This type of astronomical alarm clock was invented by Thomas Taylor, an assistant at the Royal Observatory. It is designed to wake up the astronomer when particular stars are due to pass overhead, and was a big improvement on the conventional alarm clock which had to be reset after each star transit (of which there may be some 20 each night). William Johnson, a London maker, built several copies of the clock from Taylor's designs.

The clock has an eight-day movement with anchor escapement, a ring for named star transits, a silencing lever and pull-to-wind alarm. It is inscribed, 'JOHNSON STRAND, LONDON'. The alarm times are set by inserting metal pegs into holes around the dial.