Sky & Telescope - March 2022 - 28

SCIENCE SOURCE / SCIENCE SOURCE
PIONEERING ASTRONOMY by Ted Rafferty
Mason and Dixon's
Great Venus Adv
A pair of well-known surveyors got their
start trying to solve one of the biggest
questions in astronomy.
C
harles Mason and Jeremiah Dixon are best known for
their 18th-century survey of a disputed territorial
boundary. Their names are attached to the MasonDixon
Line, which partially defines the present-day border
between the states of Pennsylvania, Maryland, Delaware,
and West Virginia. What is less well known is that the
pair were selected for this task thanks to their earlier
involvement in an astronomical endeavor to determine the
distance between Earth and the Sun.
Johannes Kepler's third law of planetary motion, published
in the early 1600s, described the relative size of
the solar system. His calculations expressed the radius of
each planet's orbit as a ratio of Earth's orbit. For example,
Mercury's orbit is 0.389 times the radius of Earth's and
Saturn's is 9.51 times. However, the absolute size of the
solar system remained a mystery - Kepler's ratios didn't
reveal the actual distances between solar system bodies.
But by determining the physical distance between just two
solar system bodies, such as Earth and the Sun, all other
distances could be calculated using Kepler's ratios.
A Question of Angles
In the 18th century, the parallax method was believed to be
the best available for establishing astronomical distances.
To measure a parallax angle, astronomers had to observe
a target from two widely separated locations and gauge
the apparent shift in that object's position against a fixed
background reference - usually the stars. The farther the
object is from the background and the closer it is to Earth,
the larger the apparent parallax shift and the greater the
accuracy of the measurement.
Early attempts to capture the parallax shift of Mars
against background stars failed. During favorable oppositions,
Mars typically only approaches to within 0.37 a.u. of
Earth, rendering the resulting angle too small to measure
precisely. Similarly, observing Mercury crossing the face
28 MARCH 2022 * SKY & TELESCOPE
of the Sun didn't yield
useful results: During
the transit Mercury is too
far, at around 0.61 a.u. from
Earth, and it's also too close to
the background reference (the Sun).
In 1677, Edmond Halley suggested that
the June 6th, 1761, transit of Venus might
offer a good chance for a precise measurement
of parallax shift. Compared with Mercury, Venus has
the advantage of being both closer to Earth (at only 0.28
a.u.) and farther from the Sun.

Sky & Telescope - March 2022

Table of Contents for the Digital Edition of Sky & Telescope - March 2022

Contents
Sky & Telescope - March 2022 - Cover1
Sky & Telescope - March 2022 - Cover2
Sky & Telescope - March 2022 - 1
Sky & Telescope - March 2022 - Contents
Sky & Telescope - March 2022 - 3
Sky & Telescope - March 2022 - 4
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Sky & Telescope - March 2022 - Cover3
Sky & Telescope - March 2022 - Cover4
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