Venus, from one side of the Sun to another

Bad Astronomy
The entire universe in blog form
April 21 2012 7:00 AM

Venus, from one side of the Sun to another

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.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


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!


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