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The Earth spins once on its axis in . . .
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The Earth orbits the Sun once . . .
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Earth & Beyond
Physics (GCSE) Revision (Universe)
| Statement | Response | Comment |
|---|---|---|
| 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
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