- Planétarium Rio Tinto Alcan
From August 27 to September 10, 2018
In September, four planets are visible at twilight between the southwest and 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 very bright at the moment and is visible at dawn until around September 10. In late August and early September, look for the tiny planet low in the east-northeast, 45 to 60 minutes before sunrise. But Mercury’s elongation from the Sun is decreasing rapidly, and during the second week of September, the tiny planet is only visible against a much brighter sky, 20 to 30 minutes before sunrise. On the morning of September 8, the thin crescent moon hangs 6 degrees above Mercury.
Venus is the bright Evening Star that pierces through the colours of twilight in the west-southwest, as soon as the sky begins to darken minutes after sunset. Presently, Venus is nearly at greatest elongation from the Sun, but the inclination of its orbit keeps the planet very close to the horizon during evening twilight: as a consequence, Venus now sets less than 75 minutes after our daytime star. 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, but it remains closer to us and appears larger and brighter than during any other opposition since 2003; at the moment, it even outshines bright Jupiter! Mars appears low in the southeast at dusk, culminates around 10 p.m. barely 18 degrees high in the south, and sets in the southwest around 2 a.m. Mars ends its retrograde loop on August 28, and returns to its direct motion, now moving eastward (toward the left) with respect to the background stars. During the night of September 19 to 20, the waxing gibbous moon will shine 4 degrees above the Red Planet.
Jupiter appears above the southwest horizon during evening twilight and spends the rest of the evening slowly descending toward the west-southwest horizon where it vanishes before 10 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 midnight. On the evening of September 17, the waxing gibbous moon shines 4 degrees to the left of the ringed planet.