Publisher Note

Equation of time is an artistic and scientific experiment initiated in 2018 at the Paris Observatory by photographer Raphaël Dallaporta. Conducted jointly with researchers from the Syrte (Système de Références Temps-Espaces) of Paris Observatory, the experiment “Equation of Time” aims to verify the differences between the true solar day, indicated by sundials, and mean solar time, indicated by clocks. While the meridian of true solar time is a straight line, the meridian of mean time has the shape of an elongated figure eight.

Every day for a year, at the exact same time, Raphaël Dallaporta took a picture of the Sun entering the Cassini room, which was built in 1671. The accumulation over the year reveals the curve in 8 of the equation of time ∞ represented by the image of the Sun on the Earth.

The equation of time is a notion that reflects the inequalities of solar time. The notion of “mean solar time” corresponds to an ideal, fictitious time, where the sun returns in the same direction after 24 hours. Every day when the Sun is visible, the Sun passes at the meridian of the observatory, in this Cassini room. We can see this spot of light that cuts the Paris meridian at solar noon.
Each of the first 365 pages of this book is centered on the position of the image of the Sun during its passage at the local mean noon in the Cassini room of Paris Observatory (48°50’11”N – 02°20’14”E). The pictures were taken by Raphaël Dallaporta as close as possible to the eyecup located at 9.933 m south of the meridian. From this point of view, the image of the Sun appears circular, the perspective reducing the variations of its elliptical projection during the year. In the following pages, astronomer and science historian Denis Savoie explains the principles of the equation of time, based on the plot of the figure-eight curve when the Earth’s revolution around the Sun at the end of a year is completed.

Conceived as a playful flip book by the Kummer & Herrman studio, the reader discovers the figure-eight curve that forms over the pages.

In the afterword, the astronomer Denis Savoie explains the principles of the equation of time, based on the curve of the figure of eight, the revolution of the Earth around the Sun after one year.

The book was published as part of the Prix Niépce Gens d’Images, of which The Eyes Publishing has been a patron since 2019, in collaboration with the Picto Foundation.

Artists’ Book

Equation du Temps

by Raphaël Dallaporta

Release Place Paris, France
Edition 1st edition
Release Date 2020
Writer: Denise Savoie
Printrun 450
ISBN-13: 979-10-92727-39-5
Subform Artist Book
Topics Light, Photography, Time
Methods Photography
Language French
Dimensions 17.0 × 24.0 × 4.0 cm
Pages 376
Technique Offset