Jasper Dark Sky Festival

This Week’s Sky: September 17 – 24

A waxing gibbous Moon visits Saturn and Mars while Venus gleams at its brightest.

Full moon rising by Gerry Pocha

Gerry Pocha captured last September’s full Moon rising next to an old farm house near Glenside, Saskatchewan.

All Week

Facing west-southwest, 45-minutes after sunset.

Facing west-southwest, 45-minutes after sunset.

It’s still possible to see four planets at dusk, but the task is slowly becoming more difficult. The challenge is Venus—the long reigning evening “star.” Your success depends on having an unobstructed view to the west-southwest since the beacon-like planet sits only a half dozen degrees above the horizon at sunset. On the plus side (as noted below), Venus this week is as bright as it can get. The other three evening worlds are much easier to catch. Jupiter, shinning at magnitude –1.8, is still conspicuous in the southwest during twilight and sets around 9 p.m., local daylight time. Scanning eastward, the next planet in line is Saturn, glowing at magnitude 0.5. Best positioned is Mars, which reaches the meridian at roughly 9:30 p.m. The Martian disc shines at magnitude –1.6, has shrunk to a little more than 16 arc seconds and exhibits a distinctly gibbous phase—all sure signs that opposition was many weeks ago.

September 17

Moon and Saturn, 45-minutes after sunset.

The Moon and Saturn, 45-minutes after sunset.

This evening the waxing gibbous Moon is positioned less than four degrees east of Saturn. The earlier you check, the closer the two will appear. For observers on the West Coast, the Moon will have drifted one degree farther eastward by the time darkness falls.

September 19

Sky watchers on the West Coast will enjoy the best view of a conjunction between the Moon and Mars tonight. The lunar disc will sit less than four degrees north of the red planet at 10 p.m., PDT, when both objects are near the meridian. From central Canada, the duo will be slightly farther apart when due south, but they’ll still manage to squeeze into a binocular field.

September 21

Venus during daylight as seen in a telescope at high magnification.

Venus during daylight as seen in a telescope at high magnification.

Today Venus reaches its peak brightness—a startlingly bright magnitude –4.8. Unfortunately, the planet is very low in the west at sunset and difficult to sight. In telescopes Venus displays a tiny, snow-white crescent. This distinctive shape is best observed in the afternoon—around 3:30 p.m., local daylight time—when the planet is highest and its tremendous brightness isn’t overwhelming.

September 22

The Sun crosses the celestial equator at 9:54 p.m., EDT. This marks the equinox and the beginning of autumn in the northern hemisphere.

September 24

The Moon is full this evening at 10:52 p.m., EDT. Being the full Moon nearest the autumn equinox, it’s known as the Harvest Moon. (For more on this, turn to page 28 of the September/October issue of SkyNews.)

Weekend Stargazer

Comet 21/P Giacobini-Zinner chart

Use this chart to locate Comet 21/P Giacobini-Zinner this weekend. The comet’s position is plotted for 5:30 a.m., EDT, for the dates shown. (The comet’s brightness is exaggerated for clarity.) Click on the illustration for a full-size version.

The September 21 – 23 weekend represents your last, best chance to enjoy Comet 21/P Giacobini-Zinner. Two factors are combining to bring the window of opportunity to a close. First, after this weekend, moonlight will start lighting up the predawn sky, where the comet is found. Second, 21/P is beginning to fade as it recedes both from the Sun and the Earth. By the time the Moon is out of the way again in early October, it’s likely the comet will be a full magnitude fainter. In other words, catch it while you can!

This weekend finds Giacobini-Zinner motoring its way through southern Gemini and into the indistinct constellation, Monoceros. The highlight of this leg of the comet’s journey occurs on September 24 (Monday morning) when it drifts alongside the 4th-magnitude open cluster, NGC2264—the Christmas Tree Cluster. Deep-sky observers will also note that the same NGC number refers to the much fainter Cone Nebula. Together, the cluster, nebula and comet should provide a fine photo opportunity. The one caveat is that the Moon sets as twilight begins. For this reason, the previous morning, Sunday, September 23, might be a better choice if you’re looking to capture the scene with your camera.

At SkyNews we love to read about your experiences and see your photos. You can share them with us by e-mailing [email protected].

Categories: This Week's Sky
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