Transit of Mercury

A transit of Mercury occurs when Mercury passes directly between Earth and the Sun, appearing as a small dark disk crossing the Sun’s face. It is visible only when Mercury crosses the Sun–Earth line near one of its orbital nodes.

Inferior conjunction: An alignment in which an inner planet lies between Earth and the Sun. Mercury must reach this alignment near an orbital node for a transit to occur.
Solar parallax measurement: The determination of the Sun's parallax from observations made at separated locations on Earth. Comparing transit timings from distant sites once offered a way to estimate solar parallax.
Johannes Kepler: A German astronomer who formulated the three laws of planetary motion. Kepler predicted the 1631 transit of Mercury, the first to be observed.
Transit of Mercury of 1677: A transit of Mercury observed by Edmond Halley from Saint Helena on 7 November 1677. Halley's timing helped inspire his proposal for measuring solar parallax with transits.
Orbital node: A point where an object's orbit crosses a reference plane. A transit requires Mercury to pass inferior conjunction close to a node in its inclined orbit.
Astronomical unit: The mean distance between Earth and the Sun, defined as exactly 149,597,870,700 metres. Solar-parallax estimates from transits helped establish the scale represented by this distance.
Pierre Gassendi: A French astronomer and philosopher who made the first recorded observation of a Mercury transit. He observed the transit predicted for 1631.
Transit of Mercury of 2003: A transit of Mercury that crossed the Sun on 7 May 2003. It was the first Mercury transit visible from much of the world in more than three decades.
Orbital inclination: The angle between an orbit's plane and a specified reference plane. Mercury's tilted orbit usually carries it above or below the Sun from Earth's viewpoint.
Black drop effect: An apparent dark ligament that can connect a transiting body's silhouette to the edge of the Sun. This visual distortion complicated precise timing of Mercury's transit contacts.

Linked from 2 pages

Solar eclipseCompared with: Mercury’s small apparent size makes its passage a transit rather than a solar eclipse.