The month begins with the moon at last quarter, meaning 21 days have passed since new moon. It will be full Sept. 23, which, coincidentally, is the day after the Autumn Equinox.
Luna's close encounters for the month begin Sept. 1, when it is less than a degree from the Pleiades (M45). Sept. 3, the moon is less than half a degree from the cluster M35. Mars and Saturn are lurking closeby - watch for them at sunset Sept. 8 and 10. By Sept. 11, Venus and the moon are within half a degree; an occultation in eastern Brazil and the eastern hemisphere - the moon passes in front of the planet. Jupiter shares the spotlight Sept. 23, and September closes with the moon again in close proximity of M45 and M35, Sept. 28 and 30, respectively.
Mercury is in front of the sun as the month opens, and becomes a morning object by the third week of September.
Venus continues as the bright Evening Star in the west at dusk. Even though the planet appears narrow, reaching a phase like the first-quarter moon, it is at its greatest illuminated extent (GIE) Sept. 23. This means that, as viewed from Earth, the illuminated area of Venus covers the most square degrees. As mentioned above, watch for the close brush by the moon Sept. 11.
Mars is low in the western evening sky, presenting a viewing challenge, as the planet slips below the horizon shortly after nightfall. Binoculars are a must, as Mars and Earth are separated by nearly twice the sun's distance; contrary to the e-mail hoax going the rounds on the Internet during the summer, Mars will never appear the same size as the full moon!
Jupiter graces the eastern evening sky, rising at mid-evening and remaining visible all night. The distance scales of the Solar System rise quickly as we view the outer planets - Jupiter is four times the distance of the sun, nearly 600 million km (33 light-minutes). Jupiter reaches opposition (on the opposite side of the sky from the sun) Sept. 21, and its largest apparent diameter of just under an arcminute (49.8 arcseconds). Uranus and Jupiter share the same approximate space, less than one degree apart Sept. 22.
Saturn is slipping closer and closer to the sun's position. By Sept. 8, it crosses into the celestial southern hemisphere, where it will remain for the next 15 years. Saturn is 10.5 astronomical units (AU), about 1.4 billion km, away.
Uranus, as mentioned above, shares the same piece of Solar System real estate as Jupiter this summer and fall. The distance to Uranus is 2.8 billion km.
Neptune rises in the east around 7 p.m., and crosses the sky through the night. Its distance from Earth is 4.3 billion km.
During the early part of Sept., watch for the zodiacal light in the early morning twilight, caused by dust particles in a Solar System orbit, backlit by the sun.
- James Edgar has had an interest in the night sky all his life. He joined the Royal Astronomical Society of Canada in 2000. He is editor's assistant and a contributor to Observer's Handbook, production manager of the bi-monthly RASC Journal and is the society's national secretary.
