Sunday, October 09, 2011

XO-1.75 seismograph

A seismograph program which uses the accelerometer of the OLPC XO-1.75 laptop

The x,y,z accelerometer readings are saved in boxes x,y,z. Action 1 computes long term averages for these readings, (boxes a,b,c), 5% of the current reading is added to 95% of the long term average to compute a new long term average.

Action 2 determines a threshold of random noise which is ignored. It sums the squares of the deviation from average (x-a)^2 + (y-b)^2 + (z-c)^2 . If this figure exceeds 10, the screen is turned red to indicate that an event is occuring.

Action 3 prints the sum of the squares of the deviations and the x,y,z deviations.

Project source

  • When an event occurs, graph the 3 channels (a)
  • Save the 3 channels events with a time stamp for exporting into a spreadsheet (b), (c)
  • Set up a network of laptops, an event is considered to have occured if all register it. (Use turtle position to share data)

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Tuesday, October 04, 2011

How low can you see that the earth is curved

At how low an altitude does the earth look like a disk, in other words, from how low does a line from the horizon to your eye trace out the surface of a cone (with angle a shown from the centre) ?

Surprisingly low. The depression of the horizon below level is noticable from an altitude as low as 218m. (Lysterfield Lake trig point 37°56'31" S 145°16'06" E, Elevation 218m)

View looking south east, the faintly visible Strezleki ranges are level with the line

View looking north west, the horizon is below the string

Looking east, the horizon is below the two tape markers

Looking west, the horizon is just below the two tape markers

The average depression was 7mm at a distance of 147cm

a = sin (a) for small a

a = 0.7/147 = 0.00476

r = 2h/a^2

r = 2 *218/(0.00476^2)

r = 19,000 km

Compared with the actual value of 6,371 km

Earth from space, image
Small angle formula

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