9 days.
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Well done that man !
Enjoy the (virtual) cigar !
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128 minutes.
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Mrs R would really love to know how she is supposed to mark 25 papers in just 4 hours!
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Better add a glass of port to go with that cigar Mike 
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4 hours.
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I would, but I’ve a meeting in the morning and are heading to bed after a music session. Good night, and thanks for the teasers.
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Looks like both Ravvie and Mike should enjoy a glass of champagne …
… whilst Mrs R will need two Asprin tablets …
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4 hours?
Well done again Gary. 4 hours it is !
Sky-Fi
From time to time the International Space Station can be seen as a bright spot of light moving over the UK. It appears in the western sky at about half past eight in the evening and moves eastwards. It seems to be travelling at about the same speed as a passenger plane, for which it is often mistaken.
Assume that passenger planes typically fly at 900 kph at an altitude of 10km. The ISS orbits at an altitude of 300km. The circumference of the earth is approximately 40,000km.
Roughly, how fast is the ISS going ? and
Approximately, how long does it take to orbit the earth ?
No need to answer both questions, one at a time will do nicely
Standing on the earth and looking at the plane 10k above me, it travels an arc of 900k in an hour. The space station is 30 times further away, and although its speed looks the same as the plane’s, it will actually travel an arc of around 27,000k in the same hour (ie 30 x 900). So its speed is roughly 27,000kph.
Based on a circumference of 40,000km, the radius of the earth is approx 6,365km. The space station is travelling in a circle of radius 6,665km (6,365 + 300), giving a circumference of approx 41,885km. At a speed of 27,000kph, it will therefore take around 1 hour 33 minutes to orbit the earth.
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A very clear explanation Steve.
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As Ravvie said, a wonderfully clear explanation. Well done !
A short follow-up …
The Earth isn’t a perfect sphere, but for an extra glass of champagne, how come its circumference is so close to the nice round number of 40,000 km ?
And in Nautical Miles (for the extra cigar), why is the circumference such an ungainly 21,600 nm ?
I know the first and I will have a guess at the second.
France decided to standardise a whole range of units based on the decimal system. The metre was derived from measuring a line from the North Pole to the Equator, passing through Paris. This length was deemed to be 10 million metres, which was a quarter of the circumference of the Earth. I don’t know if it would have made much measurable difference passing through any other point but France was keen to establish Paris rather than Greenwich as the Prime Meridian. Whilst that didn’t work out, the metric system is the main one used globally. Was champagne a subtle French clue?
21,600 is conveniently 360 x 60. Given that the Nautical Mile is primarily used for navigation, I would guess that it ties in with degrees and minutes, possibly relating to longitude at the Equator. Longitude was a big challenge and so using base 60 for distance makes sense as it was already used for time and angles. In which case it is not so ungainly. If so, maybe some rum rather than a cigar?
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Nicely spotted.
And a good, clear explanation about the metre. Plus a clear understanding of the Nautical Mile.
The Nautical Mile was originally defined as the length on the Earth’s surface of one minute of arc along a meridian . Because it was known that the Earth is an oblate sphere, the length of the Nautical Mile varied from place to place, generally being slightly longer at the Poles than at the Equator. It was later standardised in Britain at 6080 feet and then again at an international conference (in 1929 ?) at 1852m. (I use 6,000ft in rough calculations)
The UK adopted this last change in 1970 (I think)
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So I was on the right lines with nautical mile. Or perhaps not, as I was at 90 degrees considering equator rather than meridian!
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You were definitely on the right lines Ravvie. Certainly justifies the tot of rum
When does 11 plus 2 equal 1.
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