Full P7.1 physics revision OCR

P7 7.1-Naked eye astronomy

7.1 1
sun appears to travel east to west every 24 hours
Earth rotates west to east on axis once every 24 hours
Rotation makes stars appear to travel east to west in short time
Moon appears to travel east to west in longer period of time

7.1 2
Sidereal day
Time its takes for earth to rotate 360*:23h 56mins
Stars appear to travel in 23h 56m across the sky, 4 mins less than the sun
Sidereal means related to stars

Solar day
Average time sun takes to cross the sky 24h
So suns position would be in the same place 24h later
Sun is directly over earth
Earth rotated 360* its orbiting the sun so the sun isn't overhead
Earth rotated just a bit more(4 mins) so the sun is then directly over head making solar longer than sidereal

The reason it takes the earth 4 extra minutes to have a solar day is that the earth is not only spinning on its axis but is also orbiting the sun so for the same part of the earth to face the sun it needs an extra 4 minutes.

7.1 3
Mercury, venus, mars , jupiter, saturn can be seen by the naked eye
Planets move slowly against background of fixed stars in retrograde motion. E.g Venus is closer to the Sun than the Earth, so Venus' orbit of the sun takes less time than the Earths orbit of the sun. If Venus is observed over a long enough peiod (i.e. One Month), it can be seen to move compared to the background of stars.

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