Equation of Time

[tony_kitto]

Whitaker's Almanack says "The direction in which the equation of time has to be applied in different problems is a frequent source of confusion and error." I think the confusion has little to do with any difference between navigators and astronomers. I think it had more to do with the increased ownership of clocks and the changeover from apparent to mean time. In the 17th or 18th century no-one would have considered labelling a sundial as slow or fast. Originally an alternative name for the equation of time was the equation of natural days. The amount each day differs from 24 hours is accumulated to give an equation. Christian Huygens first published an equation of time in 1665. It went from zero in February to a maximum in early November. He used it to check his watches, noting the regular difference between clock time and sundial time. John Flamsteed published his equation of natural days in 1673. He calculated his times to add or subtract from apparent solar time to give mean solar time for astronomical purposes. Flamsteed fixed mean solar time by the times of the aphelion and spring equinox. Thomas Tompion sold equation tables to go with his clocks from 1683. The equation values were provided by Flamsteed. Tompion’s table. It shows "How much a good Pendulum Watch ought to be faster or slower then a true Sun-Dial, every day of the Year". The values were nor assigned + or -, instead starting in January the Watch is too fast. After April 4th the Watch is too slow, in June watch too fast again etc. . Both the clock tables and the sundial at Bath Pump Room followed the instructions "Set the watch so much faster or slower than the time by the sun, according to the Table for the Day of the Month, when you set it; and if the Watch go true, the difference of it from the Sun any day afterwards will be the same with the Table". I hope this shows the two different applications of the equation of time in the 17th century. You can find out more about Flamsteed’s equation of natural days by visiting the web-site promoting an exhibition of the founding of Greenwich Observatory at http://www.burnley.gov.uk/towneley/ttt/index.htm . Tony Kitto  [EMAIL PROTECTED]