The September equinox heralds the start of astronomical autumn which last for 89 days, 20 hours and 45 minutes plus Venus shines with second greatest brilliancy of the year.
The Sun passes from Leo (The Lion) on 16 September at 5:55 pm and
enters Virgo (The Maiden). Daylight shortens from 13:50 (13.836
hours) on 1 September to 11:37 (11.619 hours) on 30 September so we
gain 2 hours and 13 minutes of night time, by the end of the
month. Over the whole of September, the Sun approaches Earth by by
1,192,382 km (0.008 AU). The highlight of the month, though, is the
Autumnal Equinox which will occur on 23 September at 1:05 am. The Sun
will be at 33.9 degrees at Solar noon (1:05 pm) and, thereafter, begin
its southerly descent in the sky. Now, in principle, the equinox
means that night and day are equal. However, for Edinburgh and
Lothian, the equinox day will be 9 minutes and 22 seconds longer than
the equinox night. Why? Because of how we measure sunrise and sunset
along with atmospheric refraction. So, in reality, night and day are
(more or less equal to within 9 seconds) on 25 September.
The Moon begins the month in Pisces (The Fishes) and ends in Taurus
(The Bull) and approaches by 8,208 km (0.000 AU). The last quarter
starts the month on 4 September at 8:51 am in Taurus (The Bull).
Lunar perigee (closest to Earth) on 6 September at 9:36 pm finds the
Moon some 368,228 km away from Earth—around 16,172 km closer than
average—subtending an angle of 32.4 arc-minutes. The new Moon
appears on 11 September at 4:27 am in Leo (The Lion) beginning a new
synodic (Lunar) month which will last 29 days, 12 hours and 23
minutes. The first quarter appears on 18 September at 9:44 pm in
Ophiuchus (The Serpent Bearer). Lunar apogee (furthest from Earth)
occurs on 19 September at 4:01 am and takes the Moon to 404,261 km
away from Earth—around 19,861 km further than average—subtending
an angle of 29.6 arc-minutes. This is the minimum Lunar apogee all
year. We round the month out with the full Harvest Moon on 26
September at 5:49 pm in Pisces (The Fishes).
The waning crescent Moon meets with Pollux in Gemini (The Twins) on 7
September at 6:30 am. The waxing crescent Moon conjuncts with Spica
in Virgo (The Maiden) on 13 September at 7:30 pm (whilst setting).
Lastly, on 30 September, the waning gibbous Moon will be \ang{1.1}
south of the Seven Sisters (The Pleiades, M45) at 9:30 pm.
For the inferior planets: Mercury begins the month in Leo (The Lion)
and ends in Virgo (The Maiden) and approaches by 30,060,510 km (0.201
AU). The swift planet decreases in magnitude from -1.43 to -0.08
(3.46 times in brightness) over September. Earth and Mercury are
furthest apart on 2 September lying some 206 million km distant.
Mercury also reaches aphelion late in the month on the 27th. Venus
remains in Virgo (The Maiden) and approaches by 30,925,044 km (0.207
AU). The evening star (planet) increases in magnitude from -4.62 to
-4.78 (1.16 times in brightness) over the month. Venus reaches
aphelion on 4 September lying some 0.728 AU from the Sun. Venus will
be occulted by the waxing crescent Moon around midday on 14 September
but both are low on the south-eastern horizon. Venus achieves second
greatest brilliancy on 19 September in the wee, small, hours.
For the superior planets: Mars begins the month in Gemini (The Twins)
and ends in Cancer (The Crab) and approaches by 27,616,070 km (0.185
AU). The red planet increases in magnitude from 1.24 to 1.11 (1.13
times in brightness) during September. Mars rises after midnight all
month and will be visible until sunrise. Indeed, on 25 September
around 2 am, Mars, Pollux and Castor will appear to form a vertical
line in the eastern sky. Jupiter begins the month in Cancer (The
Crab) and ends in Leo (The Lion) and approaches by 41,090,868 km
(0.275 AU). The gas giant planet increases in magnitude from -1.80
to -1.87 (1.06 times in brightness) across the month. Jupiter will
be visible a few hours before sunrise all month long.
Saturn begins the month in Pisces (The Fishes) and ends in Cetus (The
Sea Monster) and approaches by 24,619,467 km (0.165 AU). The ringed
planet increases in magnitude from 0.48 to 0.35 (1.13 times in
brightness) over the month. Uranus remains in Taurus (The Bull) and
approaches by 71,995,562 km (0.481 AU). The green giant planet
increases in magnitude from 5.71 to 5.65 (1.05 times in brightness).
Uranus’ orbit becomes retrograde on 10 September. Neptune remains in
Pisces (The Fishes) and approaches by 12,120,972 km (0.081 AU). The
blue giant planet increases in magnitude from 7.69 to 7.68 (1.01
times in brightness). Earth and Neptune will be closest together all
year on 25 September and Neptune will reach opposition the following
day. For those of you with a telescope or binoculars, opposition is
the best time to view the superior planets.
The September sky graces us with 3 weak meteor showers. In all cases
the radiant point is circumpolar meaning the shooting stars should be
visible all night for Edinburgh and Lothian. The Aurigids (206 AUR)
peak on 1 September with a ZHR of 6. The waning gibbous Moon is
illuminated to 73% so observing conditions are, alas, unfavourable.
The radiant point is near the bright star Capella. The parent body is
comet Kiess a non-periodic comet with an orbital period of 20
centuries! The epsilon-Perseids (208 SPE) peak overnight on 9/10
September with, perhaps, 8 shooting stars per hour. The radiant point
is near Algol (the ‘Demon Star’) in constellation of Perseus (The
Hero). However, the thin waning crescent Moon is only illuminated to
4% so conditions are very favourable. Finally, the Lyncids (081 SLY)
peak on 13 September with a lowly ZHR of 3. On this night the waxing
crescent Moon is illuminated to 6% but the radiant point is in Lynx
(The Lynx) but not near an appropriately bright star.
At the time of our sky map, some constellations visible are Cygnus
(The Swan) near zenith, Lynx (The Lynx) in the north, Andromeda (The
Chained Maiden) in the east, Corona Borealis (The Northern Crown) in
the west (which contains our elusive recurrent nova known as the
Blaze star) and Aquila (The Eagle) in the south. The ecliptic hosts
Taurus (The Bull), Aries (The Ram), Pisces (The Fishes), Aquarius
(The Water Bearer), Capricorn (The Sea Goat) and Sagittarius (The
Archer).
The ‘Summer Triangle’—Vega in Lyra (The Lyre), Altair in Aquila
(The Eagle) and Deneb in Cygnus (The Swan)—is prominent in the
south. Also, highly visible in the east is the great square of
Pegasus (The Winged Horse) with the sentinel stars Scheat, Alpheratz,
Algenib and Markab. Circumpolar constellations—always above the
horizon—include Camelopardalis (The Giraffe), Cassiopeia (The
Seated Queen), Draco (The Dragon) and Ursa Major (The Great Bear).
