Spin doc­tor

Sky at Night Magazine - - INTERACTIVE -

In his Cut­ting Edge in May, Chris Lin­tott refers to ga­lac­tic ro­ta­tion curves, where the speed of ro­ta­tion is al­most con­stant with ra­dius as “...like [the ro­ta­tion of] spokes on a bi­cy­cle wheel.” But this would im­ply an in­crease in speed of ga­lac­tic ma­te­rial with the ra­dius. The best anal­ogy I can find is that of a CD player. The mo­tor ro­tates the disc with a high an­gu­lar ve­loc­ity when the laser head is near the cen­tre, but slows the disc down to a lower one when the head is at the pe­riph­ery. This means that data is read at a con­stant speed at the laser head as it moves out­wards along the disc. Of course, this is in spite of the CD ro­tat­ing as a solid body which would, if it had a con­stant an­gu­lar ve­loc­ity, ro­tate like a bi­cy­cle wheel at an in­creas­ing speed with ra­dius. Richard Field FRAS, via email

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