Fall officially arrives this month with the Autumnal Equinox on September 23rd at 3:50 am EDT. Hours of daylight and darkness are just about equal then, as the Sun appears to cross the Celestial Equator of the sky. From now until Winter Solstice in December day length will continue to shorten and the night sky will appear sooner for Sky-watchers.
The Sun appears to move across our sky annually, along a path we call the Ecliptic. Actually this apparent annual motion of the Sun is caused by Earth revolving around the Sun in its orbit. The ecliptic also marks the path of all the Zodiac constellations across the sky. The ecliptic has a low angle to the western horizon from September 27 to October 11th this year —— but stand nearly straight up from the eastern horizon before sunrise. This gives us an opportunity to observe the “false dawn;” also known as the Zodiacal Light.
The zodiacal light, a faint, cone-shaped glow arises from sunlight reflecting off fine dust particles in the plane of the solar system, which is why it lines up with the ecliptic. To see the light sky-watchers need to observe from a dark site shortly before twilight begins. The zodiacal light will have a broad base near the eastern horizon which tapers as it rises up toward the zenith. Books often call the zodiacal light “ethereal;” which is why we need a dark site from which to look for it.
Jupiter and Saturn continue to be the main evening planets in September; though both have shifted well to the southwestern sky. Indeed Jupiter will be very low to the horizon by 1:00 am; but there will be several good hours to see it before then. Saturn will remain in view well past midnight because it is some 30 degrees to the left of Jupiter. On September 5th the First-Quarter Moon will be seen just to the lower right of Jupiter, and the Waxing-Gibbous Moon will be just below Saturn on the 8th.
Venus and Mercury will make very fleeting appearances near the end of September, very low in the western sky, amid rather bright twilight. Venus will be easier to spot because it is brighter, but we will need to wait until October and November before Venus will gain enough altitude to be seen more easily. Mercury’s orbit will turn it back toward the Sun into early October and out of sight. Mars is too close to the Sun to see now, though it will return to view in the east before dawn in November. September’s Full Moon comes on the 14th.