Editing Equation of time (section)

=== 18th and early 19th centuries ===

The corrections in Flamsteed's tables of 1672–1673 and 1680 gave mean time computed essentially correctly and without need for further offset. But the numerical values in tables of the equation of time have somewhat changed since then, owing to three factors:

* General improvements in accuracy that came from refinements in astronomical measurement techniques,
* Slow intrinsic changes in the equation of time, occurring as a result of small long-term changes in the Earth's obliquity and eccentricity (affecting, for instance, the distance and dates of [[perihelion]]), and
* The inclusion of small sources of additional variation in the apparent motion of the Sun, unknown in the 17th century but discovered from the 18th century onwards, including the effects of the Moon (See [[barycentre]]), Venus and Jupiter.<ref name="Maskelyne64"/>

[[File:Derby Sundial C 5810.JPG|thumb|A sundial made in 1812 by [[Whitehurst & Son sundial (1812)|Whitehurst & Son]], with a circular scale showing the equation of time correction. This is now on display in [[Derby Museum and Art Gallery]].]]

From 1767 to 1833, the British ''[[Nautical Almanac|Nautical Almanac and Astronomical Ephemeris]]'' tabulated the equation of time in the sense 'add or subtract (as directed) the number of minutes and seconds stated to or from the apparent time to obtain the mean time'.  Times in the Almanac were in apparent solar time, because time aboard ship was most often determined by observing the Sun. This operation would be performed in the unusual case that the mean solar time of an observation was needed. In the issues since 1834, all times have been in mean solar time, because by then the time aboard ship was increasingly often determined by [[marine chronometer]]s. The instructions were consequently to add or subtract (as directed) the number of minutes stated to or from the mean time to obtain the apparent time. So now addition corresponded to the equation being positive and subtraction corresponded to it being negative.

As the apparent daily movement of the Sun is one revolution per day, that is 360° every 24&nbsp;hours, and the Sun itself appears as a disc of about 0.5° in the sky, simple sundials can be read to a maximum accuracy of about one minute. Since the equation of time has a range of about 33&nbsp;minutes, the difference between sundial time and clock time cannot be ignored. In addition to the equation of time, one also has to apply corrections due to one's distance from the local time zone meridian and [[Daylight saving time|summer time]], if any.

The tiny increase of the mean solar day due to the slowing down of the Earth's rotation, by about 2 [[Millisecond|ms]] per day per century, which currently accumulates up to about 1&nbsp;second every year, is not taken into account in traditional definitions of the equation of time, as it is imperceptible at the accuracy level of sundials.