January 2006

The size of Charon

In this week's edition of Nature, there is a report detailing observations of Pluto's moon Charon occulting a background star. The observations were made using part of the Very Large Telescope, the 0.5m Campo Catino Austral Telescope and the 2.5m Jorge Sahade telescope which are all based in South America.

At each of the three sites they saw the star passing behind a different part of Charon and this allowed accurate measurements of the diameter. The diameter is now found to be 603.6 km (±5.0km) which is incredibly accurate for a measurement of something so far away. Once you have a diameter for Charon you can also combine that with the mass of the moon to work out the average density. It turns out to be about 1.7 gm/cm³ which is equivalent to "an icy body with about slightly more than half of rocks". You can listen to this on the Nature podcast or read the ESO press release for more details.

Pluto is making the news quite a lot at the moment. It should get even more exciting soon as the first mission to Pluto should be launching in about 12 days time.

Tags: Pluto | Charon
Posted in astro blog by Stuart on Thursday 05th Jan 2006 (08:51 UTC) | Add a comment | Permalink

Happy Perihelion!

Today is quite a special point in the year. Obviously, it isn't the start of the calendar year (that was Jan 1st) and it isn't an equinox (those are on March 20th and June 21st this year). It is actually the point when the Earth is closest to the Sun in its orbit. In fact, according to the US Naval Observatory, that is due to occur at roughly 15:00 Universal Time (currently the same as GMT) which is … right now!

The Earth's orbit is slightly elliptical, so for some of the year it is closer than the average distance from the Sun and at other points it is slightly further away. The point in the orbit where the separation is greatest is known as aphelion and the closest approach - happening right now - is perihelion.

At this moment in history, the Earth is closest to the Sun during winter in the northern hemisphere which makes it slightly warmer than it would be if we were on a circular orbit. By the same reasoning, the southern hemisphere experiences slightly hotter summers too. But all this will change in time because of the precession of the Earth's axis (it tilts at 23.5 degrees). In about 13,000 years the northern hemisphere will be tilted towards the Sun during perihelion and summers (for us northern hemisphere folk) will be a bit hotter and the winters a bit colder.

Tags: Earth | perihelion
Posted in astro blog by Stuart on Wednesday 04th Jan 2006 (15:25 UTC) | 1 Comment | Permalink

Orion's sword

In the ancient Greek legend, Orion was a hunter. In fact he was such an important character that a constellation was named after him.

The constellation of Orion is prominent in the winter night sky and is found by looking for the three stars in a line that make up Orion's belt. From the belt hangs a small sword (or dagger) and with the unaided eye this usually looks like a few faint stars and smudge of light. Back in December I took several 15 second exposures of the sword using my digital camera set to full zoom. I had forgotten about them, so I've only just got around to adding the frames together. To take the pictures I placed my camera on a mini tripod which remained fixed during each exposure. Despite each exposure lasting only 15 seconds the apparent movement of the stars, due to the Earth's rotation, causes the stars to look like lines.

Sword of Orion
Orions sword. Created by adding six 15 second exposures. CREDIT: Stuart
In the middle of the sword you can even make out the smudge that is M42 - the Orion Nebula. My image isn't as brilliant or as beautiful as some, but I'm still impressed by such a simple setup.

Tags: Orion | astronomy
Posted in astro blog by Stuart on Tuesday 03rd Jan 2006 (21:06 UTC) | 2 Comments | Permalink

Bad Astrologers II

It is that time of year again; the start of a new one. The newspapers will be full of horoscopes for the coming year. Many people will believe these predictions despite the (western) zodiac signs being based on an out-of-date version of the sky. Oh, that and having no basis in reality but I digress.

Earlier in the year I reviewed the attempts of a bunch of astrologers, and assorted others, at predicting the results of the UK General Election. They didn't do very well. In fact they were rubbish. When it comes down to making actual predictions they don't perform any better than guesswork. Despite that, they do make a killing from all those premium rate phone lines. Why do I mention it now? Well, looking back through my website logs, I notice that that post has had a lot of visits from people looking for the astrologers mentioned. I hope that these visitors saw my poor review and stopped to get some astronomy while they were here. I can hope at least. On a related point, one visitor came here looking for the phrase "Phil Plait is trouble". I guess he is, but in a nice way ;-)

Have a great 2006 everyone!

Tags: astrology
Posted in astro blog by Stuart on Saturday 31st Dec 2005 (12:33 UTC) | 1 Comment | Permalink

The astronomy blogosphere

As usual, the holidays have been (and still are) busy. This, along with my inability to think of anything original, has meant that I haven't posted much recently. To take you through new year, here is a short round-up of what has been happening elsewhere on t'Internet.

Recently the weather seems to have been a bit poor for observing. This is quite normal in the UK, but it is most annoying now that we have long dark nights in the northern hemisphere. Christmas morning mist has been stopping Megan (UK) seeing the Lovell Telescope, so she made do with scanning in some old photos instead. Cloud has put Dave P (UK) in the doldrums while James (Ireland) has managed to get a couple of clear cold nights. I bet they weren't as cold as Tom (New England) is getting at the moment; he has been fitting out his new Sky Shack (think of a shed on a sledge) with a heater to keep him warm in the deep snow. It seems to be working a treat.

When the weather allowed, this month has seen lots of solar system observing from both hemispheres. Mars is getting smaller again, but that hasn't stopped people getting views of the major features. Venus has also been giving a good show as it overtakes the Earth on the inside and becomes a larger slim cresent. Saturn has also started to become sociable by rising earlier. This has allowed Rob (Florida) to get a nice shot of it.

Nearer to home, the Moon has been providing a good target for all. Ian (Australia) has a lunar mosaic, Math (Belgium) has a nice video trip across the Moon and Peter (New York City) has a great record of a night observing craters on the Moon. Perhaps he should enter it in Ian's competition (judge's

decision is final!) to find the best sky watching experience. Mind you, I would suggest Aditi's (India) experience of the night sky during a powercut as a contender too.

In other news, a small meteroid was caught on camera hitting the Moon, Stardust will be returning on 15th January and Uranus has gained two new rings (see Phil P or Tom). I still haven't been able to decipher that anagram yet.

Finally, Dave P has been encouraging the return of sketching (I've made you the "and finally" again Dave ;-) ) and Peter has taken up the challenge with some great sketches of the Moon. It would be nice to see more in the new year. Now, if only it wasn't foggy outside...

Posted in astro blog by Stuart on Thursday 29th Dec 2005 (18:56 UTC) | 4 Comments | Permalink

Christmas on Mars?

Back on 31st October 2006 the British Rocket Group (also known as BERG) launched Guinevere One towards Mars. The trip has taken a very quick two months and the lander should be touching down later on today. Of course, the Prime Minister (Harriet Jones) has been claiming some of the credit for this mission (preparations for which started several years before her recent appointment), so she must be hoping that it doesn't go the same way as Beagle II did. Especially as the latest data from Mars Reconnaisance Orbiter still hasn't located the crash site. Whatever happens, there is bound to be coverage on the TV tonight.

Guinevere One should be landing at about 7pm GMT and shortly after it will begin transmitting live video images - with a delay of about 20 minutes due to the finite speed of light - from Isidis Planitia. Later, several robotic rovers will study the soil around the landing site as well as provide ground-based information about the martian weather. Since the Opportunity rover stopped working during the martian winter, we've been down to just one rover on the red planet. Hopefully there will be a few more by tomorrow.

Guinevere One is also carrying a suite of messages to alien life that are similar to those on Voyager I. This is rather bizarre, as there isn't thought to be any life on Mars and certainly none that could read! This is really just a publicity stunt to get press and public attention.

Of course, it isn't the end of 2006. Most of this post is fiction created to fit in with the special Christmas edition of Doctor Who.

Posted in astro blog by Stuart on Sunday 25th Dec 2005 (00:00 UTC) | Add a comment | Permalink

Stardust

Back on the 7th February 1999, NASA's Stardust spacecraft was launched. Its seven year, 2.88 billion mile, mission was a round-trip to comet Wild 2 (pronounced Vilt 2). I remember the launch because a year or so before it there was an opportunity to add your name to a microchip that would be on board. I added quite a few names of friends and relatives to that chip. (If you missed out you may still be able to add your name to the Dawn spacecraft, although this was recently asked to stand down.)

Stardust collected samples of dust from the comet's tail in 2000 and inter-stellar dust in 2002 using aerogel 'tennis racquets'. In January 2004 Stardust made the front pages of many newspapers when the mission team released images of the comet nucleus.

Now, if you were thinking ahead you may have noticed that 1999 plus seven years brings us to 2006 and indeed there are only 23 days (or is that 24 days?) before it returns with the dust samples. The samples will be released in a capsule from the main spacecraft and hurtle through the atmosphere at 12.8 kilometres per second finally landing in the Utah desert by parachute.

If you live in the western States of the U.S. you will be able to watch re-entry early in the morning  (3:15am) on January 15th. In fact the mission team would like amateur astronomers to take photographs or movies of the re-entry to help with their analysis of what happens to the heatshield at these speeds. If you are in the right place you could even catch it passing in front of the Moon. You can find out more at the Hypervelocity re-entries website.

Posted in astro blog by Stuart on Thursday 22nd Dec 2005 (18:47 UTC) | Add a comment | Permalink

Hunting the Beagle

The BBC report the possible discovery of the UK's Beagle 2 lander on the surface of Mars. Colin Pillinger claims that various light and dark areas in a crater, seen by NASA's Mars Global Surveyor, could be parts of the ill-fated lander. It was thought that Beagle 2 probably crashed during Christmas Eve/Day 2003 because the atmosphere was thinner than had been expected so there wasn't enough braking and it hit the ground too fast. It was the first British thing to crash on Mars!

Frankly, you may need to be someone like Richard Hoagland to see much in the images on the BBC news site. I think I'll wait for higher resolution images from Mars Reconnaisance Orbiter next year before celebrating.

Tags: Beagle 2
Posted in astro blog by Stuart on Tuesday 20th Dec 2005 (12:46 UTC) | 3 Comments | Permalink

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