P7 Observing the universe notes

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P7 Observing the Universe
P7.1 Observing the sky with the naked eye
THE SOLAR SYSTEM
The solar system is the collection of planets, comets, and all other objects that orbit the sum. The sun
is a medium sized star.
The earth take 365.25 days to complete one orbit we call this a year. It
also rotates about an imaginary line called its axis.
It takes the Earth 23 hours and 56 minutes to do one whole rotation on its
axis. This is called a sidereal day. A complete solar day is 24 hours
because that's how long it takes for the same point on Earth to face the
sun again. This is because whilst the Earth is spinning on its axis, it is
also orbiting the sun, which means to catch up with the sun again it
needs to rotate a bit more for 4 minutes.
HOW THINGS MOVE ACROSS THE SKY
The sun appears to travel EAST to WEST across the sky once every 24
hours.
The starts appear to travel EAST to WEST across the sky once is a very
slighter shorter time period (about 23 hours 56 minutes).
The moon appears to travel EAST to WEST across the sky once in a slightly longer time period (about
25 hours).
The planets appear to move with the stars but change their positions in complicated patterns.
THE MOON
We can see the moon during the night because the sun's light reflects on half of the moon's surface.
This means that depending on the position of the moon, in relation to the Earth and the Sun, we can see
the moon's different phases.

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ECLIPSES
Eclipses happen with light from the sun is blocked
There are two types of eclipses. Lunar eclipse and Solar eclipse.
Lunar eclipse
As the moon orbit the Earth, sometimes it passes into the Earth's shadow. The Earth blocks sunlight
from the Moon, so almost no light reflected form the Moon and it just seems to disappear. A total lunar
eclipse is where no direct sunlight can reach the moon.…read more

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Eclipses do not happen that frequently. This is because the moon actually orbits the Earth at a tilt which
means that most of the time, the moon won't line up to cause a lunar or solar eclipse. Partial eclipses
are therefore more frequent as they don't have to line up perfectly for this. Even when there is a solar
eclipse, there is only a small region on the Earth from which it can be seen.…read more

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Earth.
Mars appears to change direction once every two years or so. Slower moving planets further out change
direction less frequently.…read more

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P7.2 How does a telescope work?
Converging lenses are CONVEX. They get fatter towards the middle.
All lenses have a principal axis, a line which passes straight through the middle of the lens.
The focal point of a lens is where rays initially parallel to the principal axis meet. (All lenses have two
focal points, one in front and one behind the lens).
The focal length of a lens is just the distance between the middle of the lends and its focal point.…read more

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DRAWING RAY DIAGRAMS
Most objects in space are so far away that their lights are almost parallel. We need to be able to draw
ray diagrams of the light being focused with a converging lens.
1. Draw one ray going directly through the middle of the lens. Carry this ray on straight through
the lens.
2. Draw a line parallel to this line going through the focal point before the lens. This line will
converge parallel to the principal axis.
3.…read more

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The distance between the two lenses is equal to the sum of the two focal lengths.
What are the limitations of refracting telescopes?
Chromatic aberration different colours are focused to different points so the image is blurry.
Large lenses sag under their own weight.
Large pieces of glass are not completely uniform which means there are slight impurities.…read more

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In a Reflecting mirror, a parabolic mirror focuses all parallel rays to one point.
This can be used as the objective of the telescope instead of a convex lens
Advantages: Reflects all colours in the same way. It is possible to build very large mirrors bigger
aperture collects more light to see fainter/distant objects. Lenses absorb some wave length as light
passes through glass so reflectors are better to use.
When a wave passes through an aperture of similar size to its wavelength it diffracts.…read more

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P7.3 What are the objects we see in the night sky and how far
away are they?
The distance to nearby stars can be measured by parallax.
Parallax is apparent change in position of an object against a distant background.
In astronomy: The parallax angle is half the angle moved against distant background stars over 6
months (at the opposite ends of the Earth's orbit). The nearer an object is to you, the greater the
angle.…read more

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All hot objects emit radiation from a continuous range of the EM spectrum. The hotter the object the
greater the luminosity overall. The hotter the object, the higher the frequency/lower the wavelength of the
radiation which is emitted the most.…read more

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