On New Year’s day, Comet Tuttle will be closest to the Earth, a mere 25 million miles away, and also at its brightest. The comet will just be visible to the unaided eye, so you will need to be observing from a very dark site.
A gallery of images, and sky maps of when and where to look, can be found at SpaceWeather.com.
[Image of Comet Tuttle taken by Pete Lawrence]

Happy solstice to all our readers!
The winter solstice this year occurs at 6am, on 22 December, 2007.
That is the time when the Earth’s North pole is pointing directly away from the Sun (which is why it is so much colder in the Northern hemisphere).
For people living in the Southern hemisphere, the South pole is pointing towards the Sun, making it summertime ‘down-under’!

On the night of the 13 December, and the morning of the 14 December, the Geminid shooting star shower reaches its peak.
The Earth will be ploughing through a stream of debris left behind by asteroid Phaethon, and we see these fragments burn up as they hit the Earth’s atmosphere, causing the shooting stars.
And they are often big fragments! I myself saw a huge fireball in the UK during the Geminid shower of 1994.
More details can be found at the NASA science website.
Details of all the major annual meteor showers visible from the UK are available on the NMM website.

Comet Holmes now appears almost twice the diameter of the full Moon in the night sky.
To see the latest images, see the gallery at Spaceweather.com.
Because the comet is so large in the sky, it is spread out, making it appear much fainter in the night sky. But it is still visible to the unaided eye when well away from light pollution.
The best way to observe the comet now is with a pair of binoculars that are large (to collect a lot of light) but with low magnification (because the comet is so large in the sky).
The apparent size and brightness of Comet Holmes is regularly estimated by amateur astronomers world wide. A list of estimates is available at the IAC/ICQ/MPC website. Using averages of these estimates, I have plotted the apparent size of Comet Holmes against time (below).

In this graph, you can see the number of days along the bottom since 24 October, 2007 – the date when Comet Holmes suddenly increased in brightness.
Up the left hand side of the graph, I show the angular size of the comet – that is how big the comet appears to us in the night-time sky. The apparent size of the Full Moon, which is half a degree across (or 32 arc-minutes) is labelled for comparison.
Up the right hand side of the graph, I show the actual size of Comet Holmes in millions of km (assuming that the comet is at a fixed distance of 1.7 AU away – although the comet is moving away from us, it has not moved too much over the last 2 months).
Note how within days of the outburst in October, the comet was bigger than the separation of the Earth and the Moon, and within weeks it was physically bigger than the Sun!
Currently, it appears about 1 degree (60 arc-mins) across in the night sky – that’s twice the diameter of the full Moon. In physical size, the nucleus of the comet is now surrounded by a cloud of gaseous water that is over 2.5 times larger than the Sun.
What an amazing comet!

New dates are available for our evening observing events.

A new daily planetarium show all about Black Holes is now available at the Peter Harrison Planetarium, here at the Royal Observatory Greenwich.
The show touches on several areas relevant to my previous research into astrophysics, which I will discuss in this blog.
The show covers observations made by NASA‘s Swift space-telescope. Swift has a collection of cameras each sensitive to different wavelengths. It has an optical camera (made in the UK at University College London’s Mullard Space Science Lab), an X-ray camera (built in the UK at the University of Leicester’s Space Research Centre) and a gamma-ray detector (the burst alert telescope, or BAT, built at NASA’s Goddard Space Flight Centre in America).
When a black hole forms at the centre of a collapsing star, we see an intense burst of high-energy gamma-rays. As the exploding gas cools down to a few million degrees (!), the gas then begins to give off X-ray radiation. Eventually, the gas cools down to a few thousand degrees, giving off optical light.
So, the three cameras on board Swift can study all three of these stages in detail.

The X-ray camera on board Swift has a single 600×600 pixel detector. My previous job was calibrating the 14 identical detectors on board the European XMM-Newton space telescope – Swift’s much bigger brother!
As a sanity check, both XMM-Newton and Swift are often pointed at the same object, and the two observations are compared, making sure that the two telescopes see exactly the same colours (colour is simply the energy of a photon, a packet of light).
This was part of my previous job as a XMM-Newton calibration scientist. Even a 0.1% difference in colour could lead to wrong conclusions being made about the speed of the gases involved, and that could lead to misunderstandings of how black holes form.
While Swift is small and nimble, allowing it to swiftly turn around to spot the formation of new black holes, XMM-Newton is much bigger, and spends its time studying existing black holes in detail.
When gas, or even a star, falls towards a black hole, it gets very hot – up to 10 million degrees centigrade! And gas this hot emits X-ray radiation.

In September 2001, XMM-Newton spotted a star being swallowed up by the black hole at the centre of our Milky-way galaxy, and took photographs of the event.
So, even though we cannot see black holes directly (finding a black star against the black background of space remains difficult!), we can still look for the tell-tale signs of their presence.
One project, called AXIS (An XMM-Newton International Survey) involved astronomers working together from around Europe. When XMM-Newton looks at a known object, it discovers hundreds of other objects around the target. (It’s a bit like taking a photograph of a friend in the street, and you end up also photographing other people in the background. But we want to know more about those other people. What colour hair do they have? How tall are they?)
The easiest way to find out what those other, unknown objects were was to look using another telescope. In June 2001 I, along with two colleagues, travelled to the Nordic Optical Telescope on the Canary Island of La Palma.
About 10 of the objects we observed turned out to be super-massive black holes at the centre of distant galaxies – black holes a thousand million times more massive than our Sun.

We have very long nights in the northern hemisphere during December – perfect for astronomy!
Here in Greenwich, the sun is setting at 4pm in the afternoon and not rising again until 8am – giving 16 hours when the Sun is below our horizon.
When you take evening and morning twilight into account, we have 14 hours of beautiful star-filled darkness… if you can get far enough away from light pollution.
The long nights are due to the Earth being tilted by 23.5°. In December, the northern hemisphere of the Earth is pointing away from the Sun, so the Sun appears much lower in the sky, and its warmth is diminished. On 3 January 2008 the Earth is actually 3.4% closer to the Sun than in June – but it certainly doesn’t feel like that here in the Northern hemisphere!
In the early evening around 6pm, we look westwards to see the patch of sky that used to be directly above us in winter. So, ironically, the Summer Triangle is visible throughout winter! In fact, from the UK, Deneb is a circumpolar star – meaning that it never sets below the horizon.
During December, we have four hours after sunset to see the Triangle before it follows the Sun below the horizon. By February however, you will only be able to see the Triangle in the early morning sky, just before sunrise. Look at my earlier posting to read about the interesting objects in the Summer Triangle.
Look towards the west this month at 8pm, and you are looking towards the centre of our galaxy, where a super-massive black hole lurks just below the horizon. You need to wait until the summer for a better view of that patch of sky, as we are on the opposite side of the Sun to the galactic centre and so the Sun hides it from view.
Rotate 180° to look east, and you are looking directly out into deep space, towards the constellation of Orion.

At the bottom of Orion, to the left of Rigel, is the Orion nebula – a cloud of gas and dust, collapsing to form the latest generation of stars, recycling materials from old, now dead, stars. And you can see that for yourself with binoculars.
Mars is also visible towards the east at 8pm, in the constellation of Gemini. Mars appears to the unaided eye as a bright red object, but you will need an expensive telescope to see any details on the surface of the planet.
And finally, Saturn is becoming sociable again! It is rising in the east at 11pm in the constellation of Leo, where you can find it just to the left of Regulus. It is cream coloured and quite bright – take a look through binoculars, and you may be able to see the rings of Saturn, which show up easily in even the smallest of telescopes.
Here’s to dark and clear skies in December!