Solar Story: understanding the Sun
Dr Rebekah Higgitt
Rebekah Higgitt began the evening with a talk entitled “Solar Observation at the Royal Observatory Greenwich”. Rebekah is NMM Curator of The History of Science and Technology.
Rebekah opened with a brief description of the ‘Solar Story’ exhibition at the ROG. The exhibition ranges from the very earliest historical observations of the Sun, through work carried out at the Royal Observatory and subsequently by NASA
Rebekah then went on to explain the historical objectives of observing the Sun. Initially astronomers were concerned to establish the scale of the solar system and, through parallax measurements make progress in establishing distance to the nearest stars. The effect of the Sun on the earth was also a matter for enquiry.
The Royal Observatory Greenwich had taken one of the earliest and most comprehensive set of daily photographs of the Sun during the period 1873 to 1973. These images are still used by researchers to construct a history of solar features, especially sunspots. This work was commenced under E. Walter Maunder.
Another important group of observations are those concerned with establishing the position is in respect of the planets and in particular transits of Mercury and Venus.
Observation of the Sun in eclipse reveals the corona and chromosphere, not observable under normal conditions as they are too faint.
At Greenwich Flamsteed himself observed using the predecessor of the present camera obscura – called the “Domus Obscurata” in those states. He observed the 1662 eclipse and made repeated attempts to measure parallax.
During the 18th century transit expeditions were deemed more accurate for establishing the distance between the Earth and the Sun. This distance (known as the Astronomical Unit) is used as the base line for measuring the distance to nearby stars by triangulation.
Curiously enough interest in the Sun itself did not take off until the 19th century. Airy used the 1860 Great Equatorial to observe granulation, sunspots etc.
Despite the fact that the observatory was funded and run principally to the benefit of the Admiralty, the ROG became a centre for pure research. Not that the Admiralty was concerned solely with traditional positional astronomy. During Pond’s period as Astronomer Royal the Admiralty began an interest in magnetism since it was believed that variations in the magnetic field of the Earth were influenced by the Sun and these of course resulted in deviations to compass readings. The magnetic hut was built (roughly where the planetarium is today) to observe variations. Geomagnetism was also thought linked to the weather.
In 1873 a new Photography and Spectroscopy Department, initially under the leadership of Maunder, was established. Maunder discovered several things: magnetic storms were related to sunspots, something previously unknown. Maunder drew the first butterfly diagrams in 1904, plotting sunspot frequencies against their solar latitude. The discovery from other historical records of a long period with virtually no sunspots in the 17th century is known to us today as the ‘Maunder Minimum’.
These first primitive researches into the origin of magnetic storms were followed up as a matter of urgency, especially during the Second World War when the RAF, Met. Office, Cavendish laboratory and other client organisations relied on Greenwich to provide news of solar storms and flares which affected radio and radar.
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Dr Marek Kukula
Secondly Marek Kukula spoke on the subject of “Solar Story”. Marek is ROG Public Astronomer. The effect of the Sun’s radiation on the Earth’s climate is one of the more controversial aspects of the climate change debate. But how does the Sun affect the climate? Ground- and space-based observations now enable us to observe the Sun at virtually any frequency of light and EM radiation. Each part of the spectrum shows us a different aspect of the Sun’s activities. Spectroscopy allows us to observe the different chemicals in the Sun — for instance Helium was detected first through spectroscopic analysis of sunlight. In the ultraviolet part of the spectrum sunspots and flares are bright.
During the 19th century various theories of what powered the Sun were put forward. Heating through the gravitational contraction of a large mass of gas was seen as one possible source, though the lifetime of the Sun would have to be very restricted if this were true. Einstein showed the way by suggesting that mass was equivalent to an immense amount of energy. The theory was confirmed by showing the sun was energised by nuclear fusion.
Our current knowledge of the Sun is very considerable but there are many gaps. Unlike distant stars, the angular resolution of Earth-based telescopes is very good. With 10 m aperture astronomical telescopes seeing is limited to approximately 1 arc second. The biggest solar telescope on earth is the McMath-Pierce instrument in Arizona. The blurring effect of the atmosphere can be eliminated by stacking repetitive photographs. Marek showed close-ups of sunspots and magnetic arches etc. Space-based photographs can cover those parts of the spectrum not observable from the ground.
The SOHO satellite (Solar and Heliospheric Observatory) was launched in 1995 and is still going strong. SOHO was particularly helpful in showing the outer layers of the Sun and enabling helio-seismology of its inner regions. As SOHO is in orbit outside the Earth’s magnetosphere, the solar wind can also be observed directly. Although we are now at solar minimum, SOHO has been able to show in greater detail than ever before the solar cycle and the latitude differences in both sunspots and their associated magnetic disturbances.
STEREO is the Solar TErrestrial RElations Observatory. Two identical spacecraft, orbiting ahead and behind Earth, enable stereoscopic observations; this offers much better observation of Coronal Mass Ejections (CMEs) which have been observed up to now only in two-dimensions with little known about their exact size and direction . This is an important breakthrough. CMEs can overwhelm the magnetosphere. They are bad news for astronauts, satellites etc and can affect power lines by inducing electrical currents which cause disruption of power supplies. Stereo has a built-in coronagraph which images the solar disc and corona. Using the Sun as a backdrop, Stereo can also show us solar system dust flows. Comets and asteroids have been discovered as they near the Sun.
Marek also spoke about the Solar Stormwatch project which is a collaboration between ROG, Rutherford Appleton Laboratories, and Galaxy Zoo. And this project will bring solar observation back to Greenwich. Marek went on to further publicise the 2010 Solar Season; ‘Secrets of the Sun’ planetarium show; spotting sunspots, and public access to the Altazimuth Pavilion used for early solar work.
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