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Events during September
· Mercury at inferior conjunction - 3rd.
· Mercury at greatest western elongation (18º) - 19th.
· Jupiter at opposition in Pisces - 21st.
· Uranus at opposition in Pisces - 22nd.
· The autumn equinox when the Sun crosses the equator into the southern hemisphere is on the 23rd.
· Venus at greatest brilliancy (-4.6 brilliancy) - 23rd.
· The Moon: New - 8th, Full - 23rd. Perigee (357,190 kilometres) - 8th, Apogee (406,170 kilometres) - 21st.
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Mercury
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Mercury passes through inferior conjunction
on 3rd September and moves rapidly out to the west of the Sun, reaching greatest western elongation (18º)
on the 19th. The planet is visible from northern and tropical latitudes as a morning object from mid-September until early October. For observors in northern
temperate latitudes this is the best morning apparition of 2010.
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.
Distance from Sun: 47 million kilometres at perihelion to 70 million kilometres at aphelion
Orbit: 88 days Temperature: -183ºC to 467ºC Diameter: 4,880 kilometres
Size: 40% of Earth
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Venus
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Venus brightens from magnitude -4.4 to -4.6 during September, attaining greatest brilliancy
on the 23rd. Unfortunately, the planet is now too low for observors in northern temperate latitudes. For those in the tropics and the southern hemisphere it is a
magnificent sight, dominating the western sky after sunset. These observors can witness Venus occulted by the three-day old waxing crescent Moon on 11th September
and making a pretty pairing in the twilight as dusk falls.
Venus moves into Virgo at the beginning of the month. On the 7th, southern hemisphere observors will see a close
grouping of Venus, Mars and Saturn with Mercury below. Venus passes 2.7º south of Saturn on 8th August and the crescent Moon joins the party on the 12th and 13th August.
Venus reaches greatest eastern elongation of 46º on 20th August and on the same day passes 2.0º south of Mars.
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.
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Mars
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At +1.5 magnitude, Mars continues to be visible low in the western evening sky after sunset but
only for observors in equatorial and southern hemisphere latitudes. Mars can be easily located during the month as it lies not far north of the brilliant Venus. Mars
moves from Virgo to 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 oppositions during autumn and winter months
when it is higher in the sky.
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Jupiter
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Jupiter, in Pisces at magnitude -2.9,
is at its brightest in September since it is at opposition on the 21st and therefore
observable throughout the night from everywhere on Earth.
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.
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Saturn
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Saturn is now too close to the Sun for observation as it moves towards
conjunction at the beginning of October.
The diagram shows Saturn´s rings closing with the south pole presented towards Earth and the far side of the rings no longer appearing clear of the planet´s body.
In 2009 the Earth passed through the ring plane making the rings invisible for a short while. It was the first time since 1997 that the planet´s magnitude has faded to
this level.
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 will remain 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 2025.
Saturn's largest moon, Titan, is visible in small telescopes orbiting outside of the ring system.
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Uranus
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Uranus is at opposition on 21st September in the constellation of Pisces.
Near opposition, the planet can be located about 0.8ºN of Jupiter.
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. If its location is known, Uranus is easily seen through binoculars. At closest approach,
Uranus is 2,856 million kilometres (1,775 million miles) from Earth.
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Neptune
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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.
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Pluto
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No longer an offical planet and never brighter than +13 magnitude, Pluto is only visible through powerful
telescopes.
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Moon
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September
Last quarter: 1st
New moon: 8th
First quarter: 15th
Full moon: 23rd
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