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| Comment |
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| 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 ) |
or...
