Transits
The Venusian orbit is slightly inclinedrelative to Earth's orbit; thus, when the planet passes between Earthand the Sun, it usually does not cross the face of the Sun. Transitsof Venus occur when the planet's inferior conjunction coincides withits presence in the plane of Earth's orbit. Transits of Venus occurin cycles of 243 years with the current pattern of transits beingpairs of transits separated by eight years, at intervals of about105.5 years or 121.5 years—a pattern first discovered in 1639 bythe English astronomer Jeremiah Horrocks.
The latest pair was June 8, 2004 andJune 5–6, 2012. The transit could be watched live from many onlineoutlets or observed locally with the right equipment and conditions.
The preceding pair of transits occurredin December 1874 and December 1882; the following pair will occur inDecember 2117 and December 2125. The 1874 transit is the subject ofthe oldest film known, the 1874 Passage de Venus. Historically,transits of Venus were important, because they allowed astronomers todetermine the size of the astronomical unit, and hence the size ofthe Solar System as shown by Horrocks in 1639. Captain Cook'sexploration of the east coast of Australia came after he had sailedto Tahiti in 1768 to observe a transit of Venus.
Pentagram of Venus
The pentagram of Venus is the path thatVenus makes as observed from Earth. Successive inferior conjunctionsof Venus repeat very near a 13:8 ratio (Earth orbits eight times forevery 13 orbits of Venus), shifting 144° upon sequential inferiorconjunctions. The 13:8 ratio is approximate. 8/13 is approximately0.61538 while Venus orbits the Sun in 0.61519 years. The pentagram ofVenus is sometimes also referred to as the petals of Venus due to thepath's visual similarity to a flower.
Daylight apparitions
Naked-eye observations of Venus duringdaylight hours exist in several anecdotes and records. AstronomerEdmund Halley calculated its maximum naked eye brightness in 1716,when many Londoners were alarmed by its appearance in the daytime.French emperor Napoleon Bonaparte once witnessed a daytime apparitionof the planet while at a reception in Luxembourg. Another historicaldaytime observation of the planet took place during the inaugurationof the American president Abraham Lincoln in Washington, D.C., on 4March 1865. Although naked eye visibility of Venus's phases isdisputed, records exist of observations of its crescent.
Ashen light
A long-standing mystery of Venusobservations is the so-called ashen light—an apparent weakillumination of its dark side, seen when the planet is in thecrescent phase. The first claimed observation of ashen light was madein 1643, but the existence of the illumination has never beenreliably confirmed. Observers have speculated it may result fromelectrical activity in the Venusian atmosphere, but it could beillusory, resulting from the physiological effect of observing abright, crescent-shaped object. The ashen light has often beensighted when Venus is in the evening sky, when the evening terminatorof the planet is towards to Earth.
Observation and exploration
Early observation
Because the movements of Venus appearto be discontinuous (it disappears due to its proximity to the sun,for many days at a time, and then reappears on the other horizon),some cultures did not recognize Venus as a single entity; instead,they assumed it to be two separate stars on each horizon: the morningand evening star. Nonetheless, a cylinder seal from the Jemdet Nasrperiod and the Venus tablet of Ammisaduqa from the First Babyloniandynasty indicate that the ancient Sumerians already knew that themorning and evening stars were the same celestial object. In the OldBabylonian period, the planet Venus was known as Ninsi'anna, andlater as Dilbat. The name "Ninsi'anna" translates to"divine lady, illumination of heaven", which refersto Venus as the brightest visible "star". Earlierspellings of the name were written with the cuneiform sign si4 (= SU,meaning "to be red"), and the original meaning mayhave been "divine lady of the redness of heaven", inreference to the colour of the morning and evening sky.
