Preface to the third edition The first edition of the book, Equation of Time and Analemmic Curve, which was still imperfect in certain respects, was promptly superseded by a second, revised, edition. This has now been further revised and expanded into the third edition which is now before you. Much more comprehensive than the second edition, this third edition has seven additional chapters. In addition to these, errors and historical inaccuracies that had crept into the second edition have been corrected. I am indebted to the technical editor Alistair Kwan for his invaluable contributions, which have enhanced the reader-friendliness of the text. Some of his comments have also been incorporated into the introduction. The third edition also aims to provide readers with a deeper understanding of the broader cultural history and context, as well as the significant achievements that have led to discoveries in the fields of mathematics and astronomy, and that have in one way or another influenced our understanding of the equation of time. It also dedicates a significant portion of the content to the development of trigonometry, in ancient times and in subsequent eras. In order to facilitate comprehension, modern mathematical notation is frequently employed in the description of ancient trigonometric procedures. These instances will be readily identifiable for the reader who is attentive to detail. Additionally, this edition presents illustrative examples of the utilisation of the equation of time, including its application in astronomical observations during the 15th century, RØMER’s determination of the speed of light, and the determination of longitude.