Several of us will be attending Astrofest at Kensington Town Hall at the end of next week. Greg Smye-Rumsby and Richard Dunn are both giving talks on Friday morning (6 February). Greg’s is titled: The Universe in three dimensions, whilst Richard’s is titled: Spying the stars: The invention and development of the telescope. Richard’s new book The telescope: a short history, is published by the National Maritime Museum this March. If you are at Astrofest, do drop in on our stand (stand 20) on the lower floor and say hello.
Today is Chinese New Year and sees us saying goodbye to the Rat and hello to the Ox. But, while most of us are relatively familiar with the idea of animals marking the 12-year cycle of the Chinese calendar, we probably know less about how this calendar works.
It is a luni-solar calendar, meaning that it is based on the apparent motions of both the Moon and the Sun. Because the lunar months (29.5 days) do not match the solar year (365.25 days), there always several days ‘left over’. This problem is solved by adding an extra, intercalary month seven times in every 19 years. The Chinese new year moves around (between 20 January and 19 February) in our calendar because it is timed to coincide with the first new moon after the winter solstice.
The 12 animals assigned to the years reappear elsewhere in Chinese calendars and astronomy. They are used to represent the 12 ordinary months, a cycle of 12 days and even two-hour periods within each 24-hour day. In astronomy, the animals are associated with the stations which Jupiter passes through in one revolution around the Sun (an 11.86-year cycle) and yet another association is with compass directions. Chinese compasses and sundials in the Museum collections therefore include the characters for the animals.
Families will get a chance to have a closer look at instruments like this during half-term (20 February 2008) when I will be displaying and talking about them at the ROG. You can find about more about details of this and other Chinese New Year events at the museum.
With next Monday’s new moon, comes not only Chinese New Year, but also an annular eclipse of the Sun. Annular eclipses are more common than total eclipses, but fewer people are aware of their existence. Both occur when the Moon passes directly in front of the Sun. But in the case of an annular eclipse, the disc of the Moon is not large enough to cover the Sun completely. The last annular eclipses to be visible from Britain were on 31 May 2003 and 8 April 1921. The next is not due until 23 July 2093. None of next Monday’s eclipse will be visible from Britain and it will probably be seen by relatively few people – not least, because most of its path is over the ocean. The picture below was taken in Spain by Darren Baskill during the annular eclipse on 3 October 2005.
Back on New Year’s Eve you may recall, an extra second, known as a leap second, was added to the day to compensate for our slowing Earth. Leap seconds are added when necessary at the end of either June or December. The International Earth Rotation and Reference Systems Service (IERS) has since announced that there will be no positive leap second added this June.
Since the system of adding leap seconds was introduced back in 1972, a total of 24 leap seconds have been added. The three most recent being at the end of December 1998, 2005 and 2008.
The Earth’s rotation is monitored by a variety of means, including satellite laser ranging (SLR). The British facility is located in the old solar dome at Herstmonceux and is operated by NERC (Natural Environment Research Council).
In a nutshell, a laser beam is fired at a satellite to which a retro-reflector has been attached. By measuring the time it takes for the beam to return, it is possible to measure where the base station is relative to the satellite and therefore detect fluctuations in the Earth’s rotation as well as tidal movements of the land (due to the pull of the Moon). Interestingly, the solar building was not only the first of the new domes at Herstmonceux to become operational following the move from Greenwich; it is the only one where active research is still carried out.
With speculation about life on Mars making the news once again, it seems timely to look back and see what was being written on the subject 50 years ago. Back in 1959, the space age was very much in its infancy and America had only recently entered the space race with the launch of Explorer 1 in January 1958. The select committee on astronautics and space exploration was set up a few weeks later on 5 March during the 85th congress. A flurry of reports were soon being tabled, including the special staff report: Space Handbook: Astronautics and its Applications, which was submitted on 29 December and published in 1959. The extract below shows the state of ‘official’ thinking about life on Mars at that time.
‘Bleak and desert like as Mars appears to be, with no free oxygen and little, if any, water, there is rather good evidence that some indigenous life forms may exist.
The seasonal color changes, from green in spring to brown in autumn, suggest vegetation. Recently Sinton has found spectroscopic evidence that organic molecules may be responsible for the Martian dark areas. The objections raised concerning differences between the color and infrared reflectivities of terrestrial organic matter and those of the dark areas on Mars have been successfully met by the excellent work of Prof. G.A. Tikhov and his colleagues of the new Soviet Institute of Astrobiology. Tikhov has shown that arctic plants differ in infrared reflection from temperate and tropical plants, and an extrapolation to Martian conditions leads to the conclusion that the dark areas are really Martian vegetable life’
This picture of Mars was taken by Tony Sizer through our 28-inch refractor back in November 2005.
Although Mars will not be visible in the evening sky until much later this year, Invaders of Mars is showing daily in our planetarium until 17 May.
We took advantage of the change in the weather yesterday afternoon to check the alignment of the Meridian laser (see Monday’s posting). Arriving in Chingford at around 17.00, we had completed our adjustments by 17.30. The picture below was taken about ten minutes later.
From time to time, we have to check and adjust the alignment of the Meridian laser. Rather conveniently, back in 1824, the sixth Astronomer Royal John Pond had an Obelisk erected at Pole Hill some 11 miles from the Observatory on the then Greenwich Meridian.
Its purpose was to provide an additional alignment check for the Transit Telescope that was then in use. Having initially adjusted the telescope to the Meridian from observations of the circumpolar stars, Pond lowered it to the horizon to determine exactly where the obelisk should be built. By this means, he was able to position it with an accuracy of about ± four inches. The Meridian defined by this particular telescope is known as the Bradley Meridian and is the Meridian used (for historic reasons) by the Ordnance Survey (OS) on its maps. It predates the current Greenwich Meridian, which runs about six metres further to the east at Greenwich and is defined by the Airy Transit Circle (ATC) of 1850. An OS trig point was erected adjacent to the Pond Obelisk and very close to present Meridian in the 1930s, and this is where I will be heading later this month.
From there I will be able to talk with my colleagues back at Greenwich and tell them in which direction (east or west) they need to move the beam. The beam will also be adjusted vertically so that it just grazes or clears the tops of the trees, thereby ensuring maximum visibility for those places further to the north. So if early one winter’s evening, you think that you can see the beam moving around a bit, your eyes probably won’t be deceiving you … it will simply be us doing one of the regular adjustments that are sometimes needed!
For those who want a bit of info about the laser itself, it’s a Millennia VS Diode-Pumped, cw Visible Laser. It has a wavelength of 532 nm and an output power > 5W. The laser unit is located beneath the Airy Transit Circle and ‘fired’ along the Meridian from above via a fibre optic as can be seen in the picture above.
Whatever the weather, the Observatory at dusk is always a pretty amazing place to be, not least because of the fabulous views across London. Unlike last night when it was misty, the skies were really clear when I snapped this shot of the Peter Harrison Planetarium on Tuesday.
As it happens we had an Evening with the Stars running that evening and our visitors had a real treat as they looked though the 28-inch telescope. If you are interested in looking through it yourself, there are more sessions scheduled for later this month and into March.
The Meridian laser marks the route of the Greenwich Meridian by night in a northerly direction from the Observatory at Greenwich.
People often ask us: from how far away it can be seen? Not surprisingly, the answer to this is highly dependent on the weather and the atmospheric conditions at the time. But under good conditions it’s a lot further than you might imagine. The furthest at which it has been reliably sighted with the naked eye is at a distance of 36.7 miles from Periwinkle Hill a couple of miles to the south of Royston.
In order to see the beam when more than a few miles away from the Observatory, you need to be standing directly beneath it (and definitely no more than a few hundred yards or so to either side), and looking south, back along the beam towards Greenwich. There are a number of elevated sites in Cambridgeshire to the north of Royston with good horizons to the south. My good friend and former colleague Robin Catchpole visited one such a site at Lolworth (a small village just to the south of the busy A14) at the end of last year. Although not visible to the naked eye, Robin was able to see the beam with his binoculars and photograph it with his compact digital camera. During the first weekend in January, when much of southern England was enjoying clear days and frosty nights, I joined Robin on the Saturday evening to go laser hunting slightly further to the north in the village of Bluntisham at a distance of 60.5 miles. Once again, the laser was visible but once again, only though binoculars … and here’s the picture that Robin took to prove it.
At this sort of distance from the Observatory, the visibility of the beam is highly dependent on the state of the atmosphere at all points along its route and unfortunately for us, within just a few minutes of our arrival the beam disappeared from view. Under exceptional conditions, I would hazard, that it would be possible to see it from as far north as Bluntisham with the naked eye.
If you want to go laser hunting yourself, you will need either a suitable Ordnance Survey (OS) map and or a GPS. Because of the way the maps are constructed, the Meridian doesn’t run vertically up the page. Instead it curves slightly from right to left. Although most OS maps do not specifically mark the Meridian, it is possible to determine more or less where it runs by making use of the graticules (the small blue crosses) that are generally marked every 5′ of latitude and longitude on the Landranger maps – the ones with the purple covers. What you will need to do, is look along the top edge of the map where the longitude scale is marked until you find 0o and then look down the page for the corresponding graticules. What you may also find useful (especially if you need to use binoculars), is a magnetic compass to help orientate yourself in approximately the right direction. The Periwinkle Hill site can be located fairly easily even if you don’t have an OS map or GPS as the beam passes almost directly over the transmitting tower that is located there.
