The vernal (spring) equinox occurred this morning at 05.14 GMT. But what actually is the vernal equinox and why does it happen?
During the course of a year the Earth completes one orbit around the Sun. From our perspective, we see this as the Sun moving through the year against the background of stars, along an imaginary line which we call the ecliptic. As the Sun moves (or appears to move) along this path, for half the year it’s seen to be above the celestial equator, which is the projection of the Earth’s equator onto the sky – this is summer in the northern hemisphere. For the other half-year it’s below this equator (northern winter).
So the Sun’s path appears to cross the celestial equator twice a year, in March and September. These times are the vernal and autumnal equinoxes – so called because at these times, day and night are of nearly equal length at all latitudes (equinox means ‘equal night’).
Find out more about equinoxes and solstices in our fact file, including why the equinoxes don’t always occur on the same day and why they don’t occur at the times when day and night are exactly equal.
H3 has returned to public display after a thorough cleaning and cataloguing. This video captures Jonathan’s feeling about the project as well as the timekeepers move back into its showcase.
As part of a large cataloguing project to research all the NMM’s marine chronometers, I am currenmtly undertaking a close study of Harrison‘s third marine timekeeper, H3 (made during the 1740s). It is proving hugely interesting and revealing as can be seen in the videos which have been tracking my progress.
The aspect I’m studying at the moment is the extraordinary (at least to me) glazed brass case of the instrument, and I’m wondering about 18th century ‘instrumental’ practice where glazed cases are concerned? The case (which the timekeeper fits in very closely indeed and which stands just over 60cms high) has a ‘top’, a middle band (attached to the timekeeper) and a ‘bottom’, the three parts held together round the middle with 32 screws. The whole thing is incredibly beautifully made, using cabinet-making techniques, and consists of precisely 501 parts, all fitted together mechanically, with no solder anywhere. The brass panels are just 2mm thick and the four main vertical edges are dovetailed (yes!) all the way from top to bottom with a total of 174 tiny dovetails rather in the way that coppersmiths tie plates together before soldering, but much finer and without solder. The dovetails are so well cut the vast majority cannot be seen, but I show a patch where corrosion and stress has revealed some of them (I have temporarily marked the lacquered surface with felt-tip pen to identify them).
The brass panels with dovetails
The structure inside forming the frames for the glazed panels are all pinned and riveted with 425 rivets, and the glass is then puttied in. The decorative moulded cornice is also ‘invisibly’ attached all round with pins, disclosed at one corner where the case was damaged in the past and was apparently heated to repair it, not very successfully.
The decorative moulded cornice
I wonder whether such large cases are unusual at this period, or are other instruments made in the mid-18th century that are housed in such cases? If so, how are they constructed? I am familiar with the 18th century grand orreries (e.g. those by Wright etc) in the lovely ‘cold-frame’ type wooden glazed covers, but can’t think of anything in metal at this period. If anyone knows of any examples please do get in touch below.
A short film documenting the dismantling of John Harrison‘s third marine timekeeper H3. It has been removed from display for cleaning and research for a new chronometer catalogue. It will be back on display in April 2011.
For the last few weeks, H2 has been in the horology conservation workshop at the Royal Observatory undergoing research. The work, which is part of the continuing research for a full published catalogue of the NMM’s collection of marine chronometers, involves the complete dismantling of the timekeeper. Every part is being studied, measured and photographed, the intention being to take a fresh look at Harrison’s work on his longitude machines.
Last year H1 was dismantled and studied, and some interesting comparisons can now be made about Harrison’s early work. It has always been believed that the simple portrayal of Harrison as a lone craftsman, was too simplistic, and we know that H1 was constructed with the help of Harrison’s brother James, and almost certainly with advice and supplies from George Graham’s contacts in London.
Harrison is known to have had help in his construction of H2, which was made in London, and the current study confirms this, with a much more professional feel to the materials and the finishing of this timekeeper; if H1 is a reminiscent of a fascinating ‘country clock’, then H2 has all the trappings of a ‘scientific instrument’.
There is no doubt Harrison had help in construction, but this doesn’t diminish the status of this extraordinary timekeeper, which teems with interesting ‘Harrisonian’ designs and construction features.
The timekeeper is now completely dismantled and before reassembly can begin there is full photography and measurement for CAD drawings to be done. Analysis is also planned on both the special alloys and the wood used in the timekeeper. It has always been said that the latter is lignum vitae, but as far as is known this has never been positively proved before.
Further updates will follow before the timekeeper returns to exhibition in the Longitude Gallery, now estimated to be sometime in late July.
The summer solstice occurs today, 21 June, at 11.38 UTC (GMT).
The summer and winter solstices mark the times when the Sun is at its furthest from the celestial equator (the projection of the Earth’s equator onto the sky). The world ‘solstice’ comes from the Latin solstitium meaning ‘Sun stands still’ because the apparent movement of the Sun’s path north or south stops before changing direction.
The summer solstice does not always fall on 21 June. Because the Earth takes approximately 365.25 days to go around the Sun, the precise time of the solstices and equinoxes occurs about 6 hours (0.25 days) later each
year, with a jump of a day backwards on leap years. The last time the summer solstice was not on 21 June was 1975 when it occurred on 22 June
at 00.27, and in the year 2012 it will occur on 20 June at 23.09.
To mark the solstice, the Royal Observatory is hosting Hour Angle by composer John Eacott, a free musical performance sonifying the exact moment of the summer
solstice. A real-time calculation of the Sun’s declination will be
transformed into live notation played by 12 musicians. The piece will
allow visitors the chance to experience the solstice in an
unforgettable way. The performance will start promptly at 12.08. Visitors are advised to
arrive early.
To mark the solstice, the Royal Observatory is hosting Hour Angle by composer John Eacott, a free musical performance sonifying the exact moment of the summer