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July 2007 Feature: Venus at Greatest Brilliancy

Dazzling Venus, the second planet from the Sun, always shines boldly. After the Sun and Moon, Venus beams more brightly than any other celestial object in the heavens. Sharp-sighted people, in fact, can even see this gleaming world in broad daylight. What's more, Venus can cast a shadow on a moonless night.
Mark July 12! On this date, Venus one-ups itself and exhibits its greatest brilliancy as the evening "star" for all of 2007. Some people prefer to say greatest illuminated extent, because the lit part of Venus' disk covers the greatest square area of sky for this evening apparition. That's in spite of the fact that Venus shows only about 25% of its lit side and 75% of its dark side. (This diagram may help you to visualize Venus and its phases.)

Venus Cycles

About 36 days before Venus attains its greatest brilliancy in the evening sky, this planet reaches its greatest eastern (or evening) elongation from the Sun. At this juncture, Venus shows about 50% of its illuminated side and 50% of its nighttime side - just like our Moon at quarter phase.
36 days after Venus reaches greatest brilliancy, the planet passes in between the Earth and the Sun at inferior conjunction. Similar to New Moon, Venus' dark side directly faces Earth while the lit side faces away. Although the Moon is transitioning from the morning to the evening sky at New Moon, the "New Venus" transitions from the evening to the morning sky.
About 36 days after Venus reaches inferior conjunction, this wondrous world shines at its greatest brilliancy in the morning sky. Once again, Venus displays a crescent that's about 25% illuminated. Then, about 36 days after Venus attains greatest brilliancy as the morning "star," the planet reaches its greatest western (or morning) elongation, its disk once again 50% illuminated - like the quarter moon.

After Greatest Morning Elongation

After Venus is farthest from the Sun at greatest morning elongation, the planet in due time swings behind the Sun, at what's called superior conjunction. Though Venus is at full phase - like the Full Moon - you can't see Venus at this time, because it's hidden behind the Sun's glare. At superior conjunction, Venus lodges on the far side of the Sun, with the Sun sitting between Earth and Venus. You might think 72 days should lapse between greatest morning elongation and superior conjunction, because it takes some 72 days for Venus to swing from inferior conjunction to greatest morning elongation. Alas, it's not quite that easy . . .

584-day Cycle

There are about 220 days between greatest morning elongation and superior conjunction, at which time Venus transitions from the morning to the evening sky. 220 days thereafter, Venus swings from superior conjunction to greatest evening elongation. Then, 36 days after greatest evening elongation, Venus attains her greatest brilliancy in the evening sky, completing the cycle.
A (mean) total of 584 days lapses from greatest evening brilliancy to greatest evening brilliancy (or from the greatest morning brilliancy to greatest morning brilliancy). The same holds true for the period of time between successive inferior conjunctions (or successive superior conjunctions), or between successive greatest evening elongations (or successive greatest morning elongations). Remember the magic number: 584 days. In this 584 Earth-day period, Venus rotates upon its axis five times relative to the Sun. In other words, a Venusian day (as measured from solar noon to noon) lasts for nearly 117 Earth-days (584 divided by 5 = 116.8).

Venus' Famous 8-year Cycle

Five returns of Venus to its greatest brilliancy as an evening star represent a period of close to 8 years. Therefore, 8 years later - on July 12, 2015 - you can expect Venus to return to nearly the place in the sky, right by the Regulus, the brightest star in the constellation Leo the Lion. Venus' phase and peak brilliancy will repeat on nearly the same date in 8 years. In this 8-year period, Venus will have rotated 25 times relative to the Sun.

Coincidence or Cycle?

Saturn shines close to Regulus and Venus on July 12, 2007. Eight years later - on July 12, 2015 - Jupiter shines right where Saturn does in 2007!

copyright 2007 by Bruce McClure

Venus' Phases through a Telescope

June 2007 Feature * August 2007 Feature