Astronomical Images : Atmospheric refraction

Gianbattista della Porta

Astronomical Images

<p style='text-align: justify;'>Gianbattista della Porta (1535-1615) is known for his work in natural magic and optics, and for his role in the establishment of the Accademia dei Lincei. When news of Galileo's work with the telescope began to circulate, della Porta was one of the first to react, claiming that the invention was based on his <i>De refractione</i>. Optics, however, was of considerable importance to astronomy well before the invention and application of the telescope in the 17th century. A typical optical problem astronomers had to tackle was atmospheric refraction, which modifies the apparent positions of celestial objects, especially near the horizon. This issue is discussed by della Porta. This woodcut shows a star at M below the horizon BAH appearing at N to an observer at A on the Earth DAE, because of atmospheric refraction through the vapours of the atmosphere FGL. Note that the true position of the star is its location as seen from the centre of the Earth, because the line of sight would then pass at right angles through the zone of vapours, without refraction. When M is further below the horizon, the refraction will be greater. As it rises, the degree of refraction decreases until it reaches zero at the zenith. This variable refraction is shown in the curve on which M lies. Perhaps the two outer curves are depicted in order to indicate that the speed at which M travels to the zenith (unmarked on the diagram, but the point at which the extension of the line CA meets with the two converging curves) is faster than that of N travelling to the zenith.</p>