length shortens by one hour and five 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 26.
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 the nights.
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 space.
Jupiter - the only planet visible in
sky; located in the southwest near Antares
in Scorpius, easily identified by its brightness.
Mars - visible between midnight and sunrise
in the eastern sky;
near Taurus in early September and in Gemini by the end of
Saturn - rises in the eastern sky each morning
following Venus by one-half to one hour; located in Leo near
the bright star Regulus; in opposition (opposite side of
the earth from the sun) to Earth on September 9, which marks
its closest point to Earth.
Uranus - best appearance in early September;
barely visible as a blue-green speck in Aquarius.
Venus - rises to the east in the early morning
hours before Saturn; reaches maximum brightness as this year’s
morning star on September 12; binoculars reveal its crescent
CONSTELLATIONS OF AUGUST