Frank Keim

The astronomical meaning of Botticelli’s Birth of Venus

Abstract

In the fresco study of Saint Augustine (c. 1480) we find first measurements of elongation angles of inner planets. The painting Birth of Venus (c. 1483-85) depicts the three phases of the inner planets: inferior conjunction, superior conjunction and greatest elongation (Mercury and Venus). At the superior conjunction the planet is fully illuminated by the Sun, the phase angle is 0°. The Venere nuda shows the angle 23.5° for the Earth. The Venus is in line with the Sun and the Earth, located behind the Sun. At the greatest elongation, the phase angle is exactly 90°, the elongation angle about 45°. For Mercury, Botticelli plotted the angle of 22°. Furthermore, he painted the two figures in such a way, that there is a balance between the covered and the uncovered parts of their bodies. Finally, at inferior conjunction, there is— seen from Earth—only a small sickle: the Venus is completely clothed. The phase angle here is 180°. But this figure still shows another phase : that of the maximum brightness of the planet that will be achieved at about 39°.- Therefore, this painting is the first known allegory of the heliocentric system in the Quattrocento.

Biography

Frank Keim was a speaker at INSAP VII Conference, Bath 2010. He has worked since 1991 at the Communication and Information center, University of Ulm.