Sun disk
The image of the sun as it appears from the earth is called the sun disk . Except near the horizon , its shape appears to be circular , the apparent diameter of the sun is around 0.5 ° (31'28 "- 32'32").
Geometric-physical aspects
Seen from earth, the sun is by far the brightest celestial body, its apparent brightness is up to −27 mag ( magnitudes ). In contrast to other self-luminous stars , it does not appear to the free-eyed observer as a point, but as a flat light source. Because at an average distance of around 150 million kilometers, around 1 AU , the radiant surface of the sun, its photosphere measuring around 1.4 million kilometers , can still be seen from an angle of around half a degree (1.4 / 150 ≈ tan 0.534 °). This apparent solar diameter of about 32 '( arc minutes ) for the visible solar disk corresponds to an actual solar diameter for the plasma sphere that is over a hundred times larger than the diameter of the earth .
Because of its high luminosity , the sun is difficult to observe with the naked eye when the sky is clear, and the view through clouds or high fog only offers the image of a round disk . Only in the telescope and with an appropriate solar filter which is limb darkening visible to us the impression of a glowing sphere conveys. The sunspots also contribute when they take on elongated form near the edge of the sun and even look a little deepened in large telescopes.
The solar disk appears to us to be about the same size as the moon disk at the phase of the full moon - the apparent diameter of both is about half a degree. Although the actual diameter of the moon is just under 3,500 kilometers, about four hundred times smaller than that of the sun, the earth is about four hundred times farther away from it than from the moon, whose mean distance from the earth is about 383,400 kilometers. The fact that the moon appears at a similar angle (29'10 "- 33'30") as the sun makes a total solar eclipse possible when the satellite moves between the sun and earth.
The fact that the apparent size of the sun disk varies somewhat - between 31'28 "( aphelion , beginning of July) and 32'32" ( perihelion , beginning of January) - was proven by measurements in the decades after 1610 with the telescope types that were just invented . The reason for these fluctuations is the different distance from the sun over the course of the year. But the eccentricity of the earth's orbit (approx. 0.017) is not only reflected in the fluctuating size of the sun disk. The variation in the orbital velocity of the earth during its orbit is also reflected in a variation in the angular velocity of the apparent movement of the sun.
Even ancient astronomers knew that the apparent movement of the sun in front of the starry sky - along the ecliptic through the zodiac constellations - was not completely uniform , despite the difficulty of measuring it. When reading sundials , these differences are taken into account with the equation of time . The first two Kepler laws provided the theoretical explanation in 1609.
As our central star but rotated , which is the radius of the solar equator little longer than that to the poles. However, this flattening of the sun as a result of its own rotation of almost four weeks is very small. It is difficult to determine from the earth's surface due to temperature-dependent disturbances in the atmosphere. It could only be proven in the last few decades, although a special measuring instrument, the heliometer , was developed for this as early as the 19th century . It was then mainly used to measure very small astronomical angle differences .
Sunrise and sunset
When the sun is high , you can only look at the solar disk with danger to your eyes . If you still want to give it a try and don't have a filter at hand, you can use your thumb , index and middle finger to form a tiny triangular aperture and make it so narrow that the disk of the sun can just be seen between your fingertips .
Almost everyone, on the other hand, knows the sight of the sun disk when it wanders through the horizon as a reddish oval around the time of sunrise or sunset . The clear deviation from the circular shape is due to the curvature of the light rays in the earth's atmosphere . This astronomical refraction causes it to be close to the horizon
- the solar disk appears to have been lifted by about 0.6 ° - more than its diameter indicates. Without the layered atmosphere of the earth, the setting sun would no longer be visible. Your apparent set is calculated for a sun position of -50 'below the horizontal plane.
- the lower edge of the sun appears more elevated than the upper edge - which leads to the oval shape. The refraction is greater near the ground and increases with the zenith distance ; at an elevation angle of 10 ° it is about 1 ', at only 0.5 ° above the sea horizon it is about 29'.
- In addition, appearances of graduated and distorted parts of the solar disk appear - up to fascinating images of apparent deposits and teardrop-shaped detachments. Depending on the weather, these are caused by refraction anomalies of different temperatures, water or dust-containing layers of air in the lower atmosphere.
Duration of rise and fall
As a result of the elliptical orbit of the earth , the size of the solar disk also changes in the course of the year, but only slightly because of the low eccentricity. The maximum difference in the course of the year is around one-thirtieth of about 1 ', so that the duration of the process in which the solar disk crosses the horizon changes only slightly. The influence of the declination δ (seasonal distance of the sun from the celestial equator ) and the effect of the respective geographical latitude B of the observation site are considerably greater .
Near the earth's equator , the sunsets pass faster than in Central Europe, and the twilight is also much shorter. Both are related to the steeper parallactic angle q under which the day arc of the sun - like the apparent path of other stars - intersects the horizon. At the equator ( B = 0 °) this angle is 90 °, at the turning circle ( B = 23.4 °) it is 90 ° - 23.4 ° = 66.6 °, while in Central Europe, for example, it is only approximately at the 50th parallel 40 °.
The set time can be roughly calculated from the intersection angle q and the sun disk diameter d with 1 / ( d • sin q ). If the solar disk were exactly 0.5 °, it would set in equatorial regions of Africa or Brazil in 2 minutes, in the Sahara in 2¼ minutes and in Central Europe in 3–4 minutes. The influence of the declination δ (−23.4 ° to + 23.4 °), which fluctuates with the seasons, must also be taken into account ; beyond the polar circle ( B = 66.6 °) the sun no longer sets every day in summer .
History and mythology
Historical astronomy
Historical astronomy knows various prehistoric measuring methods and structures in which the size of the solar disk played a role. The optimal thickness of the shadow stick ( gnomon ) of a good sundial is also related to the apparent size of the sun: if the stick is 1 m long, it should be at least 2 cm thick in order to cast a clear shadow .
Also indicates the accuracy with which the standing stones of Stonehenge were facing particular horizon points to a careful consideration of the size sun out. It has not yet been definitively researched whether the Nebra Sky Disc had similar functions.
Egyptian and Greek mythology
The Egyptian mythology knows the two lions called Akeru (also Sef and Tuau, or Xerefu), who guard the gates between sunset and sunrise in the “overworld” . They are represented as a sphinx with two heads facing away, which are connected by a symbolic solar disk.
Similar notions of a geocentric view of the world , how the sun moves from the west to the east at night, can also be found in the mythological sun chariot of ancient Greece - see Phaeton (mythology) . In stark contrast to these religious ideas are the views of materialists such as Xenophanes - who viewed the sun as a fiery cloud - or Anaxagoras , who even called it a glowing stone. These considerations, strongly hostile to the environment, could be seen as cautious beginnings of astrophysics , although mythical explanations of the sun's shape soon prevailed again in Hellenism .
These include the ancient Egyptian sun god Aton and the crocodile god Sobek , the ruler of the water. The Egyptians worshiped the crocodiles as sacred animals and idolized them in the shape of the crocodile-headed god Sobek (Souchos). This deity was a symbol of eternal survival - see also the punctual flooding of the Nile every year - and counted around 2400 BC. To the most important gods in the Egyptian pantheon . The representation as a human with a crocodile head changed in the following millennium of the New Kingdom : around 1400 BC. He wears a headdress with an incorporated sun disk and was considered to be an embodiment of the sun god Ra (also Sobek-Ra). How important the worship of the sun was to Egyptian civilization can also be seen from royal names such as Nofru sobek or Sobek hotep and from special hieroglyphics . The sun god Shamash was also worshiped in powerful Mesopotamia .
Germanic and Celtic mythology
The golden sun disk of Moordorf was found in March 1910 by Vitus Dirks near the peat ditch. He misjudged their value and gave them to his children to play with; a few years later a dealer bought them as scrap and sold them on. It was not until 1926 that the State Museum in Hanover managed to acquire the disc after there was already a risk that it would be sold abroad.
The disc has a diameter of 14.5 centimeters and a weight of 36.17 grams. In the middle it has an originally bulging hump, on the edge of which there are eight small nail-head-like bulges. This is followed by a circle made up of radial rays, a circle of again eight small humps, another circle of rays and finally a circle filled with 32 hatched triangles. Two opposite tabs suggest that the disk was originally attached to a pad. The prevailing opinion is that it is a symbol of the sun , which was venerated in prehistoric times as the giver of life.
The question of the function of this disk leads to Denmark to the Trundholm sun chariot . Its gold disc, rounded like a disc, is mounted on a bronze disc that is pulled by a horse. The animal only pulls the disc, while both can be moved by the wheels. In the religion of the older Bronze Age, the horse draws the sun over the firmament . A manufacturing technique like that used for the Moordorf disc is alien in Lower Saxony . It was probably driven on the cold route from the rear without a model. Most of the discs of this type come from Western Europe , particularly Ireland . So this disc not only provides information about aesthetics, artistic creation, metalworking techniques and religion in the Bronze Age, but is also an example of the wide-ranging relationships during this time.
Among the Celts and Teutons , the sun cross , disc wheel and horse-drawn bronze carriages with golden sun discs (finds from Trundholm , Moordorf ) testify to extensive sun worship. In the Germanic legal system, courts could only be held “when the sun was shining”. In the worldview of the entire north, the sun was the producer of light, warmth and life, fertility and, above all, the regulator and divider of time. The course of the year was accompanied by festivals. She therefore became a personal deity.
The Banc Ty'nddôl solar disk was discovered in 2002.
literature
- Volker Bialas : From heavenly myth to world law (463 p.), Ibera-Verlag, Vienna 1998
- GD Roth: Kosmos Astronomie-Geschichte (190 p.), Kosmos-Franckh, Stuttgart 1987
- H. Karttunen et al .: Fundamental Astronomy. 2nd edition, Springer, Berlin-Heidelberg-New York 1994
Web links
- Wolfgang Schröppe: The sun disks of the Bronze Age . In: Moorschmied.de.