December 1, 2015

Astro-Event: Venus at Greatest Elongation.

Venus earlier this year as seen from Astroguyz HQ. (Photo by Author).

Venus earlier this year as seen from Astroguyz HQ. (Photo by Author).


   Our nearest planetary neighbor is about to put on a brilliant dusk showing. The planet Venus reaches greatest elongation, or its maximum separation from the Sun as observed from the Earth on August 19th. From there, it will begin a long dive towards inferior conjunction with the Sun on October 28th, slendering in phase from half-lit to crescent and increasing in angular size as it does so. Venus is now the brightest object high in the west at dusk. Tonight on August 13th, a nice grouping of Venus, Mars, Saturn and the three day old Moon occurs after sunset. Now would be a good time to attempt the feat of seeing Venus in the daylight, using the nearby Moon as a guide. Greatest elongation for Venus is a maximum of 45°-47° degrees separation from the Sun. As viewed from Venus, the Earth would appear at quadrature, as we form a giant right triangle with the Sun. Venus will be the focus of action over the next year, as it is occulted by the Moon as seen from South Africa on September 11th, 2010, and has a close conjunction with Mars on August 19th (more on that next week). The orbital action is all setting us up for the climax; a transit of the planet Venus across the face of the Sun on June 5-6th, 2012!

The Astro-word for this week is Dichotomy. This is the point at which the observed “half-moon” phase is noted to occur. In a perfect world, this should occur very near greatest elongation; most tables give predicted dichotomy for Venus this month as August 17th-18th. The trouble is, the observed date of dichotomy often occurs 4 to 6 days early during evening elongations, and later during the dawn apparitions. This variation has been disputed as a real or optical illusion over the years; Venus is a dazzling object and has no surface markings by which an observer can draw any bearings real from. If it has any basis in reality, if may be caused by the obliquity of the sun’s rays along the terminator as seen from Earth. That is, this region appears dimmer next the dazzling sunlit side of Venus, confusing the eye. Use of a variable polarizing filter can aid in “dampening down” the brilliance of Venus… on what date does dichotomy appear to occur to you?


  1. [...] as Venus begins to approach the Earth and presents a noticeable crescent phase. We just passed dichotomy — the theoretical point where Venus presents a half-illuminated phase as seen from [...]

  2. [...] this glow be connected with spurious sightings of the “Ashen Light of Venus” that have cropped up over the centuries? Venus turns its night time back towards us during the [...]

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