Venus’s eight-year cycle has the author dreaming of 2029.

Sky & Telescope

As I write this in september, Venus is wrapping up another magnificent appearance as the Evening Star. All summer and fall it has beamed through the dusk as the sky has darkened before following the Sun to exit, stage West. It’s a stirring and comforting sight, and I always look forward to another visit from this stunning old friend.

Venus repeatedly passes us on an inside lane, swinging between dawn and dusk in its never-ending 19-month cycle. It approaches Earth in the evening, pulls out in front of us in the morning, then races around to come up behind us again.

Our sister planet laps us almost exactly five times every eight years. (A Venusian would say the passing repeats every 13 Venus years.) This 8:13 mean-motion resonance is not perfect — it slips by two days every eight years — and its origin is not understood. But for millennia humans have observed and recorded this repeating cycle.

In any given year, Venus slowly traces out a peculiar shape against the sky. The exact pattern will depend on your latitude. Where I live at a mid-northern latitude (see diagram), Venus in its most recent apparition first appeared in late April 2021 about 20° north of due West. It rose higher each night for a few weeks before turning south in early June. In October it will reach greatest elongation, lingering for awhile into the darkness at the extremity of its orbit as seen from Earth. December will see it reverse course, heading north and plunging downwards into the solar glare, to finally disappear in January. The whole thing makes a sort of lopsided, tilted infinity sign, with the larger lobe disappearing over the horizon.

This month, Venus again becomes the Morning Star, and it will stay that way until its next evening apparition starting in November. But the pattern then will be markedly different from what you see below, as a result of the ecliptic’s north-south seasonal tilt. The following three evening appearances will also be distinct, tracing new shapes over time.

Then something wonderful happens. The sixth appearance, beginning exactly eight years after the first, will repeat the first. And each successive return will imitate the one five cycles earlier. Thus, Venus outlines five distinct shapes in the sky, each of which repeats every eight years.

The Maya and Aztecs had different names and glyphs for each of the five appearances, which figured prominently in their origin stories and calendar cycles. Today, with our libraries full of planetary data and detailed imagery from spacecraft missions, we’re distracted from these splendid patterns. But bright Venus, undiminished even in light-polluted skies, remains there for any of us to observe.

Having trained myself to be aware of these patterns, I often use Venus cycles to mark eight-year anniversaries. When I see my brilliant companion, I know that, eight years hence, it will be right back in the same spot again.

This year I’m aware of something new and to me tremendously exciting. My colleagues and I have worked for more than 3 Venus cycles (since the late 1990s) to send new missions to Venus. And now I’m finally on a team selected to do so. We’ll build and fly a spacecraft called DAVINCI, which will be the first entry probe NASA has sent there since 1978 (S&T: Sept. 2021, p. 10). 

The launch is tentatively planned for summer 2029 — one Venus cycle from now. This evening I’ll look at the planet and know that, with luck and perseverance, the next time I see it at this same spot in the sky, our little machine will be on its way there.

This article originally appeared in print in the January 2022 issue of Sky & Telescope.


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