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G - Factary


Galileo, Galilei (1564-1642)


He is one of the most famous Italian physicists and mathematicians of all time. He is best known for discovering the four largest moons of Jupiter (now known as the Galilean satellites) using the newly invented telescope. The Galilean satellites of Jupiter are Io, Europa, Ganymede and Callisto. These are easily seen in binoculars and in fact are bright enough to see with just your eye, although the glare of the planet normally drowns them.


Galileo is also remembered for demonstrating, from the leaning tower of Pisa, that all objects accelerate (fall) towards the Earth at the same rate. We now call this the acceleration due to gravity. He is also infamous for disputes with the Catholic church, for which he spent much of his life under house arrest.


Galileo also studied sunspots and noted that the Sun rotates.


Gamma Ray


The highest energy (shortest wavelength) waves in the electromagnetic spectrum.



How short can the wavelength of a photon be? One possible answer is to consider the wavelength-energy connection. The shorter the wavelength of electromagnetic radiation, the higher its energy. So if all the energy in the universe was used to create a single photon (packet of energy), maybe that would that be the ultimate, record-breaking shortest wavelength photon! And what about the longest possible wavelength for a photon of electromagnetic radiation? How about one with a wavelength equal to the diameter of the universe?




A unit of magnetic field strength named in honour of the famous mathematician Karl Gauss and still used today by some astronomers and solar physicists, even though they should know better and use the official SI unit of magnetic field strength which is the tesla. There are 10,000 gauss in 1 tesla and the Earth's magnetic field has a strength of about 0.5 gauss (or 0.00005 tesla) at the surface.


Typical values for the strength of some magnetic fields are:


Earth's magnetic field




Small bar magnet




Strong lab magnet




Surface of neutron star






Gauss, Karl Friedrich (1777-1855)



A German who, from an early age, was brilliant at mathematics. It is said that Gauss was only five when his class was asked to add all the numbers from 1 to 100 (presumably the kind of thing teachers still do to keep classes quiet for an hour!) Gauss replied immediately with the answer 5050. Remarkably, he wasn’t just very fast at adding up, he was a true mathematician and had seen a short-cut. He realised there were 49 pairs of numbers adding to 100 (1+99, 2+98, 3+97.… 49+51), all of which come to a total of 4900, then there is a 50 in the 'middle' and a 100 on the end. So the answer is 5050 - easy!


Apart from many purely mathematical studies, Gauss made great advances in the theory of planetary motions and our ability to calculate the orbits of planets, comets and asteroids.


Geomagnetic Storm


A worldwide disturbance of the Earth's magnetic field, caused by solar activity.


Geosynchronous Orbit


The orbit of a satellite that travels above the Earth's equator from west to east so that it has a speed matching that of the Earth's rotation. Because of this it remains stationary in relation to the Earth’s surface. It is also known as a geostationary orbit. Such an orbit has an altitude of about 35,900 km (22,300 miles).


The geostationary belt seen from a direction fixed in space


The geostationary belt seen from a direction fixed in space, taken from


Giga (G)


Prefix meaning a billion or 109. That's 1 000 000 000, or one thousand million of something. Your computer probably has several gigabytes of disk storage. Somewhere between your 31st and 32nd birthdays you can celebrate having lived for one gigasecond!




Geostationary Operational Environmental Satellite. A series of US satellites to monitor the Earth and its environment. One satellite in the series measures X-rays coming from the Sun and solar scientists use those data to monitor flares occurring on the Sun.




Global Oscillations at Low Frequencies. An helioseismology instrument on board SOHO. It measures vibrations in the Sun.


Gram or gramme


A unit of mass. Originally it was defined as the mass of 1 cubic centimetre of water at 4 oC. Now, however, it's just one thousandth of a standard kilogram. The word comes from gramma which is the Latin for 'small weight'




A small region on the Sun’s surface whose bright centre shows hot gases rising to the surface and whose dark edges show where cooled gases descend towards the interior. Individual granules appear and disappear on time scales of about 5 minutes and are typically about 1000 km across. Granules can be seen surrounding a sunspot in this image.




Gravitational Potential Energy (GPE)


See also Energy.


Gravity is all around us and acts on everything that has mass. Gravity creates a FORCE which pulls everything together and to act against that force (for example climbing a hill) you must do WORK.


There are many situations you come across every day when you have to do work against gravity: cycling up hill, walking up stairs, getting out of bed (unless you're on the top bunk!), lifting a bag off the floor. Life is one long struggle against gravity!


If you are doing work, you must be using energy. Consider moving a book from the floor to a shelf. In lifting the book you are using energy to overcome the Earth’s gravitational attraction between the book and the Earth. Since energy cannot be created or destroyed (only turned from one form to another), the energy you have used in lifting the book must have gone somewhere.


In raising the book you have given the book gravitational potential energy. It's called gravitational because the work you had to do was against gravity. It's energy because you had to do work and it's potential because although the book is now at rest it still must have the energy you gave it. This energy is available to do work and so the book on the shelf has the potential to do work (for example if it falls off and smashes something on the floor!).


Let’s add a few numbers as an example. If the book has a mass of m kilograms and is raised to a height h metres against the force of gravity (the acceleration, g, due to gravity is about 10 m/s2 at the Earth’s surface) then:


Gravitational potential energy = force x distance

  = ( m x g ) x h

  = mgh joules


So if a 2 kg book is lifted 1.5 metres above the floor it has 2 x 10 x 1.5 = 30 joules of gravitational potential energy which would be released if it fell to the floor.




As many European countries were influenced at one time or another by the Ancient Greeks and Romans, many of their words have influenced English words or in some case we have copied their words directly.


You probaby know more Greek than you might imagine. You'll have come across the word thermos before - probably on a picnic - but what other Greek words do you know? You've heard of helium? That new element was first discovered in the Sun (see spectral lines) so it was named after the Greek Sun god - Helios. If you are called ‘Helen’, then you are also named after the Sun god.


The ancient Greek word for the Earth was Ge and their word for study was logos - hence 'Geology' is the study of the Earth (and that's where all the -ology words, sociology, psychology come from too - not to mention your study record 'log' book).


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