Jérôme Musialak édite SkyChart. Les horaires, hauteurs, magnitudes et classes de ciel publiés dans le Journal sont calculés par le moteur d'éphémérides de l'application, jamais repris d'une source secondaire.
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.
Maximum is at 05:12 from London, with the Moon 8.0° above the western horizon. Everything depends on a clear horizon and an alarm set for 03:15. Here is what to do tonight, while it is still light.
Tonight, shadows on the Moon are seven times as long as the relief that casts them. On 27 August they will be a tenth of it. A factor of seventy in seven nights, and it comes down to a single angle: the Sun's height above the lunar ground. Here is the arithmetic, evening by evening.
Three stars everyone points at in summer, and whose dimensions nobody ever gives. We measured them in the catalogue the app runs on: 23.85 degrees, 34.20 and 38.01. The Triangle is nowhere near equilateral, it covers one hundredth of the whole sky, and it holds two entire constellations.
The Earth's shadow will cover 96.6 % of the disc. But from Western Europe the Moon sets before the eclipse ends, seven degrees above the horizon, in a sky already turning grey. From the Caribbean and North America the whole thing happens high overhead in full darkness. Here are the times, the altitudes, and where you actually want to be.
Tonight it rises at 22:44, and it will gain twenty minutes every week. More to the point, its rings are open again: edge-on and all but gone in March 2025, they now stand at 8.9 degrees. Here is the season's timetable up to the 4 October opposition, and what instrument it takes.
Mid-August is the best stretch of the year for the core of our galaxy. It is also the moment to say what nobody says: from Paris it culminates at 12.2 degrees, and it culminates in full twilight. Here is the real window, latitude by latitude, and what the thickness of air takes from you on the way.
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.
Tomorrow morning the two planets will fit inside a circle one and a half times the width of the full Moon. They will also be four degrees above the horizon in an already brightening dawn. Here is the honest arithmetic: the hour, the altitude, the azimuth, and why half of you will see nothing.
The maximum fell last night. The stream itself stays active until 24 August. We computed, night by night, the radiant's altitude, the window of genuinely dark sky and what the Moon leaves of it. Six useful nights remain, and one date on which the door shuts.
There is nothing to buy and nothing to set up: the International Space Station is visible to the naked eye, and often brighter than the brightest star. You just have to know when. The answer depends on three things at once, and it changes every day.
Between central Paris and the Millevaches plateau there is not «a few more stars». There is a factor of thirty. Here is what the satellite atlas built into SkyChart computes for twenty real locations, and what it cannot tell you.
On the same evening, western Europe will watch the Sun set as a crescent, then the best meteor shower of the year fall through a moonless sky. That is no coincidence: it is the same Moon doing both.