A Brief History of Timekeeping by Chad Orzel

Author:Chad Orzel
Language: eng
Format: epub
ISBN: 9781953295941
Publisher: BenBella Books
Published: 2021-11-03T00:00:00+00:00

TOBIAS MAYER AND THE MOON

Johann Tobias Mayer was born in 1723 near Stuttgart. His father was trained as a wainwright but demonstrated expertise with a broader range of mechanical things, which won him a commission to build and manage water systems in Esslingen when Tobias was still an infant. As a child, Mayer imitated his father’s disassembling and sketching of mechanical devices, developing mechanical and artistic abilities that would serve him well in later life.

Mayer’s father died in 1731, leaving Tobias and his siblings in dire financial straits, but through the mentorship of the mayor of Esslingen and a mathematically inclined local shoemaker,* Mayer was able to continue his education. He developed a strong interest in becoming an artillery officer, and to that end spent a good deal of time learning to map and draw fortifications and do the geometrical calculations needed to determine the range and charge needed to hit a chosen target. While he never did manage to secure a military commission, this education led to work as a cartographer, and he eventually settled in Nuremberg in 1746. He joined the Homann Cartographic Bureau under its director Johann Franz* and began work on a new atlas. He was distressed to learn, however, just how bad the state of cartography was: only 22 locations in Germany had their latitude firmly established by astronomical measurements, and only 139 places on the entire Earth had both latitude and longitude reliably recorded in the sources available at the Homann Bureau. This shortage of information triggered an interest in astronomy, as Mayer set out to improve the techniques for determining earthbound coordinates by looking to the sky.

In pursuit of a method that would be widely applicable, Mayer began studying the moon carefully and mapped the lunar surface more precisely than anyone else had done to that time. He determined that the moon had no atmosphere (based on the lack of visible distortion as the moon passed in front of a background star) and made the best measurements to that point of the “libration” of the moon. It’s well known that the moon’s rotational period matches its orbital period, so that the same hemisphere of the moon is always facing the earth, but because the moon’s orbit is elliptical, this isn’t quite perfect. The visible surface wobbles back and forth by a few degrees of lunar longitude or latitude over the course of an orbit; as a result, the portion of the lunar surface that’s ever visible from the earth is about 59 percent of the total. Between 1748 and 1749, Mayer made repeated careful measurements of the angular distance between the edge of the visible moon and prominent features on its surface, and determined the amount of this libration.

Mayer’s ultimate goal lay in the area of mapmaking, specifically attempting to develop a method of predicting longitude based on the motion of the moon. In 1751, he moved to Göttingen to take a professorship and began a correspondence with Leonhard Euler, who was then putting the finishing touches on his model of the moon’s orbit.

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