Venus putting on 8-yearly show

Venus floats above Kapukataumahaka, Mt Cargill in the western sky. PHOTO: IAN GRIFFIN
Venus floats above Kapukataumahaka, Mt Cargill in the western sky. PHOTO: IAN GRIFFIN
There's a rather beautiful harmony between the orbits of Venus and Earth, writes Ian Griffin.

Ian Griffin. Photo: Otago Museum
Ian Griffin. Photo: Otago Museum
As I was walking my springer spaniel, Connie, in Portobello one evening last week, I became distracted by Venus. As Connie sniffed impatiently, my gaze was drawn to the western sky, where the planet is putting on a fine display after sunset.

On this unusually calm night, I was struck by Venus’ brightness and became fascinated by the complexities of its ever-changing reflections. This week’s accompanying picture was captured handheld with my phone camera while Connie tugged away at her lead. Although it doesn’t do justice to the beauty of the moment, the picture does demonstrate that modern phones have become quite good astronomy cameras!

As Connie and I carried on our perambulation I continued to contemplate the planet in the western sky. Perhaps because of the regular pattern of my paces, I began to think about Venus and its motions in the sky relative to Earth.

Did you know there is a rather beautiful harmony between the orbits of Venus and Earth? The period our home planet takes to go around the sun eight times is almost exactly the same as that taken by Venus to orbit our closest star 13 times.

If this fact isn’t exciting enough, then let me tell you something which will blow your mind. Astronomers call the time taken for a planet to return to the same place in the sky relative to the sun as seen from Earth its synodic period.

For Venus, the synodic period is 583.92 days. Guess what? It turns out that five synodic periods for Venus equates to (drumroll) almost exactly eight earth years. This means that every eight years Venus puts on a very similar display as seen from Earth. The apparition we are witnessing in winter 2021 is therefore the same as that seen in 2013.

Venus is moving from Leo into the neighbouring constellation of Virgo over the next week or so. On Wednesday, the waxing crescent moon will be visible some five degrees below the planet. This planetary pairing will make an excellent target for astrophotographers.


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