|Events during September
10th - MERCURY is in superior conjunction.
22nd - AUTUMN EQUINOX when the Sun crosses the equator into the southern
29th - URANUS is in opposition in Pisces.
* Full - 30th, New - 16th, Apogee - 404,295 kilometres - 7th, Perigee - 365,750 kilometres - 19th.
* 7th/8th and 8th/9th September - the last quarter Moon is near to Jupiter
(See September snapshot).
* 12th and 13th September - the crescent Moon lies near Venus in the early morning.
* 19th September - the Moon is close to Mars low in the south-west evening sky.
The brilliant Jupiter
, at magnitude -2.3, rises at 23:0h at the beginning of September and by 21:30h by the end of the month. The giant
planet is in the constellation of Taurus
, to the left of Aldebaran and the Hyades.
Even Jupiter is put in the shade when a dazzling Venus rises over four hours before the Sun. The morning star blazes at magnitude -4.0, moving from
Gemini through Cancer into Leo.
Mercury passes through superior conjunction
on 10th September
and remains unobservable until the end of the month when it becomes visible in the western sky after sunset.
The best times to observe Mercury in the northen hemisphere are when it is an evening star in the spring and a morning star in the autumn. In midsummer
the lighter skies make visibility difficult near the horizon.
Although fading slightly from -4.2 to -4.1 magnitude during September, Venus remains a spectacular
object in the morning sky rising four hours before the morning Sun from northern temperate latitudes and considerably less further south. The rapid eastwards motion,
carries Venus from Gemini through Cancer into
Leo during September.
The planetīs phase increases from 59% to 70% during the month and the waning crescent Moon
appears closeby in the morning sky on September 12th.
On the 8th June 2004 and 5th/6th June 2012, Venus was at inferior conjunction
and transited the sun. Transits of Venus are rare, taking place at greater
than 100 year intervals and usually in pairs. Prior to that, the last two transits of Venus were in 1874 and 1882. Transits of Venus won't occur again until 2117
Before and after inferior conjuction, when Venus is
the closest it comes to the Earth, are the times at which the planet is most brilliant and can be seen setting or rising
4 hours after or before the Sun.
Mars, at magnitude +1.2, continues to be visible in the early evening in the south-western sky. From
northern temperate latitudes the planet is low in the twilight at dusk, setting less than two hours after the Sun. Its eastwards motion carries Mars from
Virgo into Libra during September.
At opposition on the 28th August 2003, Mars was only 56 million kilometres from the Earth. It showed a
disc of 25.1 seconds of arc across which is almost as large as it can ever appear. Mars started 2003 at 310 million kilometres from
the earth at 4.5 seconds of arc and 1.6 magnitude. By opposition it brightened 50 times to reach -2.9 magnitude but faded to 0
magnitude by December. Even to the naked eye Mars was a striking object in the summer and autumn sky, easily identifiable by its
reddish hue in an area of sky poor in bright stars. Mars will not be as close again until 2018.
These favourable oppositions occur every 15 or 17 years but other oppositions occur at average intervals of 2 years 2 months during
which time the planet makes a complete circle of the Earth. In general Mars is observable every other year, being too close to the sun for favourable conditions
during other times. Brightness at opposition varies from -1.0 to -2.9 magnitude, and when furthest from the earth it fades to 1.7 magnitude. The planet can be
identified by its orange-red colour.
As in 2003, Mars comes nearest to the Earth at oppositions at the end of August. At these times it can be brighter than Jupiter,
although low in the sky in Aquarius for northern observors. In the northern hemisphere, the planet may be better seen at oppostions during autumn and winter months
when it is higher in the sky.
Jupiter remains a brilliant object in Taurus, brightening
from -2.3 to -2.5 during September, and rising mid-evening by the end of the month. The last quarter Moon appears close to Jupiter early on 8th September.
After spending the past six years in the southern skies, Jupiter moved north of the celestial equator on February 5th 2011 to spend the next six years
in northern skies.
Varying from 603 (at its closest) to 770 million kilometres from the sun, the difference in brightness between opposition and conjunction varies less than
with Mars, from about -2.9 to -1.8 magnitude. Always a bright planet, Jupiter comes to opposition a month later each year, moving approximately from one zodiacal
constellation to the next.
The 4 largest of Jupiterīs 60+ moons are easily visible through binoculars or a small telescope, ranging from 4.6 to 5.6 in magnitude. The innermost,
Io, takes 1.8 days to orbit the planet making its motion easily detectable within a few minutes.
Saturn remains at magnitude +0.8 in Virgo. From
northern temperate latitudes, the planet may be glimpsed low down in the west-south-western sky as darkness falls early in September but will be lost in twilight
before the end of the month.
The planet crossed the equator into the northern hemisphere in 1996 where it remained until 2010 with the southern side of the ring system
facing Earth. Because of its distance, its brightness varies little between opposition and conjunction but is affected by the huge ring system. Seen edge on the
rings contribute little or no light.
Every 15 years the plane of Saturn's rings passes through the Sun, illuminating first the north and then the south side. For a few days the
rings are edge on to the Sun. About the same time the Earth passes through the ring plane and, depending on the Earth's position, this may happen just once or 3 times.
During 1995/96 there was a triple crossing and the next will be 2038/39. The last single crossing was in 2009 and the next will be in 2025.
Saturn's largest moon, Titan, is visible in small telescopes orbiting outside of the ring system.
Uranus is in opposition in
Pisces on 29th September.
Brightness varies only slightly, reaching a maximum of +5.6 magnitude at opposition. Although this is
bright enough to see with the naked eye, identifying it against the stars can be difficult. At closest approach, Uranus is 2,856 million kilometres (1,775 million miles)
Neptune has an average magnitude of 7.9 which varies little with changing distance. At its closest,
Neptune is 4,341 million kilometres (2,697 million miles) from Earth.
No longer an offical planet and never brighter than +13 magnitude, Pluto is only visible through powerful telescopes.
Last quarter: 8th
New moon: 16th
First quarter: 22nd
Full moon: 30th