Venus is the only planet that changes in apparent size by more than a factor of 6, (from 9.7 arcseconds to 63 arcseconds). This is because Venus goes from being very close, between us and the sun, at around 42 million Km (26 million miles) to very far, on the opposite side of the sun, at 258 million Km (160 million miles).
Venus is the closest to Earth at inferior conjunction. Though not often, the orbital plane of Venus and the Earth “line up” and we also get a transit, that is Venus crossing the face of the sun. This occurs in “pairs” 8 years apart, but those pairs occur approximately 105 years apart.
However, inferior conjunctions are much more frequent, and for me, equally as interesting and spectacular, as that is when you get to see Venus at its largest size and as a beautiful crescent.
Inferior conjunctions happen each 19.5 months, or five times in each of the famous Venus 8-year cycles. By mathematical coincidence, because both the orbital period for Earth (365 days) and Venus (225 days) are both divisible by 5 (not exactly, but very close), there is a repeating pattern of 5 for all conjunctions, in January, March, June, August and October, as shown here:
I have tried to capture Venus at or as close as possible to inferior conjunction several times. The image below was taken within several days of inferior conjunction, on June 8th, 2020, with exceptional atmospheric stability. Venus was at 0.9% illumination.
If Venus is close enough to the sun, you will also get to see a “full halo”, as the sun’s light gets scattered throughout its thick atmosphere.
The most recent inferior conjunction was on January 8th, 2022. The “full halo” was not predicted. However, I believe the prediction is for a visual observation. With modern digital photography, and digital processing, amazing things are possible.
I set up at the Custer Institute in Southold (my house has too many trees to the South), with Venus (and the nearby sun) very low, with the sun just weeks after the Winter solstice.
Venus was above the sun, so I used the trunk of my car to create a shadow on the objective of my Celestron C5 telescope, so I could minimize internal reflections. I took many 3,000 frame videos, with each frame at 7 milliseconds. I picked out the best single video (which was earlier in the day, just before noon). Using AutoStakkert! (free alignment and stacking software), I let it choose, align and stack the best 20% of that video (600 frames). Below is the final stacked image:
When I first saw this, I thought I was seeing the full halo? So, I took that same image, and stretched it as far as I reasonably could and inverted the colors. And there it is! The full halo, just barely visible:
Image Capture Data:
- Date and time: January 8th, 2022, 11:40 AM EST (Venus at inferior conjunction, 0.3% illumination)
- Location: Southold, NY
- Telescope: Celestron C5 SCT
- Camera: ZWO ASI 183MC
- Integration: 600 x 0.007 seconds
- Conditions: -2 Celsius with clear skies
- Transparency: 7/10
- Seeing: 3/5 with some wind