Yes, it’s that time of year again. The autumnal equinox, the official start to the autumn season here in the northern hemisphere, occurs at 5:44 p.m. ADT on Sept. 22.
On this date, day and night are approximately equal in length. Thereafter, the nights grow longer, and the days shorter. To learn more about the autumnal equinox (and the vernal equinox), simply Google equinoxes.
Now that the evenings are growing cooler, you might need to don a jacket or warm sweater and, perhaps, a hat if you plan on stepping outside to have a look at the night sky. Looking WSW about 30 minutes after sunset, you cannot miss Venus shining brightly (mag. -4.0) about 10 degrees (a hand’s width at arm’s length) above the horizon. You will understand why, when it shines like this in the evening twilight, it is called the evening star. On the evening of Sept. 8, watch the waxing crescent moon slip beneath Venus.
Saturn, with its magnificent ring system is the next planet to come into view in the western twilight, appearing to the left of Venus. Saturn and Venus will move closer (visually) in the evening sky this month, until, on the evening of Sept. 19, Venus slides beneath Saturn. After that, the two planets pull further and further apart. The best view of Saturn and its ring system comes in the early part of the month, when it is much higher in the evening sky.
Mighty Jupiter walks onto the celestial stage around 2 a.m. as September opens, rising in the eastern pre-dawn sky about 30 minutes earlier each successive morning. With autumn’s steadier air masses, magnificent views of Jupiter’s clouds and four Galilean moons — Io, Callisto, Europa and Ganymede — should prove rewarding for the dedicated viewer.
Mars is conspicuous in the pre-dawn hours of September. Rising around 4 a.m., the “red planet” will be high in the sky by the time morning’s twilight brightens the eastern horizon. Shining at mag. 1.6, Mars, with its distinctive ruddy hue, should be easy to pick out. In the pre-dawn hours on Sept. 8 and 9, Mars passes through the famous Beehive (M44) star cluster in the constellation of Cancer — the Crab., a ruby sitting amid a scattering of diamonds.
For those with eight-inch or larger telescopes, Comet ISON should be visible throughout the early part of September low in the east about two hours before sunrise just north of Mars and the Beehive (M44) star cluster in Cancer. Light from the gibbous moon will interfere after mid-month.
I will have more to say about Comet ISON in my October column, as, by then, astronomers will have a better idea of whether this comet will, indeed, prove to be the “comet of the century” that everyone is hoping it will.
September’s full moon is often called the “harvest moon” since this is the month when most crops are harvested. The “harvest moon” is the full moon that occurs closest to the autumnal equinox. In two out of three years, it will occur in September, but it can, in the odd year, occur in October. Farmers take advantage of the moonlight to work late and bring in their crops.
Until next month, clear skies and good hunting.
Sept. 5 — New moon; 8:36 a.m.
Sept. 8 — Mars in Beehive (M44) star cluster
Sept. 12 — First quarter moon; 2:08 p.m.
Sept. 15 — Moon at perigee (closest to Earth); 365,258 kms.; 1:31 p.m.
Sept. 19 — Full moon; 8:13 a.m.
Sept. 22 — Autumnal equinox; 5:44 p.m.
Sept. 27 — Last quarter moon; 12:55 a.m.; Moon at apogee (farthest from Earth); 401,960 kms.;3:16 p.m.
Glenn K. Roberts is a member of the Charlottetown Centre of the Royal Astronomical Society of Canada (RASC). His column appears in The Guardian once a month. He welcomes comments from readers. Anyone who would like to comment on his column is encouraged to email him at firstname.lastname@example.org.