- Planétarium Rio Tinto Alcan
From September 10 to 24, 2018
In September, four planets are visible at twilight between the west-southwest and south-southeast. Appearing from right to left are Venus, Jupiter, Saturn and Mars. The Moon is also part of the scene from the 12th to 19th. Catch all of them in a single viewing during early twilight, about 20 to 40 minutes after sunset. But don’t wait: Venus is already very low on the horizon and the first to set.
Mercury is now too close to the Sun and is not visible. The tiny planet passes behind the Sun (superior conjunction) on September 20; it will emerge in October for what will be a very poor evening apparition for Northern Hemisphere observers.
Venus is the bright Evening Star that pierces through the colours of twilight, very low in the west-southwest as soon as the sky begins to darken after sunset. The angle between Venus and the Sun is now decreasing, and the inclination of its orbit keeps the planet very close to the horizon during evening twilight: as a consequence, Venus now sets less than an hour after our daytime star. Venus will completely vanish in the Sun’s glare by the end of the month. At dusk on September 12, the lunar crescent hangs 9 degrees above Venus: with binoculars, admire the earthshine that dimly lights the otherwise dark section of the lunar disc. The scene becomes truly magnificent when the sky darkens, 30 to 45 minutes after sunset.
Mars was at opposition on July 27. The Red Planet is now receding from Earth and slowly fading, but it remains a conspicuously bright object that appears low in the southeast at dusk, culminates around 9:30 p.m. barely 20 degrees high in the south, and sets in the southwest around 1:30 a.m. During the night of September 19 to 20, the waxing gibbous moon will shine 4 degrees above the Red Planet.
Jupiter appears during evening twilight about 15 degrees above the southwest horizon and spends the rest of the evening slowly descending toward the west-southwest horizon where it vanishes before 9 p.m. On the evening of September 13, the waxing crescent Moon will hang about 4 degrees from Jupiter.
Saturn appears during evening twilight about 22 degrees high in the south, and then gradually descends toward the southwest horizon where it vanishes around 11 p.m. On the evening of September 17, the waxing gibbous moon shines 4 degrees to the left of the ringed planet.