P7 1

P7 1

Naked eye astronomy
1. Sun appears to travel from east to south in 24 hours
2. Earth rotation is anticlockwise
3. Motion of the moon
  1. Moon appears to travel east-west across the sky once in about 25 hours
  2. Moon does a complete orbit of earth in 28 days
  3. Waxing moon = Getting bigger
  4. Waning moon = Getting smaller
  5. Crescent moon = Less than half of moon visible
  6. Gibbous = More than half of moon visible
4. Motion of the stars
  1. Stars appear to move east to west - due to rotation of earth
  2. The pole stars dont appear to move - all the other stars seem to orbit around the pole stars
  3. Stars appear to travel across the sky once every 23hrs 56min
  4. Sidereal day - 23hrs 56min - strict 360 degree rotation of earth
  5. Solar day - 24hrs - Looking from the sun - Takes a bit more than 360 degrees as the earth is rotating around the sun
5. Changing night sky
  1. Over months the 4 minutes add up and the stars appear to be in different places
  2. You can only see stars in opposite direction from the sun
  3. Stars that are behind the sun are not visible because it will be daytime when we see it
  4. So at different times of year you can see different stars
6. Motion of the planets
  1. They appear to move a little bit forwards
  2. Sometimes they are known to slow down and reverse direction known as retrograde motion
  3. This is because the earth overtakes mars on orbit and therefore we see mars in different places
7. Celestial sphere
  1. Declination is measuring up from the equator
  2. Ascension is across the equator
  3. Anything we see in the sky we can give it an angle by imagining that all the stars rest on a bigger sphere outside the earth
8. Eclipses
  1. Solar eclipses
    1. Not that frequent because of factors:
      1. Tilt of the moons orbit
      2. Only seen in a small part of the earth
      3. The distance from the earth and moon changes - elliptical orbit
      4. Sometimes shadow isnt big enough to be full eclipse
  2. Lunar eclipse
    1. When the moons light gets blocked by earth
    2. Only happens when things line up
Telescopes
1. Refraction
  1. The speed of any wave is affected by the material that it is travelling though
  2. If a wave is travelling from one medium to another then this can cause the speed to change
  3. A change in speed will cause a change in wavelength (frequency wont be affected)
  4. Wave speed = frequency x wavelength
  5. REVISE HOW TO DRAW LENSE DIAGRAM IN BOOK
  6. As seen from diagram the image will be upside down
2. Power of a Lense
  1. How strong a lens is (how quickly it can bring parallel rays of light together to a focus) can be measured by its powe
  2. Power (D) = 1/focal length (m)
  3. Power is measured in dioptrees (D)
  4. They are inversely proportional
  5. The longer the focus length the less powerful it is
  6. Two lenses in a telescope
    1. Objective lens - collects light and produces an image
    2. Eyepiece lens magnifies the image for viewing
  7. Eyepiece lens has to gave a higher power than objective
  8. The two lenses need to be the correct distance apart
3. Magnification of a telescope
  1. It is the size of the angle that something takes up in our view
  2. Another way of describing the size of something is by its angular distance
  3. With 50x magnification the angular size is 50 times bigger
  4. magnification = focal length of objective lens / focal length of eyepiece lens
  5. If the eyepiece focal length is longer then the image will be smaller
  6. A telescope can not make a distant star look bigger as it will still be a blop of light (bad quality)
  7. But a telescope is capable of spreading out stars we thought were one huge star
  8. A good telescope will produce a magnifies image but also bright clear and sharp
  9. The bigger the aperture (end of the telescope) the more light that can come in. So images will be brighter and sharper
4. Diffraction
  1. When a wave passes through a gap (an aperture)
  2. Small aperture more diffraction
  3. Big aperture less diffraction
  4. To minimize diffraction the aperture has to be larger than the wavelength of light
  5. Because different colours have slightly different wavelengths, we get focuses for different colours - this is called dispersion
  6. Because of this we use mirrors in telescopes as they dont refract
  7. The mirror used in a reflecting telescope must be concave
  8. When parallel rays of light hit a concave mirror they are reflected to a focus in the front of the mirror
  9. Using mirrors gives no problem with dispersion as every ray of light will be reflected by the same focus

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