Astronomical and Atomic Time Systems
August 04, 2015
Photographer: Mario Freitas
Summary Author: Mario Freitas
The above photo shows a subtle difference between the time that's based on the instantaneous position of the Sun in the sky and standard clock time. Notice the faded sundial on the school wall in the foreground and the clock in the background, on the top of a modern trade center in Curitiba, Brazil. This time difference occurs because of several astronomical-based reasons. First, during its elliptical orbit the Earth speeds up as it approaches the Sun and slows down as it moves away; second, the Earth's axis is tilted to its orbital plane, so the Sun's apparent motion along the ecliptic has a varying effect when viewed along the equator. Among other important factors, there's a correction between the longitude of the sundial’s location and that of the center of the time zone. The resulting difference oscillates between positive and negative values during the course of a full year, reaching a maximum that surpasses 19 degrees in early November, and generating the figure eight feature called an analemma. In late June, the sundial reads a few minutes later than the large clock, for the difference is quite small in this segment of Earth’s orbit. Since the length of the day undergoes minute unpredictable variations, Earth’s rotation is no longer valuable for high precision time measure. In 1967, the Thirteenth General Conference of Weights and Measures adopted a new definition of the time unit, based on the atomic oscillation of caesium-133 atom. Picture taken in Curitiba, Brazil, on June 30, 2015.