Venus is the brightest planet due to its proximity to the Sun and reflective cloud covered globe. Its orbit is around 70 per cent the size of Earth’s and from our perspective it never strays too far from the Sun in the sky.
It takes Venus 224 days to orbit the Sun and its day is –243 Earth days long, minus because its retrograde or backwards.
From Earth we see Venus pass the Sun twice per orbit: once on the inward part closest to Earth – known as inferior conjunction – and once on the distant part – known as superior conjunction.
On rare occasions inferior conjunction results in an event known as a transit of Venus. We won’t get another until December 2117.
After inferior conjunction, Venus pulls west from the Sun, re-emerging into the morning sky.
The re-emergence is rapid and through a scope it shows a thickening crescent phase.
Its apparent farthest distance from the Sun in the morning sky is called greatest western elongation.
After this it slowly edges back towards superior conjunction.
It takes a while to reach superior conjunction because Venus is on the far side of its orbit.
Last superior conjunction occurred on 14 August and now its edging back into the evening sky.
On 13 September mag. –3.8 Venus sits 0.3˚ from mag. –0.9 Mercury. With care it may be possible to catch the pair above a flat western horizon shortly after sunset.
The sky will still be bright, but Venus’s intense light will shine through. If using binoculars, make sure the Sun has set before looking.
On 13 September, the conjunction is virtually due west after sunset. If you have facility to do so, viewing the pair during daylight is a better proposition.
However, great care should be taken when doing this and it should only be attempted if you know what you are doing and have appropriate solar safety filters.
The two planets remain close for much of the month but typically remain above the horizon for only 30 minutes after sunset.
On 29 September mag. –3.8 Venus is separated from mag. –0.2 Mercury by 6.2˚.
A slender 1%-lit waxing crescent Moon appears 3.2˚ above Venus on this date, Mercury, Venus and the Moon forming a right-angled triangle low over the western horizon after sunset with Venus as the right-angle.
Locate Venus first, then look for Mercury. The Moon being higher will still be visible for a short time after the planets have set.
This article originally appeared in the September 2019 issue of BBC Sky at Night Magazine. Pete Lawrence is an experienced astronomer and a presenter on The Sky at Night.