Flamsteed Astronomy Society
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.
Read More at —