During September Mercury is at her morning brightest mid-month, Mars is conjunct on
the 15th, Jupiter is conjunct on the 22nd, the autumn equinox when night and day are of approximately the same length
is also on the 22nd and there is a full moon on the 28th.
We say that a planet is in conjunction with another planet or object when the two are in line as viewed from Earth. They
are of no real astronomical importance, except that a planet in conjunction with the sun will not be visible in
the night sky. Planets in conjunction with each other make an attractive spectacle.
As inner planets, Mercury and Venus have inferior and superior solar conjunctions.
Inferior conjunction is when a planet lies between the earth and the sun and superior conjunction is when a planet lies
behind the sun. The outer planets may only have superior conjunctions with the sun.
Messenger Mission to Mercury
On the 3rd August NASA launched a mission from Cape Canaveral in Florida to Mercury which will arrive in 2011 to be the
first spacecraft to orbit the planet. The Messenger probe will fly past Earth in August 2005, twice past Venus in October
2006 and June 2007and three times past Mercury in January 2008, October 2008 and September 2009 for gravity assistance
then 15 times around the sun to slow down enough to orbit the sun's nearest planet in March 2011 - a total journey of
4.9 billion miles.
Messenger is the seventh in NASA's Discovery program of low cost, scientifically focused missions of
Solar System exploration. There has only been one previous probe from Earth to visit Mercury, Mariner 10, which flew by
the planet in 1973 and 1974 mapping only 45% of the surface. Messenger has been designed to map 100% and carries
instruments to determine various scientific unknowns.
||Mercury gives her best morning displays of the year from mid-September, brightening
from -0.6 to -1.0 magnitude, visible in the east. The planet is at greatest western elongation (18 degrees) on the 9th September when it rises
2 hours before the sun. The moon is closeby on the 13th.
The best times to observe Mercury 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.
||2004 is an exceptional year for observing Venus and during September the
planet is still a brilliant -4.2 magnitude rising 4 hours before the sun. Saturn is to the north on the 1st and the moon
closeby on the 10th. Venus will remain as a brilliant morning object in the northeast sky throughout the remainder of 2004.
On the 8th June, 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 is in conjunction with the sun on the 15th September and will not be
visible this month. After conjunction the planet will move away from the sun to be visible again against a dark morning sky
in late October.
At opposition on the 28th August last year, 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 for another 15 years.
These favourable oppositions occur every 15 years but other oppositions occur at average intervals of 2 years 2 months. 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.
||Jupiter is also at conjunction this month on the 22nd and will not be observable.
Being 770 million kilometres from the sun, the difference in brightness between opposition and conjunction varies less than with Mars, from about -2.8 to -1.8 magnitude.
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 is in Gemini throughout 2004. At 0.2 magnitude the planet will rise
before 2.00h by the end of September with the moon closeby on the 9th.
Saturn 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 next single crossings will be in 2009 and 2025.
Saturn's largest moon, Titan, is visible in small telescopes orbiting outside of the ring system.
||Uranus is in Aquarius throughout 2004. By the end of September the planet
will set by 5.00h. The moon is closeby on the 25th.
Brightness varies slightly reaching 5.6 magnitude at opposition. This is bright enough to see with the naked eye but identifying it against the stars is difficult.
||Neptune is in Capricorn throughout 2004. At 8.0 magnitude Neptune sets by 3.00h
by the end of September with the moon closeby on the 24th.
Neptune has an average magnitude of 7.9 which varies little with changing distance.
||Never brighter than 13 magnitude, Pluto is only visible through powerful telescopes and we will therefore not be reporting on its position in the sky.|
||Last quarter: 6th at 17.00h
New moon: 14th at 16.00h
First quarter: 21st at 18.00h
Full moon: 28th at 15.00h