Spin doctor

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