Watch Rotating Horns of Venus at Dawn

Venus inferior conjunction
Venus inferior conjunction
Venus just 10.5 hours before inferior conjunction on March 25th. Image credit and copyright: Shahrin Ahmad (@Shahgazer)

Have you seen it yet? An old friend greeted us on an early morning run yesterday as we could easily spy brilliant Venus in the dawn, just three days after inferior conjunction this past Saturday on March 25th.

This was an especially wide pass, as the planet crossed just over eight degrees (that’s 16 Full Moon diameters!) north of the Sun. We once managed to see Venus with the unaided eye on the very day of inferior conjunction back in 1998 from the high northern latitudes of the Chena Flood Channel just outside of Fairbanks, Alaska.

The planet was a slender 59.4” wide, 1% illuminated crescent during this past weekend’s passage, and the wide pass spurred many advanced imagers to hunt for the slim crescent in the daytime sky. Of course, such a feat is challenging near the dazzling daytime Sun. Safely blocking the Sun out of view and being able to precisely point your equipment is key in this endeavor. A deep blue, high contrast sky helps, as well. Still, many Universe Today readers rose to the challenge of chronicling the horns of the slender crescent Venus as they rotated ’round the limb and the nearby world moved once again from being a dusk to dawn object.

Venus rotating horns
A daily sequence showing the ‘Horns of Venus’ rotate as it approaches inferior conjunction. Image credit and copyright: Shahrin Ahmad (@ShahGazer)

The orbit of Venus is tilted 3.4 degrees with respect to the Earth, otherwise, we’d get a transit of the planet like we did on June 5-6th, 2012 once about every 584 days, instead of having to wait again until next century on December 10th, 2117.

The joint NASA/European Space Agency’s SOlar Heliospheric Observatory (SOHO) mission also spied the planet this past weekend as it just grazed the 15 degree wide field of view of its Sun-observing LASCO C3 camera:

Venus SOHO
The glow of Venus (arrowed) just barely bleeding over into the field of view of SOHO’s LASCO C3 camera. Credit: SOHO/NASA/LASCO

Venus kicks off April as a 58” wide, 3% illuminated crescent and ends the month at 37” wide, fattening up to 28% illumination. On closest approach, the planet presents the largest apparent planetary disk possible as seen from the Earth. Can you see the horns? They’re readily readily apparent even in a low power pair of hunting binoculars. The coming week is a great time to try and see a crescent Venus… with the naked eye. Such an observation is notoriously difficult, and right on the edge of possibility for those with keen eyesight.

One problem for seasoned observers is that we know beforehand that (spoiler alert) that the Horns of Venus, like the Moon, always point away from the direction of the Sun.

True Story: a five year old girl at a public star party once asked me “why does that ‘star’ look like a tiny Moon” (!) This was prior to looking at the planet through a telescope. Children generally have sharper eyes than adults, as the lenses of our corneas wear down and yellow from ultraviolet light exposure over the years.

Still, there are tantalizing historical records that suggest that ancient cultures such as the Babylonians knew something of the true crescent nature of Venus in pre-telescopic times as well.

The Babylonian frieze of Kudurru Melishipak on display at the Louvre, depicting the Sun Moon and Venus. According to some interpretations, the goddess Ishtar (Venus) is also associated with a crescent symbol… possibly lending credence to the assertion that ancient Babylonian astronomers knew something of the phases of the planet from direct observation. Credit: Wikimedia Commons/Image in the Public Domain.

Another fun challenge in the coming months is attempting to see Venus in the daytime. This is surprisingly easy, once you know exactly where to look for it. A nearby crescent Moon is handy, as occurs on April 23rd, May 22nd, and June 20th.

Daytime Venus
Venus (arrowed) near the daytime Moon. Photo by author.

Strangely enough, the Moon is actually darker than dazzling Venus in terms of surface albedo. The ghostly daytime Moon is just larger and easier to spot. Many historical ‘UFO’ sightings such as a ‘dazzling light seen near the daytime Moon’ by the startled residents of Saint-Denis, France on the morning on January 13th, 1589 were, in fact, said brilliant planet.

The Moon near Venus on May 22nd. Credit: Stellarium.

Venus can appear startlingly bright to even a seasoned observer. We’ve seen the planet rise as a shimmering ember against a deep dark twilight sky from high northern latitudes. Air traffic controllers have tried in vain to ‘hail’ Venus on more than one occasion, and India once nearly traded shots with China along its northern border in 2012, mistaking a bright conjunction of Jupiter and Venus for spy drones.

The third brightest object in the sky behind the Sun and the Moon, Venus is even bright enough to cast a shadow as seen from a dark sky site, something that can be more readily recorded photographically.

Watch our nearest planetary neighbor long enough, and it will nearly repeat the same pattern for a given apparition. This is known as the eight year cycle of Venus, and stems from the fact that 13 Venusian orbits (8x 224.8 days) very nearly equals eight Earth years.

Follow Venus through the dawn in 2017, and it will eventually form a right triangle with the Earth and the Sun on June 3rd, reaching what is known as greatest elongation. This can vary from 47.2 to 45.4 degrees from the Sun, and this year reaches 45.9 degrees elongation in June. The planet then reaches half phase known as dichotomy around this date, though observed versus theoretical dichotomy can vary by three days. The cause of this phenomenon is thought to be the refraction of light in Venus’ dense atmosphere, coupled with observer bias due to the brilliance of Venus itself. When do you see it?

Also, keep an eye out for the ghostly glow on the night-side of Venus, known as Ashen Light. Long thought to be another trick of the eye, there’s good evidence to suggest that this long reported effect actually has a physical basis, though Venus has no large reflecting moon nearby… how could this be? The leading candidate is now thought to be air-glow radiating from the cooling nighttime side of the planet.

Cloud enshrouded Venus held on to its secrets, right up until the Space Age less than a century ago… some observers theorized that the nighttime glow on Venus was due to aurorae, volcanoes or even light pollution from Venusian cities (!). This also fueled spurious sightings of the alleged Venusian moon Neith right up through the 19th century.

Venus should also put in a showing 34 degrees west of the Sun shining at magnitude -4 during the August 21st, 2017 total solar eclipse. Follow that planet, as it makes a complex meet up with Mars, Mercury, and the Moon in late September of this year.

More to come!

-Read about planets, occultations, comets and more for the year in our 101 Astronomical Events for 2017, out as a free e-book from Universe Today.

Venus Rules the Dusk Skies at Greatest Elongation

Venus at dusk
Venus at dusk
Venus, Mars, and the waxing crescent Moon at dusk from the evening of January 3rd, 2017. Image credit and copyright: Alan Dyer.

“What’s that bright light in the sky?” The planet Venus never fails to impress, and indeed makes even seasoned observers look twice at its unexpected brilliance. The third brightest natural object in the sky, Venus now rules the dusk, a fine sight for wintertime evening commuters. Venus reaches greatest elongation tomorrow, a excellent time to admire this dazzling but shrouded world of mystery.

Venus at greatest elongation

Only the two planets interior to Earth’s orbit – Mercury and Venus – can reach a point known as greatest elongation from the Sun. As the name suggests, this is simply the point at which either planet appears to be at its maximum angular distance from the Sun. Think of a big right triangle in space, with Venus or Mercury at the right angle vertex, and the Sun and Earth at the other two corners. High school geometry can come in handy!

Venus elongation
Venus at greatest elongation (planets and orbits not to scale). Credit: Dave Dickinson

This Thursday on January 12th Venus reaches a maximum of 47 degrees elongation from the Sun at 11:00 Universal Time (UT) / 6:00 AM Eastern Standard Time, shining at magnitude -4.4. The maximum/minimum elongation for Venus that can occur is 47.3 to 45.4 degrees respectively, and this week’s is the widest until 2025.

Here’s some key dates to watch out for:

Jan 12th: Venus passes less than a degree from Neptune.

Jan 14th: Venus reaches theoretical dichotomy?

Jan 14th: Venus passes 3′ from +3.7 the magnitude star Lambda Aquarii.

Jan 17th: Venus crosses the ecliptic plane northward.

Venus and Mars reach ‘quasi-conjunction’ in late January.

January 30th: Venus crosses the celestial equator northward.

January 31st: The Moon passes 4 degrees south of Venus, and the two also form a nice equilateral triangle with Mars on the same date.

Looking west on the evening of January 31st, 2017. Image credit: Stellarium.

February 17th: Venus reaches a maximum brilliancy of magnitude -4.6.

March 26th: Solar conjunction for Venus occurs eight degrees north of the Sun … it is possible to spy Venus at solar conjunction from high northern latitudes, just be sure to block out the Sun.

Through the telescope, Venus displays a tiny 24.4” size half phase right around greatest elongation. You could stack 74 Venuses across the diameter of tomorrow’s Full Moon. When does Venus look to reach an exact half phase to you? This point, known as theoretical dichotomy, is often off by just a few days. This is a curious observed phenomenon, first noted by German amateur astronomer Johann Schröter in 1793. The effect now bears his name. A result of atmospheric refraction along the day/terminator on Venus, or an optical illusion?

Gibbous Venus
Almost there… a waning gibbous Venus from the evening of January 5th, 2017. Image credit and copyright: Shahrin Ahmad (@Shahgazer)

And hey, amateurs are now using ultraviolet filters to get actual detail on the cloud-tops of Venus… we like to use a variable polarizing filter to cut down the dazzling glare of Venus a bit at the eyepiece.

Also, keep an eye out for another strange phenomenon, known as the Ashen Light of Venus. Now,ashen light or Earthshine is readily apparent on dark side of the Moon, owing to the presence of a large sunlight reflector nearby, namely the Earth. Venus has no such large partner, though astronomers in the early age of telescopic astronomy claimed to have spied a moon of Venus, and even went as far as naming it Neith. An optical illusion? Or real evidence of Venusian sky glow on its nighttime side? After tomorrow, Venus will begin heading between the Earth and the Sun, becoming a slender crescent in the process. Solar conjunction occurs on March 25th, 2017. Venus sits just eight degrees north of the Sun on this date, and viewers in high Arctic latitudes might just be able to spy Venus above the horizon before sunrise on the day of solar conjunction. We performed a similar feat of visual athletics on the morning of January 16th, 1998 observing from North Pole, Alaska.

Venus as seen from Fairbanks, Alaska on the morning of solar conjunction, 2017. Image credit: Starry Night.

From there, Venus heads towards a fine dawn elongation on June 3rd, 2017. All of these events and more are detailed in our free e-book: 101 Astronomical Events for 2017.

Spying Venus in the Daytime

Did you know: you can actually see Venus in the daytime, if you know exactly where to look for it? A deep blue, high contrast sky is the key, and a nearby crescent Moon is handy in your daytime quest. Strange but true fact: Venus is actually brighter than the Moon per square arc second, with a shiny albedo of 70% versus the Moon’s paltry 12%. But Venus is tiny, and hard to spot against the blue daytime sky… until you catch sight of it.

The Moon passing Venus on January 31st, 2017 in the daytime sky. Image credit: Stellarium.

There’s another reason to brave the January cold for northern hemisphere residents: Venus can indeed cast a shadow if you look carefully for it. You’ll need to be away from any other light sources (including the Moon, which passes Full tomorrow as well with the first Full Moon of 2017, known as a Full Wolf Moon). And a high contrast surface such as freshly fallen snow can help… a short time exposure shot can even bring the shadow cast by Venus into focus.

If you follow Venus long enough, you’ll notice a pattern, as it visits very nearly the the same sky environs every eight years and traces out approximately the same path in the dawn and dusk sky. There’s a reason for this: 8 Earth years (8x 365.25 = 2922 days) very nearly equals 5 the synodic periods for Venus (2922/5=584 days, the number of days it takes Venus to return to roughly the same point with respect to the starry background, separate from its true orbit around the Sun of 225 days). For example, Venus last crossed the Pleiades star cluster in 2012, and will do so again in – you guessed it — in 2020. Unfortunately, this pattern isn’t precise, and Venus won’t also transit the Sun again in 2020 like it did in 2012. You’ll have to wait until one century from this year on December 10-11th, 2117 to see that celestial spectacle again….

Hopefully, we’ll have perfected that whole Futurama head-in-a-jar thing by then.