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See Venus at greatest illuminated extent

These next few evenings – September 20 and 21 – Venus’ daytime side, or illuminated side, will cover more square area on the sky’s dome than at any time during its present apparition as the evening “star.” Venus will reach what’s called its greatest illuminated extent on September 21, 2018, at 10 Universal Time. Depending where you live worldwide, you may – or may not – see Venus at the instant of its greatest illuminated extent.

Look for Venus in the western sky shortly after sunset, especially if you live at northerly latitudes. At a latitude of 40 degrees north, Venus sets about one hour after sunset; at 40 degrees south latitude, in stark contrast, Venus sets a whopping 2 1/2 hours after the sun. Click here for a recommended almanac that’ll provide the setting times for the sun and Venus in your sky.

No, that’s not the moon. That’s a simulation of how Venus would appear in the telescope at its greatest illuminated extent (2018 September 21 at 10 UTC) via the US Naval Observatory.

Venus always shines as the 3rd-brightest celestial body in Earth’s sky, after the sun and moon. However, Venus is about 2 1/2 times brighter at its brightest than at its dimmest. Surprisingly, perhaps, Venus shines most brightly in our sky when its disk is about 25% illuminated in sunshine. That’s because Venus is closer to Earth at thinner phase and farther away from Earth at a fuller phase. Take a look at the diagram below.

Earth's and Venus' orbits

Earth and Venus orbit the sun counterclockwise as seen from earthly north. Venus reaches its greatest eastern elongation in the evening sky about 72 days before inferior conjunction and its greatest western elongation in the morning sky about 72 days after inferior conjunction. Greatest illuminated extent for Venus comes midway between a greatest elongation and an inferior conjunction.

Venus first entered the evening sky at superior conjunction (full phase) on January 9, 2018, and will leave the evening sky to enter the morning sky at inferior conjunction (new phase) on October 26, 2018.

Venus last reached its greatest eastern (evening) elongation (maximum angular separation) of 46 degrees from the setting sun on August 17, 2018. Venus will next reach its greatest western (morning) elongation of 47 degrees west of the rising sun on January 6, 2019. At greatest elongation, Venus is approximately 50% illuminated in sunshine. A greatest eastern (evening) elongation takes place about 72 days before an inferior conjunction, and a greatest western (morning) elongation comes about 72 days after an inferior conjunction.

Venus exhibits its greatest illuminated extent about 36 days before – and after – inferior conjunction. Through the telescope, Venus appears about 25% illuminated in sunshine at its greatest illuminated extent. This bright world will reach its greatest illuminated extent as the morning “star” on December 2, 2018.

As Venus comes closer to Earth, its phase shrinks but its disk size enlarges. Image credit: Statis Kalyvis

Let the golden triangle help you to remember these Venus’ milestones. The two base angles equal 72o and the apex angle equals 36o. Quite by coincidence, Venus’ greatest elongations happen some 72 days before and after inferior conjunction, and Venus’ greatest illuminated extent happens some 36 days before and after inferior conjunction.

Golden Triangle

The Golden Triangle, with the apex angle = 36o and base angles = 72o

Bottom line: Venus’ greatest illuminated extent occurs on September 21, 2018. At greatest illuminated extent, Venus’ daytime side covers more square area on the sky’s dome than at any other time during its stint as the evening “star” (January 6 to October 26, 2018). It’s at and around this time that Venus shines most brightly in our sky.

Bruce McClure

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