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
Phases of the moon
- a lunar eclipse happens when the moon passes behind the earth meaning no sun light can reach it.
- so the moon seems to disappear.
- 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
Lunar and Solar eclipses
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
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.
- 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)
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.
- 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.
- 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
parallax angle diagram
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
- 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
- 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
- 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
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.
- 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
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