COMPARE DAYTIME AND NIGHTTME.WHAT DID YOU OBSERVE?HOW ARE THE STARS IN SKY VIEW DIFFRENT AT THOSE TIME?
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Answer: WHAT DO WE SEE IN THE NIGHT SKY
Sun, Moon, Planets and their moons, Comets, Asteroids Meteors, Nebula and Stars.
WHAT DO WE SEE IN THE SKY FROM EARTH
During daylight we only see our Sun and sometimes the moon.At night we see all the shining objects mentioned above against the dark sky.
HOW DO WE FIND STARS IN THE SKY
Some of the brighter stars appear to form groups in the sky, these we call constellations. Most constellations were named a very long time ago by the Greeks or Arabs. People thought they could see the shapes of animals or their gods and named the constellations after them. In most cases it is very hard to imagine how they saw the shape that the star pattern is supposed to represent but we still use the same names today. In the first 3 or 4 months of the year we see the winter constellations which include three very beautiful and interesting ones. These are called ORION the hunter, ANDROMEDA a Greek goddess and TAURUS the Bull.
Constellations are not necessarily associations of stars, some groups of stars are not groups at all but are just in the same line of sight as seen from Earth. If you were able to view what we call a constellation from some other part of the sky (say looking at Orion from a planet orbiting a star in Taurus) it would look very different from the way we see it from Earth. Some stars appear bright in our sky because they are close to us, others may appear bright because they are giant bright stars. Some dim stars are actually very bright but are a very long way from us.
We as humans always like to put things into groups so that we can sort them out in our minds, we do this with everything even with people so we do it with stars too. We are also quite good at remembering shapes and we often have good imaginations so just as we can see shapes of rabbits and elephants in the clouds it is not surprising therefore that we see shapes in the stars, these are the Constellations.To find your way around the sky, one other constellation can always be used to find your bearings, this is URSA MAJOR the Great Bear or Plough. Ursa Major is always visible in the northern sky, it is bright and easy to find. It is shaped like a saucepan with four stars forming the square shape of the pan and three more stars forming the handle. The two stars of the pan furthest from the handle are called the pointers because an imaginary line drawn through them points to the north star called Polaris. The pole star does not move in the sky and is always north of the point directly above us. By finding Polaris and turning around half a turn we will be facing south, and will then be able to use our star maps.
WHY DO STARS APPEAR TO MOVE ACROSS THE NIGHT SKY
There are actually two different reasons why stars appear to move across our sky. The first is because the Earth is spinning and second because the Earth itself is moving around the Sun.
As the Earth travels around the sun (once a year), it is as if we are sitting on a Waltzer ride at the funfair. If the whole Waltzer moved around on its track with the cars not spinning, we would see everything around the ride appear to be sweeping past once with every revolution. Standing on the surface of Earth, as it makes its journey around the Sun we see the stars passing by until after a whole revolution around the Sun, taking a year, we will have seen all the stars pass by.
At the same time as the Earth is moving around the sun it is spinning on it own axis (once a day). So just as the Waltzer cars spin, we see the the other fairground attractions pass by, then we see the centre of the Waltzer structure. On one turn of the car we might see the Big Wheel the next turn of the car we see the fairground a little further around the track with the Dodgems in view. Similarly as the Earth spins we can see the stars at that position in the orbit around the Sun by night and the Sun at the centre of the ride during the day. Each day the Earth will have moved one 365th of its orbit around the Sun. The result is that we see some different stars coming into view as the Earth spins from night to night and other stars move out of view. The fairground behind the centre of the Waltzer is blocked from our view just as the stars are invisible in the direction of the Sun because of the glare of daylight.
If the waltzer had no roof, the sky overhead would appear to be spinning around a point directly above the car. This area of sky would always be visible to us no matter where the car was on the track. This is the same for the stars around the north star Polaris. Because Polaris is close to the point in the sky directly above the axis of spin of the Earth (the north pole) it is always visible. The stars within 40 degrees of Polaris appear to rotate around it and do not set below the horizon, these stars are said to be ‘circumpolar’. Stars further than 40 degrees from Polaris disappear below the horizon for part of the year and reappear at the same time every year.
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