P7 - observing the universe - revision cards


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  • Created by: 94Y86
  • Created on: 25-05-12 11:43

Observing the sky

  • A siderial day is thetime take for a star to return to its original position, its the time it takes for the earth to spin one on its axis 
  • a siderial day is 23h 56mins 
  • a solar day is the time it takes for the sun to appear in thesame position in the sky, itis thetime it takes for the earth to orbit the sun once.
  • a solar day is 24 hours 
  • the sun and the moon appear to cross the sky from east to west this is because the earth spins from west to east.
  • the moon takes 25 hours to appear in the same position
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Phases of the moon


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lunar eclipse

  •  a lunar eclipse happens when the moon passes behind the earth meaning no sun light can reach it.
  • so the moon seems to disappear.

Solar eclipse

  • The moon is the right size and distance away that when it passes between the sun and the earth, it can block out the sun.
  • so a solar eclipse is when the moon blocks the light from the sun.

Eclipses dont happen very often  because the moon orbits the earth at a slight angle to earth's orbit around the sun. so most of the time they dont line up to cause a eclipse

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Lunar and Solar eclipses

               SOLAR                                                       LUNAR


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measuring the position of stars

  • the positions of stars are measure from angles seen from earth 
  • its like latitude and longitude on earth but for the sky 

The sky appears to turn as the earth spins so astronomers picked two Fixed positions to measure from: 

  • the pole star - a star directly above the north pole
  • celestial equator - an imaginary plane, running across the sky, extending out from the earths atmosphere.

There are two angles used to measure positions in the sky, these are : 

  • Declination - measure by degrees and is celestial latitude
  • Right accention - measure by degrees and time and is celestial longitude
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Movement of planets

  • all planets orbit the sun in the same direction but at different speeds. The closer to the sun the faster the planet will orbit.
  • without a telescope you can see the naked eye planets, these are : mercury, venus, mars, jupiter and saturn.

a way of remembering these planets : my valuable method jumps to saturn

  • the planets seem to gradually move from west to east 
  • the outer planets (mars to neptune) seem to change direction for a bit making a loop or squiggle.
  • this happens because both the planets and the earth are orbiting the sun so we are seeing their movements relative to earth. 
  • the further the planet the less frequent they change direction.
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converging lenses

  • it causes rays of light to converge (come together to a focus)
  • all lenses have a principle axis , a line which passes straight through the middle of the lens.
  • Thicker lenses bend light more, they are therefore described as more powerful. 
  • Powerful lenses have short focal lengths
  • The power of a lens is measured in dioptres (D) and is given by the formula:

Power =          1               
               focal length (m)

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Ray diagram


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simple refracting lens :

  • uses two converging lenses - objective lens and an eyepiece 
  • the lenses are alligned so that they have the same principle axis and focal points.
  • by the time the light rays from space arrive to earth the become parrallel 
  • the objective lens converges (brings the light rays together) these parallel rays to form a real image between the 2 lenses.
  • the eye lens is much more powerful that the objective lens .

Magnification = Focal lenth of objective lens /

                 focal lens of eye lens.

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Concancave mirror

  • most astronomical telscopes use a concave mirror instead of a convex objective lens.
  • concave mirrors are shiny on the inside of the curve, parallel rays of light shining on a concave mirror reflect and converge.

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Parallax angle

  • parallax angle is half the angle moved against distant background stars over 6 months (half a year).
  • the nearer the object the greater the angle 
  • parallax angle is measured in the unit of arcseconds 
  • parallax is used to calculate the distance to stars, the smaller the angle the further the star is. 
  • the distance to the nearby stars is measured in parsecs (about 3 light years)  

Distance in parsec = 1 / angle in arcsec

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parallax angle diagram


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brightness of a star

  • intrinsic brightness (how bright it would be if you went to it depends on two things:

1) size of the star        2) how hot the star is

  • the bigger and hotter a star is the brighter it is.
  • as you move away from a star itwill look dimmer this is because  less light from the star reaches us (it spreads out through space) 
  • so the observed brightness of a star from earth depends on:

1) intrinsic brightness       2) distance to the star

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Cepheid variables

  • cepheid variables are a group of stars
  • they pulse in brightness - they get brighter and dimmer over a period of several days 
  • how quickly they pulse is linked with their intrinsic brightness. the brighter the star the longer it will pulse 
  • the amount of time a star pulses is called the pulse period
  • stars with the longer pirsle have a higher intrisic brightness. 
  • the star with the longest pulse period must be furthest away
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Shapley's argument

  • the universe was one gigantic galaxy 
  • size of galaxy = 100000 parsecs across (parsec is unit of distance)
  • the sun and and solar system was far away from the center 
  • nebualea were huge clouds of gas and dust 
  • nebulae were part of the milky way
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Curtis's argument

  • universe was made of many galaxies 
  • size of galaxy = 10 000 parsecs across
  • the sun is near the centre of the galaxy 
  • the nebulae were other distant galaxies
  • the nebulae were separte from the mily way 
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Conclusion of debate

  • Neither shapley or curtis were completely right in theuir arguments, also so of their points were proven to be true. 
  • shapley was right that the solar system was far from the centre of our galaxy 
  • curtis was right that there were many galaxies in the universe , he was also right about the spiral nebulae was really far away.
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Hubble's discovery

  • hubble showed that their was objects outside the galaxy 
  • hubble helped solve the curtis-shapley debate by discovering the andromeda nebulea
  • using the largest telescope at the time (hubble telescope) he managed to see that there were many stars including cepheid variables in the nebulea.
  • Hubble was able to work out the distance to the nebulae by finding the distance to one of the cepheid variables (brightness and pulse freuency... see cards 13-14)
  • The andromeda neulea was 2.5 million light years away ( this solved the nebulea debate)
  • he studied other nebulae and found the same results (they were really far away) 
  • Mega parsecs are used instead of parsecs if the distance is enormous
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Movement of distant Galaxies

  • when a galaxie moves away from us its wavelength of the light differs - it becomes redder this is called red shift
  • by seeing how much of the light has been red shifted you can work out the reccession velocity (how fast it is moving away from us)
  • by using red shift hubble found that distant galaxies are moving away fastest


speed of reccession (km/s)  = hubble constant (km/s per Mpc) x distance (Mpc)

  • The value of the hubble constant is

70 km/s per Mpc       or           2 x 10^-18

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awesome, thanks :D



detailed and very helpful :) cgp revision guide ;)



This is really Helpful, Thankyou :)

Miss KHP


This is very good, especially for OCR triple science. It has lots of diagrams to help you visualise this information. Great use of colour. A good read! 

Once you have finished test yourself to check your progress



thank you so much my good friend



I will forever be in your debt

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