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OCR 21st centaury science - P7 (Higher)
P7.1 Naked eye astronomy
The earth takes 365 4 days to completely orbit the sun, this is a year. A
sidereal day is 23 hours and 56 minutes long which coincidentally is the same
amount of time it take the earth to make one rotation on its axis. A sidereal
day is the time it takes for a star (or constellation) to go from one place in the
sky to the same place the next night, this takes 23h 56mins.
A solar day is the time it takes for the sun to go from one place in the sky to
the same place the next day. This takes 24h. 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.
The moon reappears in the sky every 24h and 49 minutes. This is because the
moon is also orbiting the earth as well as the earth rotating, giving us a
different view of the moon and the moon is orbiting it from west to east so
during the night the moon slips back through the pattern of stars.
The planets we see in the sky seem to change position night by night relative
to the background of stars. The planets can be a steady pattern but can also go
backwards, like mars. This is because the earth is orbiting the sun at a different
rate to the other planets causing them to look like their orbiting backwards.