Brilliant Venus

As far as the planet Venus is concerned, many of the factors that affect observations of Mercury are similar for Venus. It too orbits the Sun within the Earth's orbit and it is rarely high above the horizon after sunset. Like Mercury, it is biggest when merely a thin crescent (or when transiting the Sun) and it is best when elongated as far from the Sun as possible.

Venus is the planet most similar in size to the Earth. It has an equatorial diameter of 12,104 kilometers, i.e., 95% of the Earth's. Venus orbits the Sun in 224.7 days at a mean distance of 108.2 million kilometers and it overtakes the Earth on the inside track every 583.9 days (the synodic period). But if you thought Mercury's rotation period was weird, well, you could argue that the Venusian (or Cytherian) day was weirder. Venus rotates on its axis in 243 days. Yes, that's right. The day is longer than the year. Actually, this is of little interest to the webcam imager as we cannot see the Venusian surface. Venus has a dense atmosphere.

However, the upper clouds do have a retrograde (i.e., backward, like the planet's rotation) rotation period of about four days. Visually, Venus is both beautiful and featureless. Well, not entirely featureless, as observers with keen eyesight and experience can see some subtle atmospheric details in the atmosphere, especially if they have good sensitivity in the violet end of the spectrum and are using an appropriate filter. To me, Venus is at its most mesmerising when a crescent. It is nice and big (30 to 40 arc-seconds across when at a phase between 40% and 20%, respectively) and at a decent elongation from the Sun (45 to 35 degrees).

Despite the bland appearance of the dazzling Venusian cloud tops, there are a couple of relevant phenomena that are of particular interest to the amateur observer, although most of the interest is historical. The first feature is the phase anomaly, most obvious when Venus is supposed to be at half-phase, or dichotomy. Venus looks half when it is actually a bit fatter than that. Put another way, the illuminated part looks slimmer than it actually is. For evening apparitions, with the phase shrinking from gibbous to half to crescent, dichotomy appears to happen several days early. For morning apparitions dichotomy appears to happen a few days late. It should be stressed that, to the novice, the difference is very subtle, amounting to a phase difference of maybe 2%. With digital images there is no observer bias though. The effect is undoubtedly due to the Venusian atmosphere and it varies depending on what color filter is being employed.

The second phenomenon is far more controversial. It is called the "ashen light" and can only be seen when Venus is a crescent; usually a thin crescent. The ashen light is the term used to describe the alleged visibility of the nighttime side of Venus, or the dark side if you prefer. Of course, with the crescent Moon we see this phenomenon, called Earthshine, every month. The un-illuminated side of the Moon glows faintly due to light reflected from the Earth (especially the Earth's clouds) back onto the Moon. There is no mystery about the lunar Earthshine, the near-full Earth will shine as an object of magnitude -17 as seen from the Moon and will be as bright as 70 full moons would shine in our sky (because Earth is much larger and much more reflective). However, there is no way that the Earth can reflect a significant amount of light onto Venus. From Venus, a near-full Earth would only appear as an object of magnitude -6, nowhere near bright enough to illuminate the Venusian cloud tops. So what else could the ashen light be? Possibly the most popular idea is that it is simply an optical illusion, a contrast effect between the dazzling crescent and the nighttime sky. The brain just completes the rest of the circle. However, experienced observers are well aware of the tricks the eye and brain can play and they have used tiny curved occulting bars in the eyepiece to shield the dazzling crescent from view . . . and they have still reported the ashen light from time to time. But, despite this evidence, and rather disturbingly, even experienced observers have, from time to time, recorded a Venusian dark side that is actually darker than the background twilight sky. This, of course, is impossible to justify and must be due to an optical illusion. Only the ashen light observations in which a bright ashen light has been recorded by highly experienced observers, using an occulting bar, can be taken seriously. But if this small percentage of cases is genuine, what can be the cause? There appear to be only a few possible explanations, namely: violent thunderstorms in the Venusian atmosphere; intense Venusian aurorae, produced electrically; horizontal atmospheric scattering of light; observers who have exceptional eyesight in the near infrared;

Figure 11.6. The night side of Venus imaged by Christophe Pellier on May 19, 2004, with a 356-mm Celestron 14 SCT and ATiK 1HS webcam. A short exposure on the left captures the visual crescent and an infrared (1000nm) 105 x 8 second stack captures the dark side glowing. Image: C. Pellier.

or, an illusion. To me, this latter explanation is the only one that seems likely for the majority of cases. Until I see a CCD image of the Ashen light, I am inclined to regard it as an illusion, even though I actually know experienced and genuine observers who have seen this elusive phenomenon. CCD images of the dark side glowing in the infrared have been taken and these are discussed shortly.

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook

Post a comment