Moon Passes 1.1° South of Venus Before Sunrise on November 7

Q: Why does Venus actually reach its peak brilliance when it appears as a thin crescent rather than a fully illuminated sphere?

Venus is brightest when it looks like a thin crescent because its much larger apparent size then more than offsets the fact that a smaller fraction of its sunlit side is facing Earth. When Venus is on the far side of the Sun, it appears nearly “full,” but it is also much farther from Earth, so its disk looks small. As Venus swings around the inside of Earth’s orbit and comes closer to us, the illuminated portion shrinks into a crescent, but the planet’s apparent diameter grows dramatically. At a certain point, that tradeoff produces the greatest total sunlit area visible from Earth, which is when Venus reaches peak brilliance. Astronomers sometimes call this balance point Venus’ “greatest illuminated extent” or “greatest brilliancy.” It typically happens before and after inferior conjunction, when Venus is relatively close to Earth and seen as a crescent rather than a round disk. Venus’ exceptional brightness is amplified by its atmosphere. Its global cloud deck, made of highly reflective sulfuric-acid clouds, reflects roughly 70% to 90% of incoming sunlight—higher than any other planet. That high reflectivity, combined with Venus’ closeness to Earth at crescent phase, is why it can outshine every star and all other planets. So the key is not how “full” Venus looks, but how much illuminated area it presents in our sky after distance is taken into account.

Q: The shining dot near Venus is Spica, but did you know this single point of light is actually two massive orbiting stars?

Yes. The faint point near Venus is Spica, and despite looking like a single star to the naked eye, it is actually a close binary system made of two hot, massive blue-white stars orbiting each other. Spica, also known as Alpha Virginis, lies about 250 light-years from Earth and is the brightest star in Virgo. The two stars are so close together that even large telescopes generally cannot separate them visually. Astronomers identify the pair through spectroscopy: as the stars race around each other, their light shifts back and forth because of the Doppler effect. The system is extreme. The stars complete an orbit in about 4 days, indicating a very tight separation. Estimates place the primary at roughly 11.4 times the Sun’s mass and the companion at about 7.2 solar masses. They are also intensely luminous and hot, with surface temperatures above 20,000 kelvin. The brighter star alone shines tens of thousands of times more powerfully than the Sun. Because they are so close, each star’s gravity distorts the other, pulling them out of a spherical shape into elongated, egg-like forms. That makes Spica not just a binary, but a spectroscopic binary with measurable tidal distortion. So when skywatchers look low in the eastern sky before sunrise and spot Venus near Spica, they are not seeing an ordinary single star. They are seeing one of the sky’s brightest stellar systems: two massive stars locked in a rapid, tight orbit.

3 articles · Updated · Astronomy Magazine · Nov 7

A 1.1° Moon-Venus conjunction peaks at 7 a.m. EST on November 7, with the waning crescent passing due south of Venus low in the eastern predawn sky.
About an hour before sunrise, the pair sits roughly 6° above the horizon and climbs as twilight brightens, making Venus—at magnitude -4.5—the easiest guide to the scene.
The Moon is just 1% illuminated, with its thin western crescent visible alongside earthshine lighting the shadowed part of the lunar face.
Spica, shining at magnitude 1.0, lies 1.3° northeast of Venus, giving skywatchers a second bright reference point as Venus shifts west of the star by November 10.