The Day the World Discovered the Sun by Mark Anderson

star or planet might give the casual observer hints about its distance.
    The primary reason ancient astronomers distinguished between stars and planets was that stars remained fixed in their positions with respect to one another, as observed night after night. Planets (from the Greek for “wanderer”), however, moved across the familiar stellar tapestry over the course of weeks and months.
    From the time of the ancient Mayan, Chinese, Greek, and other early civilizations, astronomers, typically also serving as astrologers, devised elaborate theories to explain planets’ wanderings—and what those wanderings might portend for kings or great events of the day.
    After the Polish astronomer Nicolaus Copernicus (1473–1543) committed the ultimate heresy of replacing the earth with the sun as the center of the solar system, the German mathematician Johannes Kepler (1571–1630)made quantitative sense of Copernicus’s framework. From a lifetime of studying detailed planetary observations by Danish astronomer Tycho Brahe (1546–1601) and others, Kepler ultimately derived three basic laws of planetary motion, which remain in use to this day.
    Kepler’s third law states that the square of the time a planet takes to complete one orbit of the sun is proportional to the cube of that planet’s distance from the sun. In a simple equation:
P 2 = a 3
(1)
    Where P is the planet’s orbital period (the length of time, measured in earth years, the planet takes to complete one orbit) and a is the planet’s average distance from the sun, measured in fractions of the earth-sun distance (astronomical units or AU). From well before Kepler’s day, detailed charts of Venus’s motions established that the planet completes one orbital period, one Venusian “year,” every 0.615 Earth years.
    Multiply 0.615 by itself and take the cube-root of the result to find Venus’s distance from the sun as 0.72 AU. 1 But what, in practical distance units such as miles, is an AU?
    Here is where astronomy remained stuck for more than a century.
    Clever attempts to leverage precision measurements of Mars’s and Mercury’s orbits brought some astronomers in the late seventeenth and early eighteenth centuries close to answering the question. 2 But planets move slowly across the sky, and tracking their motions with respect to background stars was necessarily imprecise. Discovering the sun’s distance required greater precision.
    It’s useful now to introduce an important term: the “solar parallax,” not the solar distance, is actually the quantity astronomers sought. Solar parallax is an angular measurement, representing one-half of the angular size of the earth as seen from the sun. To use the analogy of a circle, the solar parallax is like the angular “radius” of the earth, as subtended from a distance of 1 AU. Fortunately, converting between solar parallax and solardistance is relatively easy. The distance to the sun is just the radius of the earth divided by the solar parallax. In real numbers, 92,956,000 miles = 3,963.2 miles ÷ 8.79414 arc seconds. (To do this on a calculator, an extra factor of 206,265 is needed to convert arc seconds to “radians,” the natural unit of angular measurement.)
    In 1716, soon to be Astronomer Royal Edmund Halley published an astronomical call to arms, revealing that for a brief window in June 1761 and again in June 1769, the planet Venus would be moving across a kind of interplanetary yardstick—the face of the sun.
    Astronomers at different locations across earth could then time the duration of the Venus transit and compare answers (along with exact measurements of the observers’ latitude) to triangulate the sun’s distance.
    Halley’s method did not call for the observers to know their longitudes. And given how difficult longitude was to determine at the time, whether at sea or on land, Halley’s method