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Secular Variation and Calibration

A record of how the Earth's magnetic field has changed over time is required to calibrate the measured information from an archaeomagnetic sample into a calendar date. It was first realised that the direction of the Earth's field changes with time in the 16th century, since which time scientists (beginning with Henry Gellibrand) have periodically made observations of the changes in both the declination and inclination at magnetic observatories. The record of how the Earth's magnetic field has changed is referred to as a secular variation curve. The British secular variation curve is based on the observatory data as well as direct measurements from archaeological materials. The Earth's magnetic field is a complicated phenomenon and so it is necessary to develop regional records of secular variation. The regional curves are centred on specific locations; for the UK the central point is located at Meriden (Latitude 52.43°N, Longitude 1.62°W), the village located at the notional centre of England.

Secular variation curves are constantly evolving as new data becomes available. The more information there is, the better we will understand how the Earth's magnetic field has changed over time, which may allow more precise archaeomagnetic dates to be produced. A number of secular variation curves have been produced for Britain over the last 50 years, reflecting the inclusion of additional information as well as improved methods used to construct the curves. The main secular variation curves have been summarised in this section in chronological order of their development. They include:

The data used to construct the curves will be discussed as well as the relative merits/criticisms that have been noted since they were published.

The precision available for archaeomagnetic dates calibrated using the RenCurve calibration curves will then be presented for the different periods of time.