Topic
A daytime occultation of Venus, the planet at peak brightness, the equinox, the Harvest Moon and Neptune's opposition: one month, computed night by night.
On 26 September Neptune reaches opposition, 4.32 billion kilometres away: its light takes four hours to reach us. At magnitude 7.8 it is beyond the naked eye and within reach of any binoculars, provided you aim at the right spot below the Circlet of Pisces and come back two nights running. The full method, and why the exact date of opposition does not matter at all.
The full Moon of 26 September rises at 19:19 in Paris, twenty-one minutes before sunset, then only 15 or 16 minutes later on each following evening. In March the same delay reaches 84 minutes. This small geometric miracle has a name, the Harvest Moon, and an explanation that comes down to the angle of the ecliptic against the horizon. The numbers, evening by evening.
The equinox falls on 23 September at 00:05 UT. That day lasts not 12 hours but 12 h 08; true equality waits until the 25th; and above all night is advancing at its top speed: 3.6 minutes gained per day at the latitude of Paris, 5.4 at Helsinki, 2.4 at Athens. Why everything happens now, and what the equinox offers observers.
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.
At 2.5 million light-years, M31 is the most distant thing the unaided eye catches without strain. It spans three degrees, six full Moons laid end to end, climbs to 82 degrees above Paris, and the moonless window runs from 5 to 17 September. The exact star-hop, and what your own sky will let through.
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.