Orbits and elongations
Like the Moon, Venus appears to go through phases as it moves across the night sky
The motion of the inner planets in the sky is quite different from that of the outer planets. Instead of oppositions, Mercury and Venus pass through elongations. Those on the western side of the Sun are visible in the morning sky (western elongation) and those on the eastern side, in the evening sky (eastern elongation).
The graphic above shows the changes we see as Venus completes an orbit: starting at superior conjunction, when the Sun effectively lies between Venus and Earth, though the planet is too close to the Sun in the sky to observe at this point. This last occurred on 22 October 2022, and the planet is now on the Sun’s eastern side and visible in the evening sky.
A phenomenon unique to Mercury and Venus is that we can observe them passing through the same phases as the Moon, from full to new. Telescopically, Venus appears as a small gibbous disc after conjunction. The phase will decrease over time and the apparent diameter will increase as Venus moves closer to Earth. Venus reaches 50% illumination (dichotomy) when it is usually furthest from the Sun in the sky – a point called greatest eastern elongation, which happens on 4 June. The planet continues to move closer to Earth, becoming an increasingly slim crescent until it reaches inferior conjunction where it lies between the Sun and Earth. This occurs on 13 August this year. After this date, the planet transitions from an evening object and moves into the morning sky, starting a new western elongation and the whole cycle occurs in reverse.
Not all elongations are equally favourable, as the cycle happens at different times. In general, the planet is highest and best-placed when it’s in the evening sky in the winter and spring months, and in the summer when it’s in the morning sky. Every eight years, Venus returns to the same point in the sky, and this is called the synodic period.