What’s In September’s Skies

By Matt Woods

Planets:

In the upcoming month, there are some celestial movements worth noting. Mercury will gradually disappear from our view, getting lost in the Sun’s brilliance during the first few days of September. However, it will make a reappearance in the eastern morning sky around midSeptember, still situated within the constellation of Leo (The Lion). Keep an eye out for Mercury’s greatest elongation in the West on Monday, September 22nd, before it retreats towards the Sun.

As for Venus, it will commence September positioned between the constellations of Cancer (The Crab) and Hydra (The Serpent). By the month’s end, it will have journeyed to the front paws of the constellation Leo. Mars, on the other hand, will be making its way through the constellation of Virgo (The Virgin) throughout the entire month of September. To catch a glimpse of Mars, direct your gaze low in the western sky during the early evening hours.

Shifting our attention to the giant planets, Jupiter can be found nestled between the head of the constellation Cetus (The Sea Monster) and the feet of the constellation Aries (The Ram) in the night sky. Saturn, another magnificent sight, will grace the middle of the constellation of Aquarius (The Water-Bearer).

For those interested in spotting Uranus, it will rise late at night within the constellations of Taurus, precisely situated between Jupiter and the Pleiades Cluster. Lastly, Neptune can be observed during the early night hours positioned amidst the top fish in the constellation Pisces (The Fish) and the tail of Cetus.

Astronomical Events This Month:

Zodiacal Light Season Begins:

Get ready for the start of the Zodiacal Light season, commencing on the 1st of September. This amazing phenomenon occurs as the Sun approaches the September Equinox, which falls on September 23rd. At this time, sunlight scatters off dust particles dispersed along the ecliptic plane—the apparent path of the Sun’s motion on the celestial sphere as observed from Earth.

The best opportunities to witness this enchanting pearly glow are during the spring and autumn seasons, particularly at dawn or dusk. This is because the steep angle of the ecliptic in relation to our horizon allows for optimal visibility. In the Southern Hemisphere, dusk is favoured during the September Equinox season, while in the Northern Hemisphere, it’s dawn. The opposite holds true near the March Equinox.

To catch a glimpse of the zodiacal light, venture out approximately an hour after sunset or an hour before dawn. Choose a location with minimal light pollution, aiming for a dark site. Any interference from nearby cities or faint glows on the horizon can diminish the ethereal radiance of the zodiacal light. As you observe, you’ll notice a slender pyramid-shaped glow tracing the length of the ecliptic plane, creating a truly captivating display.

The September Equinox:

Equinoxes and Solstices.

Image Copyright: timeanddate.com

On Saturday the 23rd of September at 02:49 pm (AWST), the Southward Equinox - an astronomical event that signifies the onset of fall in the Northern Hemisphere and the arrival of spring in the Southern Hemisphere will occur. At this moment, the Sun’s declination, as observed from Earth, reaches 0°. The equinoctial points, where the celestial equator intersects with the imaginary path of the Sun known as the ecliptic, defines this significant occurrence. Throughout the 21st century, the September Equinox will fall on either September 22nd or 23rd, until the year 2092 when it will occur on September 21st every fourth year.

The term “Equinox” originates from Latin and translates to “equal nights.” During this event, day and night are nearly equal in duration across the globe, and the Sun rises precisely due east and sets due west as seen by an observer. The Full Moon nearest to the September Equinox holds the title of the “Harvest Moon”, a period when farmers utilise the extended twilight to gather their crops. In 2023, the Harvest Moon will grace the sky on the 29th of September.

It’s worth noting the term “Equilux”, which distinguishes the true Equinox from the point when the duration of daylight matches the duration of night-time. Several factors contribute to this disparity, including the time it takes for the physical diameter of the Sun to clear the horizon, atmospheric refraction, and the observer’s precise location within their respective time zone. The Equilux occurs within a few days of either Equinox, accounting for these variations.

Things To Look At This Month:

Helix Nebula:

The Helix Nebula, also known as NGC 7293, located in the constellation of Aquarius, is a mesmerising planetary nebula. Among the closest planetary nebulas to Earth at a distance of approximately 655 light-years, it shares a striking resemblance in appearance to nebulas such as the Cat’s Eye Nebula and the Ring Nebula. These nebulas possess similar size, age, and physical characteristics, akin to the Dumbbell Nebula. In popular culture, it has been likened to the “Eye of Sauron” or the “Eye of God”.

Unlike the star-forming regions found in nebulae like The Orion Nebula (M42), a planetary nebula marks the late stage in the life cycle of a star. It occurs when a star expels a shell of material in a nova-style explosion. The intense ultraviolet radiation from the progenitor White Dwarf star causes the previously expelled shell of gas to fluoresce brilliantly, creating an emission nebula. From our perspective, the gases expelled from the star give the impression of a helix structure, as if we are peering down its winding path. Spanning a modest 2.5 light-years, the expansion of the entire planetary nebula is estimated to have commenced around 6,560 years ago.