Moon Passes 2° South of Venus in Virgo as Crescent Pair Lines Up August 16

Q: Why did Venus appear to grow larger even as its illuminated phase shrank during its recent twilight dance with the Moon?

Venus looked larger because it was moving closer to Earth, even as the sunlit fraction of its disk visible from Earth was decreasing. That effect is a normal result of Venus’ orbit inside Earth’s. Around Aug. 15, Venus reached greatest eastern elongation, about 46 degrees from the Sun, making it prominent after sunset. At this stage, Venus appears roughly half lit, near dichotomy. After that point, as Venus continues along its orbit toward inferior conjunction in October, Earth’s view shifts more toward its night side. The visible phase shrinks from gibbous toward crescent. At the same time, the planet’s distance from Earth is decreasing. A nearer Venus subtends a larger angle in the sky, so telescopes show a bigger disk. The sky guide cited Venus at about 48% illuminated with an apparent diameter of roughly 25 arcseconds and noted that its angular diameter would keep growing as the phase waned. So the two changes are linked but driven by different geometry: phase depends on how much of Venus’ day side faces Earth, while apparent size depends on how far Venus is from Earth. As Venus swings closer on the inside track of the solar system, it can look larger even while less of it is illuminated.

Q: What rare cosmic illusion makes the upcoming Venus-Spica alignment look so close when they are actually light-years apart?

The “closeness” is a line-of-sight illusion known as a conjunction, or more broadly an apparent alignment caused by perspective from Earth. Venus and Spica can look nearly side by side on the sky’s dome because both lie close to the ecliptic, the narrow path along which the Sun, Moon and planets appear to move. From Earth, objects along that path can seem to bunch together even though they are separated by enormous distances in space. In reality, Venus is a nearby planet within our solar system, while Spica is a blue-white binary star about 250 light-years away. Their upcoming approach is not a true near-miss in space, just an optical effect created by our viewing angle. That perspective effect is the same reason astronomers often describe such events as conjunctions rather than literal alignments. The objects only appear close together against the fixed background of the sky; they are not physically close to one another. The rarity comes from timing and geometry. Venus is moving eastward through Virgo and will appear to tighten its gap with Spica night by night before their early-September conjunction. Such close apparent pairings are attractive but fleeting, and a much rarer event — an actual Venus occultation of Spica — will not happen again until 2197.

3 articles · Updated · Astronomy Magazine · Aug 16

August 16 brings a close Moon-Venus pairing in Virgo, with the waxing crescent Moon appearing beside Venus in the western twilight after sunset.
By darkening twilight, Venus shines about 9.5° to the Moon’s right; the planet is a 48%-lit crescent with an apparent diameter of 25 arcseconds after reaching dichotomy last week.
Spica, Virgo’s brightest star, sits to the Moon’s upper left and offers a third marker in the scene as the sky fades from twilight.
The skywatching stretch continues into early August 17, when Mars rises in Gemini and stands just over 1° north of Eta Geminorum, showing its motion against nearby stars.