The 2019–’20 eastern elongation of Venus, Part I: Observations of the dayside

2021 May 31

Paul G. Abel

Introduction

The 2019–’20 eastern elongation of Venus was an excellent one for observers based in the northern hemisphere. From January to early May, the planet dominated in the evening skies and its high altitude made it easily accessible. With most people confined to their homes due to the COVID-19 coronavirus pandemic, amateur astronomers made good use of their enforced increase in telescope time, which resulted in a large volume of observations being communicated to the Director.

The elongation began on 2019 Aug 14, when Venus passed through superior conjunction. The planet was visible in the evening skies for the last part of 2019, when it was rather low down for UK observers, but well placed for southern hemisphere astronomers. By the start of 2020 it was becoming better placed, and observations started to increase. The planet reached greatest eastern elongation on 2020 Mar 24, with theoretical dichotomy predicted for 2020 Mar 26. As we shall see, observed dichotomy took place four days earlier.

The planet then continued to move back towards the Sun, until it came to inferior conjunction on 2020 Jun 3. It subsequently reappeared in the morning sky, marking the start of the 2020–’21 western elongation, when the planet was well placed for observation in the morning sky for northern hemisphere observers.

Coverage of the 2019–’20 eastern elongation was particularly good; regular observations were received from 48 Section members. Their names, along with details of the instrumentation and locations, are given in Table 1. It is pleasing to see a small increase in the number of visual observers for this elongation, and the Director is happy to report that a number of members new to the Association contributed UV images. Many high-quality IR and UV images were communicated to the Director throughout the elongation, as were some well-executed drawings.

Upon reviewing and analysing the observations received, it became clear that Venus has been unusually dynamic during this elongation. Various anomalous atmospheric phenomena were recorded, along with unusual appearances of the terminator and cusps. A small number of experienced observers obtained striking images close to inferior conjunction and some also had success in imaging the nightside of the planet.

With so much material to cover, it has been necessary to split this report into two parts. The first deals with the analysis of observations concerning the sunlit hemisphere, while the second will be devoted to observations of the nightside.

The images and drawings presented here are a small number of those submitted to the Section. However, many ‘raw’ observations (ones which have not been analysed in any way) can be found in the Section newsletter, Messenger, covering this period.^1,2 Unless otherwise stated, the convention of putting south at the top is followed in all images and drawings.

Cloud features

In earlier times, Venus acquired a reputation for being a bland world, with little to see in the way of features. While it is true that those visual observers sensitive to shorter wavelengths are more likely to see any compelling atmospheric details, in the last few years a number of UV filters have become available, allowing imagers with modest apertures to capture striking cloud formations in the Cytherean atmosphere.

Both visual and digital observers recorded unusual cloud formations. The cloud markings on Venus tend to be a mixture of streaks, brighter spots, variations on well known ‘Ψ markings’,³ and occasionally more subtle features. During this elongation, it seems to have been the case that the more subtle markings and cloud structures were observed visually along with a number of brighter spots, as can be seen in Figure 1.

The atmospheric features of Venus are most accessible at shorter wavelengths and, as a result, imagers who possess UV filters generally capture the most detail. Many of the Section’s digital observers do possess such filters and as a result, unusual, intricate cloud formations were captured during the elongation.

Figure 2 gives a selection of the more notable cloud patterns. Wesley’s images (Figure 2b & c) show some of the most striking formations the Director has seen in amateur photographs. Two colour images of the planet were made by Casquinha and Peach (Figure 2k & l).

On 2020 Mar 22, Manos Kardasis communicated to me his observations of a highly unusual wave-like structure in the atmosphere of Venus, which he had imaged on the previous day (Figure 3). This remarkable bow-shaped wave had previously been recorded by the Japanese Akatsuki spacecraft; an analysis of the observations made by Kardasis and other amateur imagers can be found in a 2020 paper by McKim, Kardasis and the Director.⁴ In that paper we note the very slow drift of the feature with respect to the surface and conjecture a volcanic origin for the phenomenon.

Terminator

Throughout the elongation, the terminator appears to have been geometrically regular in both images and visual observations. It is not uncommon for some observers to report slight irregularities to it near the time of dichotomy; this is thought to arise due to the presence of dark clouds on or near it at that time.

The most interesting phenomenon concerning the terminator during this elongation was the presence of the ‘cusp terminator anomaly’ – a name first introduced by the Director. Usually, the terminator follows a well-defined ellipse from pole to pole; however, during April and early May there was a notable deviation at the cusps, whereby it appeared much flatter there than usual. This was the anomaly and observations of it are given in Figure 4.

The Director first observed the phenomenon on 2020 Apr 24, when the effect was quite notable, visible in both white light and with a W15 (Wratten number) filter (Figure 4d & e). The Director contacted Leatherbarrow, who had clear skies in Sheffield: He was able to confirm the effect visually and successfully capture an image of the phenomenon, as can be seen in Figure 4b. Leatherbarrow also produced an over-exposed IR image which greatly emphasises the effect, as shown in Figure 4c. McKim observed the planet earlier in the evening and his drawing appears to show a similar, albeit somewhat more muted, effect (Figure 4f). The anomaly also seems to be faintly present in an observation made by Aerts on 2020 Apr 20 (Figure 4a).

The Director next observed the phenomenon on 2020 May 6 and it seemed to be strongest in the W21 (orange) filter, particularly at the southern cusp (Figure 4h–k). Interestingly, Foulkes took an image in IR on the previous evening (2020 May 5) which appears to show the anomaly quite nicely (Figure 4g). These observations would seem to imply that the anomaly was most apparent in IR and longer wavelengths. This appearance of the cusp caps is not new – upon reviewing past elongation reports, one can find many instances of the phenomenon, usually when the phase of the planet is between 40 and 50%.

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