Local news from Observatoire Solaire…
This month sees the confirmed publication and availability of our next publication, a biography of one of the great French astronomers of the 19th century, François Felix Tisserand. We will launch the booklet on our website here on the 16th February.
We are also delighted to be able to let you know that our book on the Sun has been accepted for expert review by ‘The Observatory’ (a publication closely associated with the Royal Astronomical Society) and we very much look forward to their review in about 2 months.
A la une...
Ce mois notre prochaine publication sera disponible sur notre site (le 16 février). C'est la biographie d'un astronome français du 19ième siècle, François Félix Tisserand.
Et à la fin de mars il y aura une critique de notre livre sur le Soleil dans l'Observatory, une publication liée au Royal Astronomical Society
The cold spell in Europe, and solar irradiance
Observatoire Solaire is currently based in Scotland although the warmer clime, and rather sunnier skies (always an advantage for a solar observatory) of southern France hold great appeal. But, as you’ll have seen on the news, Europe has recently experienced an unusually cold spell with snow as far south as the Greek islands. Although exceptional, this weather is a natural consequence of the northern winter season, aided and abetted by the path of the jet streams (high altitude and high speed wind). It has nothing to do with radiation emanating from the Sun varying!
As readers of this blog will know, the Earth revolves around the Sun in a near-circular orbit. Our orbit though is not precisely a circle, it’s an ellipse with eccentricity of 0.01671. This eccentricity gives an aphelion distance (the point where the Earth is furthest away from the Sun) of 152.1 million km, whilst at perihelion (closest to the Sun) we’re 147.1 km from the Sun. These distances are ‘measured’ from the centre of masses, i.e. the centre of the Sun to the centre of the Earth.
The date at which Earth is at perihelion this year was the 4th January, at 14:17hrs (GMT – Greenwich Mean Time). Next year, it will be on the 3rd January around about 05:34hrs. (The orientation of the direction of the aphelion and perihelion points varies slightly each year due to a gravitational perturbation effect known as ‘precession’….we may look at precession in a future blog!)
A difference between perihelion and aphelion distance of 5 million km does give a range of just under 7% in the received solar irradiance. Although not insignificant, hemisphere seasonal effects and Earth atmospheric circulation make this variation a very much second order effect on our weather. In passing, the irradiance also varies by ~0.2% depending upon the solar (magnetic/sunspot) cycle.
But quantitatively, how much energy does the Earth receive from solar radiation? The solar irradiance is related to solar luminosity and is defined as the energy received by an object when at a distance of one Astronomical Unit (AU) from the Sun (an AU is the mean distance of the Earth from the Sun). Solar irradiance is measured in units of kilowatts per square metre (i.e. kilojoules per second).
The earliest direct measurements of the solar irradiance were made by Claude Matthias Pouillet (b.16th Feb 1790 d.14th Jun 1868). Pouillet was the son of a paper-maker in Cusance, a very small Burgundy village, and was clearly a very remarkable man and scientist. In 1838 he became professor of physics at the Sorbonne in Paris, a position he held until 2nd December 1851 when he was relieved of his position (we would nowadays say fired!) because he refused to swear an oath of allegiance to Napoléon Bonaparte’s republic regime.
Claude, pictured left, was an outstanding physicist and worked mostly, but not exclusively, on atmospheric physics and meteorology. His seminal work Éléments de physique expérimentale et de météorologie was published in 2 volumes (and eight parts) in 1827 and 1829.
In 1837 he designed and built the first the pyrheliometer (effectively a small telescope with a thermopile at the focal point) and used this to measure the solar irradiance. The measurements Pouillet made, at Earth surface level, were remarkably accurate.
His were the first scientific (and repeatable) estimates of this key parameter and his published (1838) values of 1.228 kilowatts/square metre are within 10% of the value measured today. Nowadays we have satellite technology to accurately measure the irradiance, and the Earth receives 1.361kilowatts/square metre.
More details on the life and works of Claude Pouillet will be found in our future book (currently in production) on the French astronomers of the 18th and 19th centuries.
The source of the Sun’s energy, and thus solar irradiance, was for many years unknown. As reviewed last month, Anaxagoras considered the Sun to be ‘a hot stone’. More recent scientists of the 18th and 19th centuries knew that a ‘hot-stone’ model just didn’t help.
What we can be confident of though is that the sun has generated approximately the same amount of energy and had the same luminosity for the past few billion years. How can we make this deduction? The earliest fossilised form of multi-cellular life so far found (in 2008 in shale rocks in Gabon) have been dated to around 2.1 billion years in age. Even earlier, there is strong evidence of simple bacterial life (cyanobacteria) found in 3.5 billion-year-old Archean sediments of Western Australia. So, by taking this evidence for how long life has existed on Earth, we can be reasonably certain that global atmospheric temperature has been within 273 to 373 Kelvin (0 to 100 degrees Celsius).
Hermann von Helmholtz (b.1821 d.1894) and William Thompson (b.1824 d.1907), usually referred to as (Lord) Kelvin, both proposed that the Sun’s energy could be produced by the conversion of gravitational potential to thermal energy. As the Sun slowly compresses under its gravitational self-attraction/collapse, the hotter it becomes and thus begins and continues to radiate (thermal) energy. This ‘works’, but only for a limited time. Even a mass as large as the Sun (which in passing is a very modestly sized star, ‘merely’ 333,000 times the mass of the Earth) would have a very limited lifetime if its energy were to originate from this process.
The Kelvin-Helmholtz timescale for the Sun (i.e. how long it would take for the Sun to convert the total available gravitational potential energy to kinetic (thermal) energy) is just under 38 million years. (We look at this in detail in our Sun book; the mathematics are available in the extract of the book here on this webpage). This of course doesn’t match the fossil record of life on Earth.
Other mechanisms – such as chemical reactions or radioisotope decay (proposed by Ernest Rutherford in 1904) – all have the same problem; they ‘work’ but would only generate energy at the level of the current solar irradiance for a few million years at best.
Only following advances in atomic physics and the identification of nuclear forces in the 1930s did nuclear fusion become known and the ‘obvious’ solution. Obvious? Elementary particles predicted by fusion theory (and specifically the neutrino flux) have shown this to be the mechanism generating the Sun’s power. However, difficulties with a full theory to match the observed evidence were only solved by further advances in atomic physics, with ‘neutrino oscillation’ being detected in 1998.
We look in much more detail at the solar irradiance and the nuclear fusion processes operating at the core of the Sun in our book. Other references and interesting further reading (in French) include:
Éléments de physique expérimentale et de météorologie
Biography of Claude Servais Matthias Pouilet
La détermination de la constante solaire par Claude Matthias Pouillet