Empirical Earth · Test It Yourself

How far to the Sun — the first measurements of the astronomical unit

The claim

“The Sun is small and close, a few thousand kilometers overhead, and nobody has ever measured its distance. The 150-million-kilometer figure is an assumption, not an observation.”

What is measured

Four methods, two of them naked-eye geometry, land on about 150 million kilometers. A local Sun a few thousand kilometers overhead would fail all four at once, and it does.

What would show this is wrong

producing a parallax of the Sun, a Venus-transit timing, or a radar echo delay consistent with a Sun a few thousand kilometers away, or by showing that the half-moon, Mars-parallax, transit, and radar methods return distances that disagree. They do not; they converge near 150 million kilometers.

Sources

  1. Aristarchus and the half-moon method. In the third century BC Aristarchus of Samos used the right triangle formed at the half-moon to compare the distances of the Sun and Moon. He measured t link
  2. The 1672 Mars parallax. During the close approach of Mars in 1672, Giovanni Cassini in Paris and Jean Richer in Cayenne measured the planet’s position against the stars at the same moments. link
  3. The transits of Venus, 1761 and 1769. Edmond Halley proposed timing a Venus transit from widely separated latitudes to derive the solar parallax. International campaigns observed the 1761 an link
  4. Radar ranging and the modern value. In 1961 several groups bounced radar off Venus and timed the echo, fixing the scale of the solar system directly and far more precisely than the optical c link
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