People often ask why the Greenwich Meridian is where it is and which came first, the line or the Observatory. The short answer is that the Observatory came first and the line is where it is because of the Observatory. But it's a bit more complicated than that!

The Greenwich Meridian

A meridian is any imaginary line running from the North Pole to the South Pole. Astronomers who wanted to plot the positions of stars and planets set their telescopes up on a meridian, which they used as a reference line for their measurements and charts.

In 1850 the Astronomer Royal at that time, George Airy, ordered a new and much more accurate telescope, which he designed himself. It was much larger than previous telescopes so he had to find firms who were capable of making it.

The seventh Astronomer Royal, George Biddell Airy

For the metal castings he chose the engineers Ransomes and Sims of Ipswich. They specialised in agricultural machinery, but had some of the best manufacturing facilities in the country and were related to Airy through his mother. The lenses and other optical parts were made by a well-known firm of specialist opticians in London, Troughton and Simms. They had made other telescopes for the Observatory, so Airy was confident of their ability.

Airy's transit circle, completed in 1851

The new design was for a type of telescope called a transit circle. The name comes from the fact that it was fixed on a north-south line for the observation of stars as they crossed, or transited, the meridian. It also had a circular scale for measuring the height of the stars above the horizon. The line through the centre of this telescope became the Greenwich Meridian.

While researching a book I've been writing on the history of the telescope, I've come across a number of humorous prints like this one, Robert Sayer's Viewing the Transit of Venus of 1793.

It is one of the many satirical prints produced in England in the 18th century and seems to show a well-heeled couple observing Venus as it passes between the Earth and the Sun. So at first glance this appears to be just the sort of message that science's supporters were putting out - that science was worthy of study by the more learned and 'polite' members of English society.

But if we look closer, something rather different is going on. The scene isn't topical at all - the most recent transit of Venus had taken place over twenty years earlier in 1769. And the statue of the satyr on the right hints at rather more sexual interests, emphasised by the way in which the man lightly fingers the telescope, suggesting that it is akin to what contemporaries called the 'staff of life'. Perhaps he is hoping for a more bodily transit of Venus, recalling stories of the amorous encounters of the Greek goddess of love. This telescope, then, has become a most impolite instrument.

Keep coming back for more Telescopes stories throughout the International Year of Astronomy 2009.

This week H1 is to be dismantled for repairs in the conservation workshop here at the Royal Observatory. The Harrison timekeepers are so popular that they're rarely off display.

Harrison's first timekeeper, H1

The problem with the timekeeper occurred last year when the connection broke between one of the balances and its lower balance spring. The balances and springs are the vital 'beating heart' of the timekeeper, and being complete and well adjusted is fundamental to the good going of the instrument.

H1's right balance and its broken spring connection

As Senior Specialist in Horology at the Museum it's my job to undertake this exciting and delicate work, and it was with some trepidation that we prepared H1 for its 40 metre trip yesterday morning to the workshop. It's now on the 'operating table' (a medical trolley we use for transporting) and my first task is to take photographs all round the timekeeper before any dismantling takes place.

During the work, there is a screen set up in H1's showcase with a short slide show telling visitors what's happening. If staffing allows, the on-going work on H1 can be seen from the Time for the Navy gallery, which is next to the workshop and has a glass-backed showcase through which parts of the workshop can be viewed.

Working on H1 is going to be very special for me as one of my great heroes in the Harrison story is Lt.Cdr R.T.Gould, who rescued the timekeepers from oblivion back in the 1920s and 30s. It was almost 90 years ago, in 1920, that he first started that work. I'll be contributing two or three entries a week to this blog, so keep an eye out if you're interested!

Dismantling of H1 has progressed smoothly this last week, and there has been considerable interest in the project from colleagues and the public. Last Thursday the Guardian newspaper's delightful Maev Kennedy paid us a visit with photographer David Levene, and an extraordinary double page 'centrefold' picture appeared in the paper on Monday with H1 pictured from a fascinating and engaging angle. If I'd known my rough notes, visible next to the timekeeper, would be readable in a national newspaper I'd have tried to make them less inscrutable!

Also on Monday we had a visit from the current students and tutor, Matthew Read, from the Clock Course at West Dean College, who were duly impressed by being able to inspect H1 partially apart, and were consequently able to better understand some of its intricacies.

So far, the dials are all off and the front plate has been removed, revealing the 'motion work' (the gears which ensure the hands go round at the correct rate). The calendar wheel had a particularly lovely surprise, in that I found Harrison had decorated the little part on the wheel called the 'jumper' with a most beautiful, rather Celtic style tail and with a front end shaped in the form of a little bird's head!

None of this can be seen when the clock is complete of course, and it must have been included entirely to please himself, and future clockmakers. Well, it certainly pleased me John!

Next will be the main train of wheels and all their roller bearings; it will be interesting to see if much wear is evident after nearly half a century of virtually constant running.

Today has been designated Ada Lovelace Day - an international day of blogging to draw attention to women excelling in technology. It is named after Byron's daughter, Ada Lovelace (1815-1852), the mathematician who is credited with writing the first ever computer programme for Charles Babbage's Analytical Engine, a never-completed computing machine. Taking the idea of technology broadly, I thought this would be a good opportunity to begin a series of occasional blog entries on women connected to the ROG throughout its history.

The first woman who was seriously involved with life and work at the ROG was the wife of the first Astronomer Royal, Margaret Flamsteed (c.1670-1730). She was the daughter of a London lawyer, Ralph Cooke and granddaughter of John Flamsteed's predecessor as rector of the Surrey parish of Burstow. She was probably baptised in Hackney in 1670 making her about 22 when she married the 46-year-old Astronomer Royal on 23 October 1692. Despite the age gap, there seems to have been a sincere attachment between the two, if the words 'beloved Wife' in Flamsteed's will and Margaret's determination to secure her husband's pothumous reputation are anything to go by.

The Royal Observatory from Crooms Hill, in about 1680 - the time that Margaret Flamsteed began living there.

Margaret Flamsteed was clearly a well-educated woman, both literate and numerate. We know from Flamsteed's records that she was occasionally of practical assistance during observational and calculating work, and manuscripts in the RGO archive in Cambridge record her study of mathematics and astronomy. These studies may have reflected those undertaken by Flamsteed's paid assistants and paying pupils - men and boys that Margaret would have taken care of as part of her general management of the household on Greenwich Hill. She seems to have been a particular asset when acting as hostess to the Observatory's visitors, no doubt smoothing over her husband's sometimes prickly temper.

It is, however, as Flamsteed's widow that Margaret is best remembered. Firstly she worked with the Observatory's former assistants to publish the full version of Flamsteed's book, Historia Coelestis, an abridged version of which had, to Flamsteed's fury, been published by Edmond Halley in 1712. Rather more sadly for us, she also removed and sold all of the astronomical and horological equipment that Flamsteed used at the Observatory. This she was probably justified in doing, as Flamsteed had either paid for the equipment himself or had received it as a personal gift from his patron Jonas Moore. It was to take several more Astronomers Royal before the questions of ownership and copyright that dogged the Flamsteeds were sorted out.

Today, I'm pleased to say, sees the publication of a new book I've written on the history of the telescope. The book has been brought out in celebration of the International Year of Astronomy 2009 and to mark the 400th anniversary of the invention (or at least the announcement) of the telescope in 1608.

I've tried to tell as broad a story as possible. So I've talked about the development of telescopes for astronomy and their huge impact in changing our conceptions of the universe but I've also looked at the more humble story of the hand-held telescopes and binoculars used for all sorts of things on land and at sea. If you think about it, after all, most of the telescopes ever made were for these more mundane purposes. Another aspect I've included is the cultural impact of the telescope, whether as a symbol in art and literature, or as an inspiration for science fiction and other writers and film-makers. So as well as Galileo, Newton and Hubble, you'll find Alfred Hitchcock, H.G. Wells and Virginia Woolf. And don't forget to look out for Tom Swift and his megascope space prober.

For those wanting to look even deeper into the telescope's history, you may also be interested in a forthcoming conference, The Long View, this July.

Dismantling of H1 has progressed well and the timekeeper is now almost totally in pieces. As parts come off I'm taking the chance to look at them closely and am developing a much more detailed idea of the restoration carried out by Gould and the Chronometer Section in the 1960s.

In spite of his descriptions of the restorations in his notebooks, Gould did not provide complete data, and I am now busy counting teeth on wheels, measuring parts and keeping a tally of the total number of parts in H1. Some statistics are proving quite surprising. For example, just the two fusee chains (which link the driving springs with the wheels of the clock) between them total almost 4,000 bits!

Now that the wheels and roller bearings are all out it is becoming clear that the timekeeper is in very fine condition. Almost no wear is perceptible on the bearings, and Harrison's 'rolling contact' arrangement has stood the test of time extremely well. The only evidence of long use is what one might call the 'computer mouse' problem, where the rolling contact has caused thin layers of dust to become compacted on the rollers (just as it does inside the mouse sometimes).

Next is extensive photography and measuring, and I have to decide on the best (and most conservative) way to repair the broken balance spring connection.