Did you manage to see the Moon hide Venus on Monday night?

Unfortunately, the weather was not kind to us here in Greenwich, with thick cloud rolling in just minutes before the occultation began. However, we did manage to get a few photographs, just before the cloud came in.

Royal Observatory Astronomers Darren Baskill & Tony Sizer managed to grab the following photographs.

Although the clouds did spoil our view of the Moon and Venus, they did manage to provide a beautiful sunset. This was the view from the Royal Observatory South building, about 10 minutes before the occulatation was due to start.

There are only a few more chances left to see the International Space Station fly over the UK before the new year. The following times are when the station is flying over the UK (if you live elsewhere, take a look at SpaceWeather.com satellite flyby website)...

The Moon, Jupiter and Venus have been a beautiful sight over the last few evenings. Here are some photographs showing the view from Greenwich.

On the evening of the 2nd of December, we had the following view... notice the seperation of the Moon and thew two planets.

Just a day later, and the Moon has move a significant distance in the sky, as it orbits around the Earth. The following photograph shows the view on the evening of the 3rd of December, 2008.

This final photograph was also taken on the evening of the 3rd of December. What a beautiful sight we have been treated to over the last view days!

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.

Jupiter & Venus are the highlights this month, visible low in the west in the evening sky, just after sunset. While Jupiter stays low throughout the month, Venus gets higher in the evening sky every day, as it orbits around the Sun.

And on the 27th of December, Venus is next to Neptune, making Neptune unusually accessible. Just find Venus, the brightest thing in the sky, and then look to the top-right slightly (about two finger widths at arms length) and see if you can see Neptune. If you live well away from any light pollution, you may be able to see it with you own eyes (although it is at the very limit of human visability!); otherwise, try using binoculars or a telescope.

It is also the time of year when we are further away from the centre of our Milky-way galaxy, and the galactic centre is hidden behind the Sun.

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. Read last months' posting to read about the interesting objects in the Summer Triangle.

Look East in the evening, 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.

Here's to dark and clear skies in December!

Time by Wire: 175 years of the Greenwich Time Service David Rooney The Institute of Engineering and Technology 3 December 2008 >> go to webcast>> recommend to friend

David Rooney, Curator of Timekeeping at The Royal Observatory, Greenwich was honoured to give The Wheatstone Lecture 2008. In his lecture, David Rooney explored the curious history of time distribution from Greenwich, revealing little-known stories behind one of Britain's best-known measurement systems: Greenwich Mean Time.

The movie below shows how Jupiter, Venus & Mercury will look at 5pm in the evening sky everyday throughout December 2008, as produced using the excellent (& free!) planetarium software Stellarium.

At the start of the month, the brighter Venus is to the left of Jupiter - both are easily visible with the unaided eye in the evening twilight. Between 4pm and 5pm is the best time to look.

As December progresses (see the date in the top left corner), Venus gets higher and higher in the evening sky. As the Earth orbits around the Sun, it is moving to the opposite side of the Sun to Jupiter - which is why Jupiter appears to be moving towards the Sun. Just after Christmas, Mercury comes into view as well.

New Year's Eve is the highlight, however, with Jupiter and Mercury being VERY close together in the twilight sky, as well as the Moon and Venus!

This movie shows how we will see the planets "dance" during December (2008). First off, we see the view that we will have when we look towards the South-West. The movie then plays again, this time retaining the 'old' positions super-imposed so we can see exactly where the planets have moved from. Finally, these two scenes are repeated for convenience.

To understand why the planets are moving around as they do, take a look at the view from above...

This image was produced using the free Celestia software. The yellow arrow indicates which way all the planets orbit around the Sun, and note that the planets closer to the Sun orbit much quicker than the planets further out. Mercury wizzes around the Sun in just 88 days; Venus in 225 days; the Earth in 1 year; and Jupiter takes almost 12 years to orbit the Sun!

From above, we can see that, as the Earth orbits the Sun, Jupiter begins to be hidden by the glare of the Sun as the Earth moves to the opposite side, while Venus & Mercury become easier to see as they get further away from the Sun.

After seeing all this movement, it is hardly surprising to learn the origin of the word 'planet' - it's ancient Greek meaning wanderer!

Today, the 12th of December 2008, we have the earliest sunset of the year in the Northern hemisphere. In Greenwich, the Sun sets at 15:50GMT.

The earliest sunset occurs on a different day to the solstice due to a combination of the Earth's elliptical (oval-shaped) orbit and it's tilt. Put together, these two effects are known as the Equation of Time, the difference between what an averaged mechanical clock will say and what a Sun dial will say.

On Friday the 12th of December, we have the biggest Full Moon for 15 years!

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The Moon orbits the Earth in an elliptical (oval-shaped) orbit, and during tonight's full Moon, it will be closer than usual - only 356,600km away from Greenwich! (which is about 363,000km away from the centre of the Earth).

On average, the Moon is 378,000 km away, and at furthest, it is 399,300 km away from the Earth's surface. So tonight's full Moon is 6% closer than average, and so 11% brighter than average! (Or, to put it another way, it is 11% bigger & 20% brighter than when the Moon is at its furthest point away from us).

At 12.03pm GMT today, the poles of the Earth were aligned with the Sun, which is what we call the solstice. While in the Northern hemisphere, the North pole was pointing away from the Sun (it was the Winter Solstice), in the Southern hemisphere, the South pole was pointing towards the Sun (the Summer Solstice), which is why it is winter in the North and summer in the South.

The image below shows a stacked series of images taken 20 minutes apart throughout the 17th December, 2005 (within just a couple of days of the winter solstice), showing how the Sun moved across the sky during that day, from the South-East to the South-West. A timelapse movie version is also available, showing that at this time of year, while the Sun is very low in the sky, the full Moon is very high in the sky.

The Winter Solstice also marks the shortest day, although the earliest Sun-set and latest Sun-set occur on different days (the 12th and 30th of December, 2008).

There are lots of special events to mark the solstice, including a gathering at Stonehenge in Wiltshire (see photographs on the BBC News). Under the early Julian Calendar, the winter solstice actually occurred on the 25th of December, but because that calendar was not accurate, the solstice slipped to the 21st (the accuracy of the calendar was corrected with the introduction of the Gregorian calendar in 1582).

However, we still traditionally keep the 25th of December as the day for celebration, and Christmas now includes a rich mix of celebrations, including Roman (Saturnalia, and Dies Natalis Solis Invicti, the birthday of the unconquered sun), Nordic Pagan festivals and the Christian celebration of the birth of Christ (see more about the origin of Christmas at this Wikipedia article).

Whichever way you choose to celebrate this time of year, we hope you will enjoy yourselves and have a very happy festive season!

Like last month, the Moon will be drifting past two planets on its Moonthly orbit around the Earth. Unlike last month, the Moon will be passing by a different pair of planets!

Last month, the Moon drifted past Venus and Jupiter, forming a wonderful smiley face in the evening sky! This unique event was witnessed around the world,and Naveen L Nanjundappa, Vice President of the Bangalore Astronomical Society, took the wonderful photograph below (many more photographs are on the BBC News website, and SpaceWeather.com).

This month, the Moon will be passing by the planets Jupiter and Mercury on the 28th and 29th of December. Their exact alignment and the best time to look will depend on your location, but the three will be close to each other. For the UK, the best time to look is on the evening of the 29th, just after sunset (4.30pm is the best time), low to the South-West, when the crescent Moon is just to the top left of the bright Jupiter, and slightly fainter Mercury.

In fact, for just a brief period after sunset on the 29th, the Moon is so close to Mercury that it actually hides it! (an occultation) Unfortunately, that event will only be visible from a small region of the Pacific Ocean, a few thousand miles East of Japan!

However, for people living in western/central Australia, just after their sunset (which occurs 6 hours later than Eastern Japan), they will get to witness the Moon covering Jupiter instead!

It just goes to show what a difference a few thousand miles can make!

On new year's eve, the Moon will have moved along to Venus, higher and brighter than both Jupiter and Mercury. To see where the planets will be every night, take a look back at my December's Dance of the Evening Planets post.