|Events during September
|3rd - Mercury at greatest western elongation (18║).
23rd - Autum Equinox when the Sun crosses the equator into the southern hemisphere.
26th - Uranus at oppostion in Pisces.
28th - Mercury in superior conjunction.
MOON: Full - 12th, New - 27th, Apogee - 406,065 kilometres - 21st, Perigee - 357,560 kilometres - 28th.
Mercury reaches greatest eastern elongation (18║) on 3rd September. The planet is
consequently visible from northern and tropical latitudes as a morning object from late August until mid-September. For observors in the northern hemisphere,
this is the most favourable morning apparition of 2011.
Mercury is at its brightest in mid-September when it attains magnitude -1.2. After the middle of the month, Mercury draws back towards the Sun and
passes through superior conjunction on 28th September.
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.
Venus, magnitude -3.9, was in superior conjunction
in August. By the end of September, Venus emerges low in the western sky after sunset but may only be gimpsed by observors in equatorial and southerly latitudes.
Observors in northern latitudes will have to wait until December for a true Venusian (evening) spectacle.
On the 8th June 2004, 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. The last two transits of Venus were in 1874 and 1882. June's transit
began at 7.20h and lasted 6 hours until 13.20h, the total event visible from Europe as a small black disc crossing
the lower part of the Sun from left to right. The next transit will be in late June 2012. After that, transits of Venus
won't occur again until 2117 and 2125.
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. The dates of the next two inferior conjunctions are October 28th 2010 and October 26th 2018.
Mars, at magnitude +1.4, continues to be visible as an early morning object, rising soon
after midnight from northern temperate latitudes but later for those living further south. The planet┤s rapid apparent motion takes it through
during the month.
There is no opposition of Mars in 2011.
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 brightens from magnitude -2.6 to -2.8 during September as it approaches
opposition next month. Moving retrograde
through Aries, Jupiter rises soon after sunset and is visible throughout the hours of darkness.
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 moons of Jupiter are easily visible through 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, at magnitude +0.9, is in Virgo
and no longer visible to observors in northern temperate latitudes. However, in early September it may be seen low in the western sky after sunset from the
tropics and southern hemisphere.
Saturn moves more slowly than Jupiter and can remain in the same constellation for several years. The brightness of the planet depends on the
aspect of its rings, as well as its distance from Earth and the Sun.
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 the 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 at opposition in
Pisces on September 26th. With a magnitude of +5.7, it is barely visible to the naked eye but is easily located
with binoculars. At opposition Uranus is 2,854 million kilometres (1,773 million miles) from Earth.
Brightness varies slightly, reaching a maximum of +5.6 magnitude at opposition. This is bright
enough to see with the naked eye but identifying it against the stars is difficult. At closest approach, Uranus is 2,856 million kilometres (1,775 million miles)
Neptune is a morning object in Aquarius
visible in binoculars or a small telescope.
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.
First quarter: 4th
Full moon: 12th
Last quarter: 20th
New moon: 27th