Imperfections Revealed by Sunlight

April 10, 2013

Moonwall2b

Photographer: Mario Freitas
Summary Author: Mario Freitas

In 1610, Galileo Galilei published his Sidereus Nuncius, the first scientific treatise based on observations of the sky through a telescope. Among the remarkable conclusions that were drawn by Galileo is that the Moon is not a perfectly spherical body. The many small spots he observed, which varied as the angle of solar illumination varied, he reasoned were shadows: the magnified Moon had many mountains and valleys. So, our satellite had “imperfections” similar to the Earth’s. Thus the Earth itself, like the Moon, should move in space like a celestial body. This represented a stunning revolution in the Aristotelian world.

The above two pictures depict an analogy for the optical phenomenon observed by Galileo. During a commemoration in Casa Selvatica (a cultural center in Curitiba, Brazil), I captured the image at right of the solar disk projected on a wall by a pinhole in the roof above the staircase. Details (left photo) reveal minuscule “craters” and “hills,” due to the roughness of the wall. The direction of the shadows on the vertical wall allows us to verify the solar zenith angle of 28 degrees calculated for the time and date of the photo (1:55 p.m., March 3, 2013) and time zone and geographical coordinates of the observing site (GMT -3; 25°26’44.32”S; 49°16’18.42”W). The difference between the azimuths of the Sun (336 degrees) and of the wall (309 degrees) gives an angle of 27 degrees.

Analogies can play a powerful role in making science education meaningful.

Photo details: Top - Camera Maker: Panasonic; Camera Model: DMC-LX5; Focal Length: 5.1mm (35mm equivalent: 28mm); Aperture: f/2.0; Exposure Time: 0.017 s (1/60); ISO equiv: 200; Software: ACD Systems Digital Imaging. Right - same except: Exposure Time: 0.200 s (1/5); ISO equiv: 400; Software: Microsoft Windows Photo Viewer 6.1.7600.16385.