People tend to assume that the time between noon one day and noon the next is exactly 24 hours. In fact, the length of a day varies slighty throughout the year. Watches are based on the convenient assumption that all days in the year are exactly 24 hours long. Sundials take the days as they are, varying in length from 24 hours and 30 seconds in late December and 23 hours 59 minutes and 40 seconds on 15 September as stated above. Find out the longitude of the place where the sundial will be installed You can do this from any good atlas - longitude lines run from north to south on the atlas.
Many atlases give the latitude and longitude against the place names in the index.
The latitude and longitude obtained in this way will not be the exact latitude and longitude of your sundial, although it will usually be good enough for these purposes unless you live in a large city. For a more exact method, please refer to our page on Finding your Latitude and Longitude. Find out the longitude of your standard meridian The world is divided into time zones 15 deg. Thus, the standard meridian for the United Kingdom, Ireland and Portugal is the prime meridian of 0 deg, while the rest of continental Europe keeps European Time, for which the standard meridian is 15 deg. Use the Spot-On Sundials Solar Noon Calculator.