length shortens by one hour and twelve minutes this month.
The times for sunrise and sunset are calculated for a flat
horizon, so actual times may vary depending upon the surrounding
landscape. Twilight progresses in three stages. Civil twilight
occurs approximately one-half hour before official sunrise;
you can easily function without artificial light. Nautical
twilight, about one hour before sunrise, may require artificial
light to assist movement. Astronomical twilight occurs about
one and one-half hours before sunrise. A faint glow appears
above the horizon. The same progression applies to dusk.
The shortened days and waning moon of early September provide
excellent conditions for star gazing through the middle
of the month. On September 3 a third quarter moon rises
with the Pleiades in the eastern sky at 11:32pm. New
Moon occurs September 11. A waxing crescent returns to
the western night sky September 15 to the west of Libra.
The moon can be a guide for locating many constellations.
On September 17 find it near Scorpius; on the 19th it
begins to move through Sagittarius; on the 22nd it is
in the center of Capricornus; on the 24th in Aquarius;
on the 27th in the middle of Pisces; on the 28th in Aries;
and on the 30th in Taurus. Full Moon occurs September
An imaginary line called the ecliptic traces the path of
the sun across the sky relative to the background stars.
Earth’s orbit around the sun creates the seasons
because the equator is tilted from the plane of the ecliptic
at an angle of 23.5 degrees. An extension of Earth’s
equator into the celestial sphere creates the celestial
equator. The fall equinox occurs at the point in the
earth’s orbit where the plane of the ecliptic intersects
the celestial equator. Neither the northern hemisphere
nor the southern hemisphere tilts towards the sun. As
a result, the direct rays of the sun fall perpendicular
to the equator on this day. Sunrise will be due east
and sunset will be due west. The autumnal equinox occurs
September 23 at 3:51am MDT. The length of day and night
should be equal, but according to the sunrise/sunset
table, daylength is seven minutes longer on September
23. This occurs because the atmosphere refracts sunlight
around the curvature of the Earth. At sunrise we see
the sun before it reaches a horizontal plane with where
we are standing. At sunset we continue to see the sun
after it dips below the horizon. On September 26 the
sunrise/sunset table does show equal periods of day and
night. By this time, the days are actually shorter than
Astronomers are currently watching four galaxies collide
in the region of Ursa Major. Three are elliptical galaxies
of approximately the same size as our own Milky Way; the
fourth is ten times larger. The collision, as seen today,
occurred five billion years ago. In real time, the event
would be long over. Two other colliding galaxies are being
watched in the southern sky in the region of Capricornus.
These two galaxies are rich in gases which are spawning
new stars. In another five billion years the Milky Way
Galaxy is expected to collide with the Andromeda Galaxy.
The gravitational forces that interact in these collisions
sling many stars into intergalactic space. Some of these
stars will be pulled back into new positions within the
galaxy, while others will remain as isolated stars in intergalactic
Jupiter - brightest
object in the southwestern sky at a -2 magnitude; low
in the evening sky near Sagittarius; sets earlier each
Mars - reddish-orange object shining at
magnitude 0; rises one to two hours before midnight, in
Gemini, throughout October.
Saturn - rises soon after Venus in the
early morning sky until October 13 when Saturn begins to
rise first; Saturn, Venus and the bright star Regulus (in Leo) cluster together
the first half of the month; October 10 is their closest approach; Saturn shines
magnitude 0, fainter than Venus but brighter than Regulus at magnitude 1.
Venus - rises shortly before 4:00am, preceding
Saturn until October 13 when Saturn rises one
minute earlier; Venus and Saturn move away from Regulus after October 10 and
closer together from October 14-16 with a conjunction on October 15; reaches
position in the morning sky on October 28; shines at magnitude -4 in its half-moon
CONSTELLATIONS OF OCTOBER
Corona Borealis Cygnus
three brightest stars in the evening sky are Vega (in
Lyra), Altair (in Aquila), and Deneb (in Cygnus).
These 1st magnitude stars mark the Summer
Triangle. Cygnus, flying toward the
southwest, is also called the
Hold the star chart high
above your head
and match the compass directions to the
direction you are facing. Adjust the
star chart by orienting Ursa Major
(Big Dipper) to match its
position in the sky.
The star chart
approximates the night sky from astronomical twilight
to midnight. As the night and the month progress,
the constellations will shift toward the northwest.