a covering of grass-like features. Their name comes from the Latin word 'spiculum' meaning a small, sharp point.




See Coronal Streamer.




Solar Ultraviolet Measurements of Emitted Radiation. An ultraviolet spectrometer aboard SOHO which studied the solar spectrum at wavelengths up to 160 nm.


Sun - the distance to it.


The distance between the Sun and Earth varies during the year, but on average it is approximately 93,000,000 miles or 150,000,000 kilometres.


BrainiacHow do we know how far the Sun is from Earth and who first measured it?
It's a long story but the basics are:


In the early 17th century, from observations of planetary positions and movements, Johannes Kepler and others were able to draw up a scale model of the solar system in which all the relative distances of the planets from the Sun were known. For instance, if the Earth-Sun distance was taken as one unit (now called the astronomical unit - AU) then Mercury is only 0.4 units from the Sun and Jupiter is 5.2 units from the Sun. The question is 'how big is a unit' in miles or kilometres?


The answer starts with finding out the size of the Earth. In the 3rd century BC the Greek scientist Eratosthenes worked that out pretty accurately by observing the different heights of the Sun in the sky as observed from different places on the Earth at the same time of day.


Nearly 2000 years later (science moved relatively slowly in those days!) the Dutch astronomer Huygens had an idea to measure the Earth's distance from the Sun using observations of the planet Venus.


He knew that viewed from Earth the planet showed phases (just like the Moon) as it orbited the Sun and he realised that when it was showing precisely a half-moon shape then the Sun-Venus-Earth angle must be 90 degrees. Now the geometry of right-angled triangles is easy. We can measure the angle between the Sun and Venus at that time (with a protractor), but we are still short of one side of the triangle. Huygens made an inspired guess (often a good place for new science to start!). He assumed Venus was the same size as the Earth and from its measured size in a telescope he could then work out its distance. Amazingly he then got nearly the right answer for the distance of the Earth from the Sun because Venus IS about the same size as Earth!


However Huygens isn't credited with the first real measurement of the AU since really his result was just a lucky guess. The credit normally goes to the astronomer Cassini (the one the gap in Saturn's rings is named after). He realised that once you have a firm and known base (the known size of the Earth), it is possible to play geometry for real. Using the effect of parallax it is possible to look at other objects in the sky from different places on Earth and see if their position appears to change relative to the distant stars. If it does then geometry will again allow you to calculate real distances. Cassini chose a time when the planet Mars was close to Earth and he sent a colleague to South America so they could both observe the position of Mars at the same time and look for any parallax effect. Although they succeeded in this way of getting the first real measurement of the AU, the observation was very difficult and the result not very accurate.


Using the same principle, observations of the planets Mercury and Venus when they cross in front of the Sun, called transits, can also be used. Because it is closer to the Earth, Venus gives the most accurate result. The first time this was tried was in 1761 and carrying the astronomers and equipment was the main reason behind Captain Cook's first voyage to the Pacific.


Transits of Venus are quite rare but occur in pairs every 100 years or so. The last one was on June 8th 2004 and the next is due on the 5/6th June 2012.





An instrument which uses the shadow cast by the Sun to tell the time.


BrainiacBecause the Earth’s axis is tilted in space and because the Earth does not travel at a constant speed around the Sun, the time shown on a sundial can differ from true ‘clock’ time. The correction needed to convert sundial time to clock time is called the Equation of Time.

Clock time = Time on sundial - Equation of time

The plot below shows the value of the Equation of Time throughout the year. Be careful when using this formula about what happens when the correction is negative (numerically two negatives make a positive!)

Equation of time




Image of SunspotsA dark spot on the Sun. More technically, it's a temporarily disturbed area in the solar photosphere that appears dark because it is cooler than the surrounding areas. Sunspots are formed by concentrations of strong magnetic fields. They usually occur in pairs or groups of opposite polarity that move together across the face of the Sun as it rotates. The number of sunspots visible at any time can be used as a measure of how 'active' the Sun is (see solar cycle).



Why are cool things darker? Because cool things contain less energy and if they have less energy they have less to give off (radiate) and so appear darker. Don't forget it's all relative - if we could see sunspots away from the Sun they would appear very bright because they are still have a temperature of nearly 4000 °C!


Find out more in our sunspot section




A very large explosion that marks the end of the lifecycle of some stars.


Crab Nebula supernova




An instrument on board the SOHO spacecraft which analyses variations in the solar wind by observing the radiation from hydrogen atoms. The acronym comes from Solar Wind Anisotropies


BrainiacAs our galaxy, the Milky Way, rotates, the Sun and its planets move through space. However, this 'space' (interstellar space, the space between the stars) is not empty. It contains a very thin 'mist' of hydrogen atoms. As our solar system travels through this mist, the particles in the solar wind bang into the hydrogen atoms in the mist and the effect of this interaction is what the SWAN instrument measures. Isotropic means 'the same in all directions' so the opposite 'anisotropic' means 'not the same in every direction'. SWAN is therefore looking to see how anisotropic the interaction of the solar wind with interstellar hydrogen is. By studying this it can look for changes in the structure of the solar wind itself.


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