Mercury Transit 2019: How to Watch the Rare Event Live Online

Some good reports for the Mercury transit coming on Monday by This celestial event provides an opportunity to teach basic astronomy too - the heliocentric solar system and astronomical unit. I offer this note. There are citizen science groups observing this Mercury transit to measure the solar parallax and using trigonometry - record the distance between Earth and the Sun, i.e. the astronomical unit (done in 2016 and also previous Venus transits in 2004 and 2012). The method using Venus and Mercury transits - recorded in astronomy since the 1700s. In the geocentric world of Claudius Ptolemy, Tycho Brahe, the great upheaval period with Galileo, telescopes did not exist or just beginning to be used. Only later after Galileo's days, telescopes were used to record and measure the solar parallax - showing the astronomical unit. Using Venus and Mercury transits was proposed in 1716 by Edmond Halley astronomer. Cassini and Richer used telescopes to measure the Mars parallax when Mars was at or near opposition in 1672. Tycho Brahe failed to measure the Mars parallax in the late 1500s because Mars distance from Earth, too far away for Tycho using unaided eye instruments to measure. Astronomers never understood just how far away until the Mars parallax was obtained using telescopes in 1672. Tycho Brahe wanted to measure the Mars parallax to show Mars was always farther away from Earth than the Sun - to refute Copernicus system. Copernicus showed at times, Mars would be closer to the Earth than the Sun using his heliocentric solar system but the real distances were unknown. Starting in 1672, astronomers began to understand the true size and dimension of the solar system and distances to the planets and the Sun. Previously, the geocentric firmament astronomy was always a small universe with the Sun much closer, perhaps 1200 earth radii vs. some 23455 earth radii today using that unit of measure. Claudius Ptolemy geocentric astronomy placed the sphere of fixed stars some 20,000 earth radii distance or 0.85 AU using the modern, astronomical unit, closer to Earth than the Sun in the modern heliocentric solar system. The depth of the Heavens... The Mercury transit provides opportunity to teach some of this critical, astronomical history to folks.

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