Topic
Morning star or evening star: elongation, phase, magnitude and the exact window Venus leaves you above the horizon, evening by evening.
On 23 September Venus reaches magnitude -4.78, its brightest of the year, while only 22 % of it is lit. The paradox resolves into a multiplication: the crescent thins, but the disc grows faster, 42 arcseconds across at 59 million kilometres. The calculation, the observing window that is melting away, and what binoculars make of the crescent.
On Monday 14 September around midday, a 12 % crescent Moon will pass in front of Venus, magnitude -4.75, under a Sun standing 40 degrees high. From Paris the planet vanishes at 11:26 and re-emerges at 12:34; from London the Moon rises with Venus already hidden. The disappearance itself takes a minute and a half: Venus is a disc, not a point. City-by-city times, and the method for seeing a planet in daylight.
Venus peaks at magnitude -4.8, yet its lead over the Sun melts from 61 minutes to 15 in a single month. In between, the Moon will hide it in broad daylight on the 14th, the equinox falls on the 23rd, and the full Moon of the 26th will rise barely later each evening. The month's programme, computed for Western Europe.
Between 21:10 and 21:55, low in the west-south-west, a crescent lit to 11.9 % passes within seven degrees of Venus. It is also the best evening this month to see the dark limb of the Moon lit by the Earth. Here are the two quarter-hours that matter, and the arithmetic of what is doing the lighting.