To return to the subject of how orbits appear when plotted geocentrically (that is, with the Earth stationary).
This is how Mars’s Earth-based movements appear over a span of two years: one loop toward us at Mars’s opposition, contrasting with a wide swing around the far side of the Sun.
Now Venus: what pattern do its geocentric motions make? The pattern is shaped by the famous Venus cycle.
(By the way, I had this post ready to go a week ago. Then Microsoft struck. I’ve only just got back my internet connection.)
Over 8 years, each phenomenon – each relative position of Earth, Venus, and Sun – occurs 5 times, and then over the next 8 years they repeat 5 times almost identically. For Venus, the tight inward loops are the inferior conjunctions, in which Venus passes between us and the Sun; the wide swings are centered on the superior conjunctions, when Venus passes around the far side of the Sun. So the general pattern is (as Anthony Barreiro commented) a “lovely five-petalled rose.” The tight loops are the stamens of the rose, the wide swings are the petals.
When I try to plot a geocentric picture in the same way as the one for Mars, but over eight years (2016-2023) to show the complete rose, it is bewilderingly cluttered: five overlapping tracks, eight-times-twelve little Venus globes at monthly intervals – already too much without the ecliptic-plane grid and other details. So here is the oicture in a more simplified version: still calculated in three dimensions, but, by moving the viewpoint to the north ecliptic pole, it becomes a flat plan of Venus’s path.
Earth is in the middle; the vernal-equinox direction is to the right; the yellow spots are the Sun at the beginning of each month. The rest is the rhythmic motions of Venus.
You’ll still have trouble deciphering which part of the track is for which year (I’ve used white, cyan, magenta, yellow for 2016, 2017, 2018, 2019, and again for 2020, 2021, 2022, 2023) but it doesn’t matter greatly. You can see the five inferior conjunctions, in their five directions. If you trace across the circle from each loop to the next, you see that they are not adjacent to each other but 2/5 pf the way around, like the five points of a pentagram.
(A pentagram, from the cover picture story of Astronomical Calendar 2015.)
The directions of the five inferior conjunctions determine their differing characters, by determining their places in Venus’s “true” (heliocentric) orbit, which is tilted and slightly elliptical.
2017 Mar 25: in Pisces; Venus passes 8 degrees north of the Sun; distance from us 0.28 AU (astronomical units, Sun-Earth distance); diameter of Venus’s (mostly dark) disk 60″ (arc seconds).
2018 Oct 26: in Virgo; 6° degree south of Sun; 0.27 AU; 62″.
2020 Jun 3: in Taurus; 0.5° north of Sun; 0.29 AU; 58″.
2022 Jan 9: in Sagittarius; 5° north of Sun; 0.27 AU; 63″.
2023 Aug 13: Cancer-Leo-Hydra border; 7° degrees south of Sun; 0.29 AU; 58″.
March 2017 is the next of those wonderful occasions, like March 2009, when we in the northern hemisphere have a chance to see Venus pass so far north of the Sun that it may be visible near to both sunset and sunrise of the days near, and even the day of, inferior conjunction. And years of the 2023 type are the similar opportunities for south-hemisphere dwellers.
Having brought Venus into conjunction with petals and the southern hemisphere, I may be forgiven for remembering the limerick that rhymes in éili@.
There was a young gal from Australia
Who went to a ball as a dahlia.
When the petals unfurled
It was known to the world
That the dress – as a dress – was a failure.