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Don’t miss the moon and Venus on December 5 or 6!

Waxing moon and Venus closer after sunset on December 5. Read more

Tonight for December 5, 2013

Depending on where you live on the globe, the moon and the planet Venus will pair up most closely for the month on the evening of December 5 or December 6. But no matter. Both evenings will be beautiful, with Venus now at its brightest for this 2013 evening apparition.

You’ll be looking in the southwestern or western sky after sunset, and you simply can’t miss nearby Venus. It’s always the third-brightest object to grace the heavens, after the sun and moon.

What’s more, Venus is now shining at or near its greatest brilliancy. According to astronomical almanacs, Venus displays its greatest illuminated extent on Friday, December 6. Greatest illuminated extent means the lit portion of Venus’ disk covers the greatest square area of sky during its present apparition as an “evening star” after sunset – or, coming in February 2014, its apparition as the “morning star” before sunrise.

Phases of Venus

This isn’t the moon. It’s the planet Venus as seen through the telescope. As Venus wanes in the evening sky it gets closer to Earth. The lit portion of Venus covers the greatest area of sky when its disk is about one-quarter illuminated. Image credit: Wikimedia Commons

Believe it or not, Venus’ disk is only around 26% illuminated in sunshine at its greatest illuminated extent. Even so, the illuminated portion of Venus covers more square area of sky than at a greater or lesser phase. By the way, you need at telescope to observe Venus’s phases. Venus is now waning toward new phase.

Venus is farthest away from Earth when at full phase, so its apparent diameter is then at its smallest. At new phase, Venus comes closest to Earth and exhibits its greatest diameter, yet Venus’ nighttime side is facing Earth and its daytime side is turned away from us at that time. Venus’ diameter is a solid 6 times larger at new phase than when it’s full.

So Venus’ greatest illuminated extent depends on that delicate balance between phase and the apparent size of its diameter. Venus shines most brilliantly when its disk is about one-quarter illuminated, blazing away at an apparent magnitude of -4.7. That’s about 6 times brighter than Jupiter, the second brightest planet, 20 times brighter than Sirius, the brightest star of the nighttime sky, and 70 times brighter than Arcturus, the brightest star in the northern half of sky.

Stellar luminosity: The true brightnesses of stars

Diagram of the phases of Venus

Unlike the moon, Venus wanes from full to new in the evening sky and waxes from new to full in the morning sky

Bottom line: Let the waxing crescent moon be your guide to Venus on December 5 and 6, and then watch Venus beam at its greatest brilliancy throughout early December 2013!