Double-faced

An inner planet (Venus, Mercury) is double-faced if it can be seen with the naked eye in both bright dawn and bright dusk . This only happens every few years when the planet comes into its lower conjunction (i.e., passes between the sun and earth), but far north of the sun ; for an observation from the southern hemisphere of the earth a very southern position is accordingly necessary. Then it goes down for a few days both just after her and just before her.

This constellation is related to the significant orbital inclination of the two planets relative to the ecliptic (plane of the earth's orbit). The Venus has an orbital inclination of 3.4 degrees, Mercury of 7 °. But because both planets come relatively close to earth in their lower conjunction, their apparent northerly angular distance can increase to almost 8 °. As a result, despite approaching the sun, they rise almost ½ an hour before it and set after it.

Venus last had such a constellation at the end of March 2017 . This double vision was only feasible when the sky was very clear - and therefore not near the city.