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Earth & Beyond

Physics (GCSE) Revision (Universe)

StatementResponseComment
The Earth spins once on its axis in . . . a day (of 24 hours) Actually 23h 56m 4s
The Earth orbits the Sun once . . . each year of 365.25 days Actually it is a little more than a quarter of a day so we have to miss a leap year unless the century divides by 400.
The stars in the night sky stay in fixed patterns called . . . constellations They do move but very slowly so we see no appreciable change even after 100s of years.
The planets which are visible to the naked eye look like stars but . . . they move slowly across the constellations. Mars moves completely around the sky in just under 2 years.
Stars emit their own light, but planets . . . are seen because they reflect sunlight. The brightest planet is Venus, because it is the closest one to us and it reflects a lot of light from its clouds.
Where we see a planet depends on . . . where it is in its orbit relative to the Earth. We can overtake Mars and the other outer planets because they move more slowly than the Earth does.
The orbits of the planets are best described as . . . ellipses or slightly squashed circles. Apart from Pluto, the orbits of the planets are very nearly circular.
Comets have orbits which are far from circular. They can best be seen when they are . . . close to the Sun ( at perihelion ) This is because they are closer to the Earth but also because they have developed long tails and become much brighter.
The tail of a comet always points . . . away from the Sun. Because the solar wind and the pressure of sunlight blow the dust and gases away from the comet's nucleus.
Satellites can be put into orbit around the Earth. They can be used to . . . send or recieve information, monitor the weather and observe the universe. The type of satellite determines its orbit.
A telecommunications satellite is best placed . . . in a geostationary orbit above the equator. The period of the orbit is the same as the Earth's 24 hour day. It appears to be stationary above the same point all the time.
A spy satellite is best placed . . . in a polar orbit so it can see the whole Earth during a 24 hour day. It makes one orbit every 90 minutes so it does 16 complete orbits every day.
A satellite which is close to the Earth's surface has a . . . short orbital period. This is because it needs to move faster to avoid being pulled down by Earth's gravity.
The Earth, Sun and Moon all attract each other with a force called . . . GRAVITY All bodies with MASS attract each other - even humans!
As the distance between two bodies increases, the force of gravity between them . . . DECREASES Isaac Newton said it varies as the inverse square of the distance.
Mercury is the closest planet to the Sun. It orbits once every 88 days. Pluto takes 248 years, why? Mercury feels a strong force of gravity close to the Sun, it is much weaker at the distance of Pluto. Pluto also has much further to travel but it is moving much more slowly than Mercury.
A smaller body will stay in orbit around a larger one because . . . the combination of its high speed and the force of gravity prevents it either falling to the surface or flying away into space. Kepler's laws describe this perfectly.
To stay in orbit around a planet a satellite must have . . . the correct combination of speed and distance. The square of the period is proportional to the cube of the distance.
The further away an orbiting body is . . . the longer it takes to make a complete orbit. See Kepler's laws - not on the Physics paper but astronomers know them very well!
Our Sun is just one of millions in our galaxy which is called . . . the Milky Way We believe it is a spiral galaxy.
The stars in a galaxy are usually millions of times further away from each other than . . . the planets in the solar system. "You may think that it is a long way to the chemists, but that's just peanuts to space."
The Universe as a whole is made up of at least . . . a BILLION galaxies. You can almost certainly multiply this by 100 !
Galaxies are often millions of times further apart than . . . stars within a galaxy. If you put two grains of sand inside a cathedral they would still be closer together than stars in the galaxy - relatively speaking.
Stars, including the Sun, form when enough gas and dust in space is pulled together by . . . gravitational attraction. Large clouds of hydrogen gas are called nebulae.
Smaller bodies than stars may also form from clouds of gas and dust. They are called . . . PLANETS That's how Earth formed over 4.6 billion years ago.
One way to search for life on other planets would be to . . . send robot spacecraft to take pictures or return samples. Care must be taken to avoid contamination - both of the planet and our own when the samples return.
Living organisms might reveal their presence by . . . chemical changes they produce in a closed system. The atmosphere of Earth has free oxygen produced by green plants (photosynthesis).
Intelligent lifeforms may be able to communicate with us by . . . sending radio signals we can detect with our radio telescopes. Jodrell Bank in Cheshire has been 'listening' for intelligent signals for years - without success (so far!)
The search for extra-terrestrial intelligence (SETI) uses radio telescopes to try to find . . . meaningful signals in a narrow band of wavelengths - not just NOISE. If we detected binary code containing prime numbers we would be pretty impressed.
Individual stars, including our Sun, do not stay the same forever. The main reason for this is because . . . they eventually run out of fuel in their cores. Hydrogen is the fuel used by stars for most of their lives.
Stars are very massive so the force of gravity which holds them together is . . . very strong. It causes their cores to be highly compressed.
The high temperatures in their cores create . . . high pressure which holds the star up against gravity. This is a delicate balance, like balancing a ping pong ball on the jet of air from a hair dryer - try it!
The forces of gravity and pressure within a star remain balanced during . . . the main stable period of its life, which may last for billions of years. Our Sun is already 4.6 billion years old and good for another 4 billion ( we hope!)
When a star runs short of hydrogen fuel, its core begins to . . . Shrink Paradoxically, this cause the core to heat up - try squeezing the air in a bicycle pump.
When a star like the Sun reaches the end of its stable period, it will become a . . . RED GIANT! The Sun will swell up until it swallows the Earth and possibly Mars too.
When a star like the Sun reaches the end of its life, it will shrink to become . . . a WHITE DWARF These are very hot but small stars about the size of the Earth.
The matter from which a white dwarf is made is . . . millions of times denser than any matter on Earth. A teaspoonful would weigh more than a tonne.
Stars which are much more massive than the Sun do not die quietly. They may . . . explode as supernovae. They scatter their ashes throughout the galaxy.
We cannot see black holes directly, but we can detect . . . X-rays emitted by hot gas as it spirals into the black hole. Cygnus X-1 was the first black hole to be discovered in this way.
During a star's lifetime, nuclei of lighter elements gradually . . . FUSE together to produce nuclei of heavier elements. Hydrogen nuclei undergo fusion reactions to produce helium in the Sun.
The energy released by nuclear fusion is . . . radiated away into spcae by stars. This is how the Sun 'shines'.
Nuclei of heavier elements ( such as gold) are present in the Sun and planets. This suggests that . . . the solar system was formed from the material produced when earlier stars exploded. "We are stardust . . . "
Edwin Hubble discovered in the 1920s that the light from distant galaxies is redshifted. This suggests that . . . most of the galaxies are moving away from us. Some nearby galaxies are actually getting closer, but this is only a little local disturbance.
The further away a galaxy is, the greater its . . . redshift. If the Universe is expanding it is doing so uniformly.
Hubble's discovery of redshift strongly suggests that . . . the Universe began with a Big Bang. If you wind the tape back you will get a Big Crunch ( or gnaB giB )
Created by: J Thomson on 2005-04-25



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