Venus reaches its greatest illuminated extent or greatest brilliancy in the evening sky on December 6, 2013. That means the planet’s daytime side is covering more square area of sky than at any other time during Venus’ present apparition as the “evening star.” And it means that Venus is brighter now than at any other time during this evening apparition.
You might think Venus appears brightest when we see its disk as most fully illuminated from Earth. Not so. If you were to observe Venus with the telescope today, you’d see that Venus’s disk is only a touch more than one-quarter illuminated by sunshine. A full Venus is always on the far side of the sun from us, so its disk size at full phase is always small. It’s only when we see Venus as a crescent that this world comes close enough to us to exhibit its greatest illuminated extent, at which time its daytime side covers the greatest area of sky.
Venus entered the evening sky on March 28, 2013 and will leave it on January 11, 2014. Where will it go? It’ll pass more or less between us and the sun on January 11. Astronomers call that an inferior conjunction of Venus.
Take a look at the chart below. Venus transitioned from the morning to evening sky when Venus swung more or less behind the sun as viewed from Earth (superior conjunction) on March 28, 2013.
Then some 292 days (9.6 months) later, Venus will transition back to the morning sky when it passes more or less in between the Earth and sun (inferior conjunction) on January 11, 2014. It’s not going to pass precisely between us and the sun at this inferior conjunction. If it were, Venus would transit the sun, as it did in June of 2012.
Bird’s-eye view of Earth’s and Venus’ orbits
Because Venus orbits the sun inside Earth’s orbit, we can never see Venus opposite (180o) the sun in our sky (like the full moon). We can’t even see Venus 90o from the sun (like the half-lit quarter moon). At most, Venus strays no farther than 47o from the sun in our sky. This is called Venus’ greatest eastern elongation when Venus appears the evening sky and greatest western elongation when she predominates over the morning sky.
Venus reaches its greatest elongation in the evening sky about 72 days before inferior conjunction and its greatest elongation in the morning sky some 72 days after inferior conjunction. If you look at Venus through a telescope at these times, you’ll see that its disk is about 50% illuminated by sunshine.
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 these times. Thirty-six days before inferior conjunction, it’s Venus’ brightest appearance in the evening sky; thirty-six days after inferior conjunction, it’s Venus brightest appearance in the morning sky.
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 72 days before and after inferior conjunction, and Venus’ greatest illuminated extent happens 36 days before inferior conjunction and 36 days after. (See above diagram of Venus’ and Earth’s orbits.)
Bottom line: Enjoy Venus at dusk and early evening on December 6, 2013. Even though this world is only about one-quarter illuminated in sunshine now, as seen from Earth, Venus is nonetheless shining at its brightest now!