Mohawk Valley Astronomical Society

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Twilight Jewel

by Perry Pezzolanella

Venus beckons to be observed being the brightest object in the twilight sky besides the Moon. It is often the closest planet to Earth, coming as close as 24 million miles, but is frustrating to observe. This is because Venus is completely covered by thick carbon dioxide clouds laced with sulfuric acid. There is no way to observe its towering mountains, rolling plains, and grand canyons, but Venus still has a lot to offer.

Venus orbits the Sun closer than Earth; therefore, it can never be seen all night. As Venus travels from the far side of the Sun towards Earth, it goes through a series of phases from nearly full to a very thin crescent, all the while growing larger in size. It can range from 10 to 67 arcseconds across, and appear like a dazzling white jewel as bright as magnitude -4.9. The crescent becomes sliver thin as Venus passes between the Sun and Earth, and then shrinks in size as it reappears in the morning twilight going from crescent to full. The phases can be seen with a small telescope, and binoculars can resolve the crescent. A fun challenge is to try and see the large, fingernail-thin crescent with the unaided eye. A few sharp-eyed people with 20/15 or better vision have claimed to see it!

A fun project is to determine when Venus is at half phase. It always appears like a plump crescent when it is predicted to be at half phase. The thick clouds may create a foreshortening of the shadows along the terminator and create the illusion of a crescent. Seeing any detail in the clouds is challenging and a deep blue (W38A) filter may help, or a violet filter (W47) on larger telescopes. These features will be elusive and appear as faint streaks. An ultraviolet filter (W18A) or a special Venus filter will bring out some detail, but these are strictly used for photography and will not work for the eyes.

Venus may display cusp caps and cusp extensions. A cusp cap will make the normally pointed tip of the crescent, usually just one of them, appear unusually bright or blunted. It might be bordered by a dusky collar and can last for weeks. These may be caused by strong circulation patterns in the clouds. Another effect due to the thick clouds and strong circulation are deformations in the terminator. Usually the terminator has a soft, smooth edge, but notches may appear. Using various filters may enhance the effect. The thick atmosphere can cause cusp extensions where the very thin crescent tips become very prolonged and appear like the crescent on the Turkish flag. This phenomenon is quite easy to see. Even more beautiful, but difficult, is when the horns unite to form a delicate ring of light around the night side. This can only be seen when Venus is at its closest and largest near inferior conjunction, between the Earth and Sun.

A final oddity known as the Ashen Light is still a mystery. There may be a very faint reddish glow visible on the night side of Venus that is best seen when it is a thin crescent. The cause may be from airglow, thunderstorms, or the faint glow of the intensely hot surface. Something seems to be glowing when the night side of Venus should be pitch black and blend in with the background of space. The question remains: Is it real?

Experienced observers with sophisticated equipment can image the surface of Venus! The ferocious, nearly 900ºF surface, glows brightly in the infrared. Astronomers equipped with IR detectors can actually image the night side of Venus through selected wavelengths where the heat is transparent to the clouds. These images reveal the hotter lowlands glowing faintly brighter than the darker, cooler highlands!

Venus is far from being a boring, cloudy planet hardly worthy of a glance and should be given serious observing time. With only a few spacecraft exploring Venus the past several decades, it might be a rare possibility for the casual observer to discover something unusual. Venus is still full of mystery and a dedicated observer may be rewarded with a few surprises!