|Events during December
· The winter solstice, when the Sun reaches its most southerly point over the Tropic of Capricorn, is on the 21st
· The Geminids meteor shower is at its height from 12th to 14th December. A predicted average hourly rate of 60 per hour with unfavourable visibility due to moonlight.
· The Moon: Full - 12th, New – 27th.
· The Full Moon this month is the closest of 2008 and it will appear bigger and brighter than usual. It will be 14% larger and 30% brighter compared
to the Moonīs other extreme when it is furthest away and at its smallest.
The Moon’s orbit is an ellipse that once every month at perigee comes closest to Earth and then two weeks later at apogee is at its furthest from Earth.
The distance to the Moon at perigee and apogee also varies slightly from month to month and some perigees are closer than others. The Moon doesn’t
necessarily reach perigee when it is Full but this month the lunar cycles synchronise to give us a Full Moon just 5 hours before it reaches perigee,
and that perigee happens to be the closest perigee of the year. On this day the Moon is 356,566 kilometres away, a couple of thousand kilometres closer
than in November at perigee, and 50,000 kilometres closer than it will be two weeks later at apogee on the other side of its orbit.
In addition any December Full Moon in the northern hemisphere, is the highest Full Moon of the year,
shining through less atmosphere and above any haze that can dim it.
|Mercury becomes visible to observors in tropical and southern latitudes during the second half of December. Observors in
northern latitudes will not see Mercury until early in the new year.
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.
|A brilliant -4.2 magnitude moving northwards in declination, Venus is a spectacular object in the early evenings, completely
dominating the western sky for several hours after sunset. Venus is magnificent to watch as it sets.
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.
|Passing slowly through superior conjuction on the 5th December,
Mars continues to be unsuitably placed for observation this month.
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.
|At -2.0 magnitude, Jupiter is approaching the end of its 2008 apparition but is still a bright object in the western evening sky
during December. Remaining in Sagittarius this month, Jupiter will move eastwards into Capricorn early in the new year.
Venus is moving steadily away from Jupiter, ending the year at aout 20š distance. Mercury is moving quickly towards Jupiter and will pass 1š south on New Yearīs Day.
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. 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, remaining at magnitude +0.9 in Leo, continues to be visible as a morning object in the eastern sky before sunrise.
On the last day of 2008 Saturn reaches its first stationary point then follows a retrograde motion.
By the end of the year Saturn will be seen low above the eastern horizon before midnight.
Saturnīs rings continue to close with the south pole presented towards Earth and the far side of the rings no longer appearing clear of the planetīs body. Next year the
Earth will pass through the ring plane making the rings invisible for a short while.
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 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.
|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 has an average magnitude of 7.9 which varies little with changing distance.
|No longer an offical planet and never brighter than +13 magnitude, Pluto is only visible through powerful
First quarter: 5th
Full moon: 12th
Last quarter: 19th
New moon: 27th