The 2023–2024 western elongation of Venus: interim report

On 2023 Aug 13, Venus passed through inferior conjunction and returned to the morning skies, embarking on the 2023–’24 western elongation. The planet is now steadily moving away from us with the phase increasing. On 2023 Oct 22, Venus reached dichotomy – the point where it is exactly 50% illuminated and looks like a ‘half-moon’ in telescopes. The Director and a number of Section members were able to observe the planet during this time.

A reasonable number of Section members continue to observe Venus in the morning sky, and it is thanks to them that this interim report can be made. Since inferior conjunction, some interesting phenomena have been observed and we report on them here. In all images/drawings, north is at the bottom and south at the top.


Figure 1. Nightside images of Venus.
(a) Discrete surface features. Taken on 2023 Sep 6, 04:36 UT with a 444mm Newtonian reflector. (Martin Lewis, UK)
(b) Further individual surface features. Taken on 2023 Oct 3, 18:55 UT with a 508mm Newtonian and 1000–1200nm filters. (Anthony Wesley & Phil Miles, Australia)
(c) Two images taken on 2023 Sep 9 at 04:36 UT (left), and 2023 Sep 10 at 04:44 UT (right). Both were obtained with a 355.6mm Schmidt–Cassegrain (SCT) using a 1000nm long-pass filter. A long, thin cloud seems to be present in the north on the morning of Sep 10. (Peter Tickner, UK)
(d) Another transient feature – possibly a cloud – detected on the nightside. This IR image was taken on 2023 Oct 10, 05:36 UT; a 355.6mm SCT was used with a 1000nm long-pass filter. (Peter Tickner, UK)



Figure 2. Observations showing the cusp anomaly. (a) 2023 Oct 16, 355mm SCT, ZWO ASI 290MM (left: 04:27 UT, UV Chroma Bessel U; right: 04:52 UT, IR685 Baader) by Clyde Foster. (b) 2023 Oct 15, 203mm Newtonian reflector, ×100 (top: 10:26 UT, W15 filter, bottom: 10:33 UT, IL).


Figure 3. Observation of a feature similar to the wave discontinuity, seen on the nightside for the first time. The image was taken on 2023 Oct 7, 05:53.8 UT, with a 355.6mm SCT and a 1010nm FWHM 38nm filter.

Observations of the nightside

The hot nightside of Venus glows in infrared (IR) – this may be captured using IR imaging techniques provided the bright daytime crescent is not too large. Imaging the nightside of Venus requires a degree of skill, but a number of Section members routinely do this work when the phase is small. This allows us to monitor the nightside for signs of active volcanism.
Figure 1 shows recent nightside observations. In both (a) and (b), discrete surface features can be clearly identified. In particular, the dark patch near the centre of the disc in (b) is likely to be the Ovda Regio. In (c), Tickner’s image appears to show a thin transient cloud to the north. A smaller transient feature appears in his image of 2023 Oct 10 (d). Peter has been very active in pursuing nightside observations, and he has written a number of articles in the Section newsletter, Messenger, about this.1

The cusp anomaly

When the planet is in the crescent stage, occasionally the terminator at the poles deviates from its usual path and becomes much flatter – this is the cusp anomaly. The anomaly was last seen during the 2019–’20 eastern elongation.2 The Director observed the effect a number of times before dichotomy in October. Figure 2(a) shows a drawing made by the Director on 2023 Oct 15 with the cusp anomaly present. It is present again in images taken by Clyde Foster the next day (Figure 2(b)).

The wave discontinuity?

Finally, we present an interesting observation made by António Cidadão of Portugal. His image from 2023 Oct 3 shows what looks like the wave discontinuity on the nightside of the planet. We have reported on this phenomenon recently,3 but this is the first time it (or something like it) has been seen in nightside images.

The Director wishes to thank all who have sent their observations in to the Section and would urge members to continue to monitor Venus, as it is clear that the planet is very active!

Paul G. Abel, Mercury & Venus Section Director


1 BAA Mercury & Venus Section web pages:
2 Abel P. G., ‘The 2019–’20 eastern elongation of Venus, Part I: Observations of the dayside’, J. Br. Astron. Assoc., 131(4) (2021)
3 McKim R. J. & Abel P. G., ‘Observations of another Venusian atmospheric discontinuity, 2022 May to September’, J. Br. Astron. Assoc., 133(1) (2023)

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