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Starts With A Bang

What Did We See When The Moon And Venus Aligned On September 9, 2021?

If you had a clear western horizon, you had your shot at this view!

On September 9, 2021, the Moon and Venus nearly overlapped.

A simulated view of the post-sunset skies on September 9, 2021, from mid-northern latitudes. The Moon and Venus will be visible briefly after sunset, particularly if you have a clear western horizon. Overhead, Arcturus should be the first star to emerge at night: the brightest star in the northern celestial hemisphere. In the northwest skies, the stars of the Big Dipper should appear as the Sun descends further below the horizon; after 8 PM, the skies darken, but the Moon and Venus will set shortly thereafter. (E. SIEGEL / STELLARIUM)

The two brightest nighttime objects appeared separated by just 4° in Earth’s post-sunset skies.

This image, from February 20, 2015, shows a thin crescent Moon, with its face illuminated by Earthshine, with bright Venus nearby. Venus can come near a thin crescent Moon due to the geometry of its orbit, but can never come close to a full Moon, given that it never strays more than 47 degrees from the Sun in the sky. (GASPA/ullstein bild via Getty Images)

The full Moon can shine over 1500 times as bright as Venus, although September 9’s crescent was merely ~10 times as bright.

The Moon and Venus nearly overlapped entirely on December 1, 2008, as viewed from many parts of the world, including Europe. The craters on the Moon can be seen in extraordinary detail near the terminator: the day/night boundary on the Moon, where the sunlight comes in at low angles, highlighting the Moon’s crater walls. (Mike Hewitt/Getty Images)

Most of the Moon remained dark, with Sun-induced shadows highlighting its crater walls.

The Moon takes a little over 27 days to orbit 360º around Earth, and a little over 29 days to go from new Moon to new Moon again. As the phases cycle, the portion of the Moon illuminated by the Sun, and the amount of Earthshine falling on the Moon, changes. When the Moon is a thin crescent, the shadow-line between day and night can reveal its craters spectacularly. (WIKIMEDIA COMMONS USER ORION 8)

The Moon’s thin crescent arrived just 2 days after its “new” phase.

The cycle from new Moon to full Moon to new Moon again coincides with increases and decreases in apparent size as the Moon moves along its elliptical orbit. Because it moves faster at perigee and slower at apogee, but has a constant rate of rotation, we see slightly more than 50% of the Moon over the course of a lunar month: this is the phenomenon of lunar libration. When the Moon is only ~2 days “old,” or after the new Moon, it makes for excellent viewing in the post-sunset skies, but remains only a minor source of light pollution. (WIKIMEDIA COMMONS USER TOMRUEN)

Simple geometry sufficiently demonstrates the Sun’s much greater distance than the Moon’s.

The Moon’s phases are caused by reflected sunlight, allowing us to geometrically compare the Moon’s distance from Earth with the Sun’s. The fact that the Sun’s incident rays strike the Moon teach us that the Sun must be much, much farther away — at least hundreds of times — than the Moon, in order to explain our observations. (ANDONEE/WIKIMEDIA COMMONS/CCA-SA-4.0)

Cloud-rich Venus, on the other hand, exhibits different phase properties.

An infrared view of Venus’ night side, by the Akatsuki spacecraft. The over-illuminated portion, at left, is due to direct sunlight shining on Venus itself. Venus’s brightness is greater than that of any other planet as seen from Earth, and it approaches our world closer than any other planet does. When it is on the other side of the Sun, only a few other planets appear smaller. (ISAS, JAXA)

Through binoculars or a telescope, a small gibbous appears: a mostly full Venus.

On the evening of September 9, 2021, the post-sunset skies reveal a gibbous Venus. Venus is close to its minimum apparent size and still in its gibbous phase. As it approaches maximum elongation on October 29, it will rise higher in the post-sunset skies and be brighter and visible for longer, however its phase will first increase, peak, and then shrink as that moment approaches. (NASA/JPL SOLAR SYSTEM SIMULATOR; GARY M. WINTER)

This demonstrates Venus’s tremendous distance from Earth: comparable to the Sun, not the Moon.

When looking at an interior planet, it will never appear to ‘wander’ too far from the Sun. The inner planets orbit more quickly than the Earth does, and exhibit phases that showcase the great distances involved in the geometry between the Sun, Earth, and the planet in question. (WIKIMEDIA COMMONS USER WMHERIC)

The phases of Venus, throughout the year, reveal the Solar System’s scale.

The phases of Venus, as viewed from Earth, can enable us to understand how Venus appears to move east-to-west from the perspective of Earth. As Earth and Venus both orbit the Sun, Venus does so at a faster pace, which means that as it emerges (in a counterclockwise orbit) from behind the Sun, it will appear to move away from the Sun and higher in the post-sunset skies. If you view Venus in the skies right now, in late May, 2020, you’ll find it in a thin crescent phase. (WIKIMEDIA COMMONS USERS NICHALP AND SAGREDO)

At a September 9 distance of ~157 million kilometers, Venus was ~400 times the Earth-Moon distance.

The phase and illumination profile of Venus on September 9, 2021. Its very different phase from the Moon, given the angle between Earth, Venus, and the Sun, indicates that Venus is much farther away from Earth than the Moon is. In fact, on this date, Venus is approximately 400 times as distant from Earth as the Moon is. (NASA SOLAR SYSTEM SIMULATOR)

However, Venus is 5.4 times as reflective and 12.1 times the area of the Moon.

Mercury, Venus, Earth, Moon, Mars, and Ceres shown to accurate scale. In terms of brightnesses, Venus is the most reflective planet in the Solar System, reflecting 65% of incident sunlight back into space. Of the worlds shown here, the Moon is one of the least reflective, as on average it only reflects 12% of incident sunlight back into space. (KOPPELO/WIKIPEDIA, DERIVED FROM NASA IMAGES)

Its larger intrinsic brightness creates fascinating spectacles alongside the Moon.

This photo shows planet Venus and a crescent moon above a mountain near Drome, France. Although the brightness of the Moon is greater and spread out over a larger area, when the Moon is in a thin crescent phase, bright Venus can shine nearly as bright, making for some spectacular sights. (Francois LE DIASCORN/Gamma-Rapho via Getty Images)

From Earth, the Moon and Venus are both night sky objects capable of casting shadows.

The shadow of Vincent Jacques’s telescope as cast by Venus’s reflected light in Breil-sur-Roya, France. With an astronomical magnitude of around -4, Venus is the next brightest night sky object after the Moon, and is bright enough, when the Moon is absent, to cast a dim but noticeable shadow. (VINCENT JACQUES; EPOD/USRA)

Only during lunar eclipses can Venus outshine the Moon.

The Moon, during a total lunar eclipse, will drop in brightness from its full magnitude (between -13 and -12, as seen from Earth) down to its minimum brightness (about magnitude 0 at totality), where the brightest planets and even some stars can outshine it. Although an eclipsed Moon can never be seen in the same portion of the sky as Venus, Venus, whenever visible, will always outshine a totally eclipsed Moon. (KEN HASEKAMP)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


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