Venus, from one side of the Sun to another

By Phil Plait | April 21, 2012 7:00 am

The dance of the planets fascinates me. All the planets orbit the Sun, keeping their own time depending on how far they are from our star. From our vantage point on Earth, circling the Sun once per year, the planets move across our sky slowly, stately, taking weeks or months to get from one side to the other.

Venus is closer to the Sun, and takes only 225 days to orbit it. From Earth, that makes its motion pretty complicated. Sometimes we see it at night, setting after the Sun, and then half an orbit later it passes the Sun in the sky and becomes a morning object. As I write this, Venus is high in the west after sunset, an intense beacon in the twilight sky.

In 2010, starting in March and continuing on until September of that year, Turkish astrophotographer Tunç Tezel pointed a camera to the west and took a photo of Venus every few days. He captured its motion across the sky in this amazing composite photograph:

I had to shrink it a bit to get it to fit, which made Venus look a little dimmer than on the original. Click to encythereanate and see it much better.

In this series, Venus came out from behind the Sun (on the far side of its orbit relative to Earth) near the center of the picture. It moved up and to the right over the next few months, then in late May it turned the corner and started to head to the lower left at that shallow angle. Finally, in the fall, as it came between us and the Sun, it took that last dip on the left (the diagram on this page may help).

There’s a funny thing about Venus’s orbit. Eight Earth years = 2922 days (6 regular 365 day years plus 2 leap years of 366 days). Interestingly, 13 Venus years = 13 x 224.65 (to be more exact) = 2921 days. In other words, the orbital configuration of Venus and Earth cycles every 8 of our years!

Think of it like two cars going around a racetrack, with the outer one going around 8 times every time the inner one goes around 13 times. If you were standing by the side of the track, you’d see the inner car (Venus) circle the track 13 times, while the outer car (Earth) goes around 8 times. But from Earth, during that same time, we’d see the inner car pass us only 5 times, because we’re moving around too — I know, this is hard to picture. The point it, there are five paths Venus takes across our sky, and that pattern of five repeats.

The path Tunç captured in the photo above for 2010 was the same path Venus took in 2002, and is the same it will take in 2018. It’s strange, but it’s how the numbers work out.

Is it a coincidence? That’s hard to say. It seems like an unlikely one, but it’s also hard to see how it could’ve come to be in the first place. Sometimes gravity forces situations like this, as when the Moon spins once on its axis for every one orbit around the Earth. But Venus is so far away from Earth, it’s hard to see how the two planets could interact that way.

One thing we do know is that this repeating pattern was known by the Babylonians, something like 3500 years ago. They were keen observers, and could trace the motion of the planets in the sky. Venus is incredibly bright and obvious, so them tracking it over multiple cycles is not surprising, and noticing the pattern isn’t surprising either. They weren’t dumb, they just didn’t have telescopes and computers! And in fact ancient people were tied more to the sky, since they could use the cycles of the Sun, Moon, planets and stars to predict seasons and even things like when rivers flooded, so they knew when to plant their seeds.

Most of us have lost that connection to the sky, and that’s a shame. Look at what beauty, both artistic and mathematical, comes from watching it!

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
MORE ABOUT: orbits, Tunç Tezel, Venus

Comments (25)

Links to this Post

  1. Science News « Diverkat | April 25, 2012
  1. Pete Jackson

    The eight-year cycle is also manifested in the eight-year period between pairs of Venus’ transits across the face of the Sun as seen from Earth. This year we will see one on June 6 (V day and D day both!), and the prior one was eight years ago on June 8, 2004. Make sure that you see it (through projection or filters of course) because you’ll have to wait until 2117 and 2125 for the next pair! It’s a glorious sight which reminded me of the opening sequence of the film “2001 A Space Odyssey”.

    Another curious fact about Venus is that the solar day on Venus is 116.75 Earth solar days long (Sun rises in the west and sets in the East, if you could ever see the Sun through the clouds). This is very close to being exactly 1/5 of the Venus’ synodic period (the time it takes Venus to overlap the Earth once in their 225 and 365-day respective solar orbits) of 583.92 days. The consequence is that the same side of Venus faces Earth at each inferior conjunction when Venus passes between us and the Sun. There has been speculation and papers written on the possibility that this forms a spin-orbit tidal lock (with some oscillation due to other gravitational and dynamic effects) but the conclusions are that it is basically a coincidence; especially since spacecraft orbiting Venus have not found whopping gravitational anomalies in Venus structure that would be necessary for the Earth’s weak tidal influence to come into play.

  2. There is a lot going on in this beautiful picture. Over the course of those seven months (or so, exact start/end dates aren’t given) the Sun sets at different spots along the western horizon and the angle of the ecliptic with respect to the horizon changes. Understanding these changes and mentallyoverlaying a visualization of them on this photo is very interesting. Wish I had the time to do a real overlay to help people visualize them!

  3. If you’re still finding the two-cars-on-a-racetrack image hard to picture check out my video about the transit of Venus which uses a computer-generated handheld orrery (!!!) to make the cosmic clockwork a bit more comprehensible!

    Admittedly it did take slightly less long to make than that amazing photo…

  4. timbebinder

    If there are any cats in this picture, I don’t see them. Let’s try this again.

  5. Giordano Bruno

    Nice capture of the retrograde motion.

  6. Glauco

    41 shots, right? That’s a hell of a work, but totally paid off by the final image.

  7. Chris

    I have to say that that it sounds like a coincidence and being human nature you are looking for integer multiples when there is no higher physics at work here. By your own math the two cycles are not exact anyway, but differ by a day.

    Really we can find patterns and coincidence anywhere we look for them. For instance if I take the year on Venus and divide by year on Mercury and Multiply by the 8/13 ratio of earth to Venus years I get pi/2 to 0.05% accuracy. Same as the accuracy of the Earth to Venus years

    (224.65/87.969)*8/13 = 1.57153263

    pi/2 = 1.57079633

    Weird? Yes Coincidence? Yes

  8. Howard

    How is it claimed that this photo was produced? The easiest and “most honest” way would be to use multiple exposures, but I would have expected at least one stray aircraft in the picture then, and probably some evidence from the foreground of multiple exposures. I don’t see any fuzziness in the trees due to different growths of leaves over the course of time, nor any blurring in the sign that hangs at an odd angle (as though it might have slipped and may yet slip more) from the building just left of center.

  9. Laurel

    Cool to see it all laid-out like that. And it’s neat how apparent this movement is to serious skywatchers — the Mayans knew the 8-year cycle as well!

  10. I think encythereanate is my new favourite word.

  11. There is a bit longer explanation about the picture in “Encythereanate” link given in the post. But I have to mention that, after aligning the pictures, I cut the foregrounds (as it was changing and there was also a construction rising) and registered remaining Venus images on to a single base picture, which I chose to be the one on 22nd April.

    In one earlier effort dating back to 2006-7, not only did I apply the same compositing method, but also I make a movie of all the frames with the changing foreground:

  12. Kevin

    It immediately occured to me while reading this post that 8 and 13 are Fibonnacci numbers. Is this a coincidence? Maybe. As everyone here probably knows, the ratios of consecutive Fibonnacci numbers aproach a constant known as the Golden Ratio, or phi. Phi is found throughout much of nature, from the spirals of nautilus shells to the arrangement of seeds, and leaves in plants. There is very good reason for the ubiquity of phi in nature. A quick calculation shows that the orbital periods of Earth and Mars do not have a phi relationship, although Mercury and Venus have an approximate phi-squared relationship. Still, I wonder if the Golden Ratio in the motions of moons and planets occurs more frequently than mere chance would allow?

  13. Troy

    Regarding 7. Howard’s question “how was it produced” it says the photos were digitally combined. Pretty obvious that Venus’ location in the field was preserved for each evenings shot but the terrestrial features were from only one picture.

  14. Spocko

    Maybe if we could actually *see* the sky we might appreciate it more. :(

  15. Eleven replies and not one of you mentioned Fibonacci numbers. How did THAT happen? 13 to 8 (not to mention the 5 inner track passages). People have noticed this many times before. Apparently such relations use Fibonacci, Lucas and perhaps other related number series, all related to the Golden Ratio.

  16. eyesoars

    I was under the impression that the solar planets were all more or less running in synchrony with Jupiter; that drifting off harmony with Jupiter would tend to cause unequal gravitational effects from it to push the orbital period back to simple harmony.

    If that was the case, then both Earth and Venus would necessarily have common multiples.

    Not so?

  17. Cris (without an H)

    “And in fact ancient people were tied more to the sky” — and as Spocko hinted, ancient people had a whole lot less light pollution to contend with.

  18. frankenstein monster

    Is this atmospheric refraction ? Wonder why the arc on the sky isn’t a straight ellipse. The lower part is probably due atmospheric refraction, but the part far above horizon does look only slightly less deformed. Any idea what it could be ?

  19. @ ^frankenstein monster : Perhaps an effect of latitude? Maybe?

    Or an artefact of outr perspective and the ellipticity (right word?) of our planets orbit – relatve to Venus, natch, given the Cytherean orbit is about the most circular in our whole solar system I think.


    Very similar to an analemma only Venus not our daytime star. Neat work – cheers! :-)


    @ 9. Invader Xan : “I think encythereanate is my new favourite word.”

    Yup, I love that one too. The adjective/adverb ‘Cytherean’ for pertaining to Venus is one of my faves. :-)

    @4. Giordano Bruno : “Nice capture of the retrograde motion.”

    Ahem, sorry I could be mistaken I guess, but I’m pretty sure that retrograde motion only applies to the planet’s outside Earth’s orbit never Venus or Mercury. Its caused by Earth’s orbital motion exceeding the outer planets causing them to “backtrack” in the sky after all isn’t it?

    I like your username though! :-)

  20. Chris

    @16 Messier Tidy Upper

    At least according to Wikipedia “When a planet travels eastward in relation to the stars, it is called prograde. When the planet travels westward in relation to the stars (opposite path) it is called retrograde.”

    So it appears Venus and Mercury can have retrograde motion since Venus is definitely going the “wrong” way.

  21. Inner planets retrograde as well, but the motion is mostly invisible as they pass inferior conjunction and lost in the glare of the Sun. SOHO Lasco images catch inner planet retrogrades nicely. During transits, Mercury and Venus advance from celestial east to west, hence retrograde. Next opportunity: Transit of Venus on 5-6 June 2012.

  22. Eden Keeper

    @19. Messier Tidy Upper & 5. Giordano Bruno: The link given (in the 2nd paragraph below the photo) by BA indicates Cytherean retrograde motion, which is hard to see in the glare, while annotation with the photo source gives an end date prior to the retrograde.

    @19. & 18. frankenstein monster: A major portion of the motion in a Cytherean anelemma is seasonal. If Sol was the fixed reference frame, including both position and solar axis, for such a photo rather than the horizon then the relative effects of orbital radii, eccentricity, inclination, etc. would dominate.

  23. TeaPot562

    In the urban part of Southern California, we have too much atmospheric pollution to see Venus when it is just a few degrees away from the sun at Sunset. Currently, it is high and brilliant in the Western sky, tending Northwest as the evening wears on.
    Loved the photographs and the explanation.

  24. Messier Tidy Upper

    @22. Eden Keeper : Hmm .. ok, thanks. :-)


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