Thursday, 4 September 2014

Venus at perihelion.

The planet Venus will reach perihelion (the closest point on its orbit to the Sun) at 3.3p pm GMT on Friday 5 September 2014, when it will be 0.718 440 AU (107 477 000 km) from the Sun. This is not actually a great deal closer than Venus's aphelion (furthest point on its orbit from the Sun), as Venus has the most circular orbit of any planet in our Solar System, with its distance from the Sun varying by only 1 462 000 km over the course of a Venusian Year; to give a sense of scale to this, the Moon orbits the Earth at an average distance of 384 399 km, so the distance between Venus's closest and furthest distances from the Sun differes by only 3.8 times the average distance at which the Moon orbits the Earth.

The relative positions of Earth, Venus and Mercury on 5 September 2014. From the point of view of an Earth based observer the Venusian perihelion will make no difference to the planets visibility. JPL Small Body Database Browser.

Because Venus is closer to the Sun than the Earth, its year is somewhat shorter, only 224.7 Earth days compared to 365.25 for the Earth, and Venus's last perihelion fell on 23 January 2014. Despite a Venusian Year having passed since this date, the planet has not yet completed a full day, as a Venusian day is actually longer than its year, at 243 Earth days, with Venus rotating clockwise on its axis while orbiting the Sun anti-clockwise. Exactly why this occurs is uncertain, as all other planets both orbit and rotate in an anti-clockwise direction; it is uncertain whether the planet has reached this situation by slow evolution of its orbit over billions of years from an unstable condition within the original protoplanetary disk from which all the planets formed, or whether its modern orbit results from an encounter with some other body at some point in the past. Either way Venus is unique in that (were it possible to see the sky from the planet's surface) the Sun would appear to pass across the sky from west to east, and that the time it took to complete a circuit from the perspective of a viewer on the surface would be significantly from the length of the planet's 'day'. This is because the shorter Venusian year affects the movement of the Sun across the sky more than the longer day, so that it is possible to talk about a 'Solar Day' on Venus (the length of time it takes a feature on the planet's surface to pass from directly under the Sun round to back directly under the Sun), only 116.75 days.

See also...


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