Nov 26, 2010


Total lunar eclipse
WHEN: 21 December, 05:00 to 08:30

During totality the Moon is completely in the Earth's umbral shadow

A TOTAL ECLlPSE of the Moon takes place in the early hours of 21 December. Such an event can only happen when the Sun, Earth and Moon are perfectly aligned, Viewing the eclipse will be tricky since the Moon will be setting as the eclipse really gets underway.
The Earth's shadow has two parts due to the fact that the Sun isn't a point source of light as seen from Earth. The main 'dark' or umbral shadow is in the form of a cone stretching out behind the Earth in the opposite direction to the Sun, as shown in the diagram. Surrounding this is another region of less intense shadow known as the penumbra. Look back at the Sun from within the penumbral shadow and you'd see only part of the Sun's disc covered. In the umbral shadow, the entire Sun appears covered.
At the distance of the Moon, the cross-section of these two shadows appears as two concentric circles: all inner dark umbral circle about three times the apparent diameter of the Moon, surrounded by a fainter penumbral circle about five times the apparent diameter of the Moon. The Moon enters this bull's-eye cross-section on the morning of 21 December.
The penumbral eclipse starts at 05:27 GMT when the Moon is about 23° up in the west as seen from the centre of the UK. However, because the penumbral shadow isn't very dark, it doesn't much change the visual appearance of the Moon. Having said that, the left-hand edge of the Moon will begin to take on a subtly darker shade just before the Moon enters the darker umbral shadow at 06:32.
The main umbral part of the eclipse begins with the Moon 14° above the horizon, and this marks the start of the partial phase of the eclipse. Totality occurs at 07:40, when the Moon's entire disc will be immersed in the umbral shadow. Its altitude at this time will be just 5° and the sky will be starting to get bright.
The Moon never goes completely dark because some light from the Sun is refracted, or bent, as it passes through the Earth's atmosphere. The blue component of this light gets scattered, leaving mainly red to fill in the umbral shadow. The result will be a reddened Moon, close to the northwestern horizon. Mid-eclipse occurs at 08:12 and from the centre of the UK, the Moon sets soon after.

Geminid meteors
WHEN: Peak on 13/14 December 

THE GEMINIDS is  probably the finest meteor shower of the year with a dependable Zenithal Hourly Rate (ZHR) of around 100 meteors per hour. They all appear to emanate from a point in the sky known as the radiant, which in the case of the Geminids lies close to the star Castor, as shown in the chart. The shower is active from 6-19 December, with peak rates occurring in the early hours of 14 December, close to when the radiant is at its highest point in the sky.
The best way to observe the shower is to lie on a sunbed and look up at a large area of sky. Being December, it's important to make sure you wrap up warm before venturing outside for any length of time. Choose the darkest location you can find away from any lighting, lie back and wait. The only thing you need to enjoy the Geminids is your eyes and plenty of patience! Although you could see a meteor anywhere, looking in the direction of Orion for a couple of hours after midnight and then switching to look at Leo until dawn would be our recommendation.
The first quarter Moon will look gorgeous on the night of 13 December, sitting just 6° north of mag. -2.3 Jupiter. However, its light will drown out some of the early meteor. Thankfully, the Moon sets at around 00:30 on 14 December, and from this time until dawn is the best period to view the Geminids. The shower produces medium to slow meteors with a good proportion of bright trails. This means that the Geminids make excellent photographic subjects.

Asteroid 7 Iris and M67
WHEN: Visible all December; view around midnight 

THE ASTEROID 7 IRIS  will be located in the constellation of Cancer the Crab throughout December and will pass quite close to the mag. +6.9 open star cluster M67. A collection of stars loosely held together by their mutual gravity, this object is one of the more ancient open clusters in our Galaxy, and is thought to be around 4 billion years old.
lris brightens from mag. +8.9 at the start of December to +8.3 by the end and should be easy to pick out with binoculars or a small telescope using a low-power eyepiece. This is a fairly sparse region of the sky so it shouldn't be too difficult to spot Iris as it moves from day to day among the background stars. The best way to confirm you've seen it is to observe the suspected region several days apart, drawing or photographing the main 'stars' you can see. If one of those stars moves between your observations then that should be Iris.
The asteroid's track swings quite close to M67, taking it within a degree of the cluster's centre between 13- 25 December. At its closest approach on 2L December, it sits about 45 arcminutes from the cluster's centre, unfortunately coinciding with the full Moon!

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