Transit of Venus

A transit of Venus occurs when Venus passes directly between Earth and the Sun, appearing as a small silhouette against the solar disk. The events occur in pairs eight years apart, separated by more than a century.

Inferior conjunction: An alignment in which an inner planet lies between Earth and the Sun. A transit can occur only when Venus reaches inferior conjunction near a point where its orbit crosses the Sun’s apparent path.
Jeremiah Horrocks: An English astronomer who made the first recorded observation of a transit of Venus in 1639. His observation demonstrated the event and helped launch its use for astronomical measurement.
Halley's method: A method for estimating solar parallax by comparing Venus transit timings observed from distant locations. It supplied the central logic for the eighteenth-century expeditions.
Transit of Venus of 1874: A transit of Venus observed during an international campaign to refine the solar parallax. Photographic techniques were used extensively in efforts to improve on earlier timing measurements.
Expedition science: Scientific research carried out through organized journeys to collect observations or specimens in distant places. Transit campaigns coordinated astronomers, navigators, and instruments across continents and oceans.
Orbital inclination: The angle between an object's orbital plane and a chosen reference plane. Venus’s tilted orbit usually carries it above or below the Sun at inferior conjunction.
Edmond Halley: An English astronomer who proposed using Venus transits to determine the solar parallax. His method turned the transits of 1761 and 1769 into a global measurement campaign.
Astronomical unit: The mean distance between Earth and the Sun, defined as exactly 149,597,870,700 metres. Estimating solar parallax allowed astronomers to determine this fundamental distance.
Transit of Venus of 1882: The last Venus transit of the nineteenth century, observed by astronomers around the world. Its extensive documentation illustrates the scale of the final major pre-modern transit campaign.
Parallax: The apparent shift in an object's position when viewed from different locations. The transit’s apparent path differed by location, making parallax a route to solar-system distances.

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