THE FIRST DAY OF AUTUMN
Autumn officially begins at 7:04 p.m. on Sept. 22. At this time, the sun’s apparent annual path, the ecliptic, intersects the celestial equator (the projection of Earth’s equator onto the night sky).
This point is known as the Autumnal Equinox. The word “equinox” literally means “Equal Night,” for on an equinox the lengths of daylight and darkness are almost equal. There would be an equivalence of day and night if our planet were a perfect, uniformly dense sphere. Earth is an oblate spheroid with an uneven distribution of land on its surface. Consequently, Earth is slightly pear-shaped and light does not fall on our planet evenly.
You won’t be able to see the Moon at week’s beginning, for it will be new on Sept. 17. During the week, our satellite will pass through the waxing crescent phase, an epoch that immediately precedes first quarter (Sept. 24).
Observe the crescent moon in the western evening sky by mid-week.
LIGHT ON THE ZODIACAL LIGHT
Around the time of either equinox, we in the northern latitudes have the opportunity to observe a sight which tropical observers see all year: the zodiacal light.
This is a light swath visible just after sunset or before sunrise.
The Zodiacal light resembles a broad pyramid-shaped light swath. The pyramid’s vertex is aligned along the ecliptic band. The base appears to emerge from the horizon.
This light is caused by the reflection of sunlight along the dust-enshrouded ecliptic band. Possible only when the ecliptic forms a sharp angle with the celestial equator, the Zodiacal light is a rare sight for us.
Do not mistake this glow for the auroral shimmer, an infrequent phenomenon directed toward the north magnetic pole.
VENUS AND THE LITTLE KING STAR
If you’re awake in the early morning, keep a watch on the eastern pre-dawn sky, where you will find the brilliant planet Venus close to Regulus, the “Little King” star that marks the southern point in Leo the Lion’s famous sickle. “Leo’s Sickle” resembles a backward question mark. It is visible in the post midnight eastern sky.
VENUS will be quite a bit brighter than its stellar companion. These two objects will appear to be within three degrees of each other between Sept. 18 – 22.
They reach their minimum angular separation distance of 0.5 degrees on September 20.
By observing these two celestial objects over the course of a few nights, you’ll be able to track the motion of our brilliant sister world against the backdrop of stars.
THE PEGASUS SQUARE
If you’re a student of mythology, or if you have ever been to a Mobil gas station, you will recognize the famous mythological icon of Pegasus – the snow-white horse with wings. Said to have been born out of the blood of the slain Gorgon Medusa, Pegasus is a famous autumn sky pattern that you can find tonight in the east.
Pegasus’s main body is represented by a large four-star square called “The Great Square of Pegasus.”
Pegasus’ neck and head are marked by stars that protrude from the southwestern square star.
By mid-evening, you can see both the Summer Triangle in the western sky and the Great Square high in the east. Autumn is a perfect time to view the sky if you like shapes.
MERCURY AT GREATEST ELONGATION
On Sept. 18, Mercury will reach its greatest eastern elongation from the sun (27 degrees). Although it will be almost as far from the sun as possible, Mercury is still quite difficult to observe. Try your luck with Mercury, which will be quite low in the morning twilight sky. Mercury is quite close to the sun and therefore will always be close to our parent star in the sky. Its close proximity to the sun makes Mercury a difficult sight to observe.
Mercury is the only naked-eye planet which many observers – even seasoned ones – have never seen.