Edge of the sun
As the solar limb is called in astronomy and geodesy the visible edge of the solar disk . From the astronomical point of view , it is the upper limit of the photosphere , that outer layer of the sun that still shines in integral (white) light.
From the point of view of the surveyor, however , the edge of the sun is an ideal long-term goal with which a surveying network can be absolutely oriented on about half of the days . Since the sun appears practically circular , the direction to the center of the sun can be obtained simply by measuring to its right and left edge.
Visual appearance
If you look at the sun in a telescope (with a dark filter or with the projection method ) you can see:
- mostly a few sunspots - which become oval towards the edge and look a little deepened
- a noticeable darkening of the brightness at the edge of the sun
- rapidly changing waviness of the edge due to the unrest in the air
- when the sun is low, an oval image of the sun instead of a circle.
While the first-mentioned phenomenon is primarily of interest to professional and hobby astronomers , the last one particularly appeals to photographers . It goes back to the astronomical refraction , which increases very strongly towards the horizon .
The second phenomenon is due to the radiation of a gas -like body together and are each photo of the sun a plastic impression. A solid body in spherical shape hardly shows such a darkening of the edge .
The third phenomenon is a consequence of the atmospheric turbulence and is directly related to the quality of the astronomical view . The air turbulence is usually stronger during the day than at night - and naturally particularly intense when the sun is shining. Because if the sky is not or only slightly cloudy and a lot of energy hits the earth's surface, the resulting temperature differences cause more air turbulence than when the sky is overcast. The stronger the edges of the sun "roll", the stronger z. B. also the updrafts and downdrafts when gliding or in the mountains.
Safe methods of solar observation
The simplest method of observing the sun is to project it through the eyepiece onto a sheet of white paper. The focus is done on the eyepiece or by changing the distance. The only disadvantage is the grain of the paper, which becomes harmless if it is gently circled.
The use of solar filters (coated glasses or foils) is more complex and expensive . However, they should not be placed behind the eyepiece because they could melt or shatter in the heat there. The filters must therefore be mounted in front of the lens , which, however, requires larger dimensions. Special eyepieces for more comfortable observation are the Herschel or Pentaprism and the geodetic Roelof prism .
If flares or protuberances are to be observed in addition to sunspots and the edges of the sun , special color filters (e.g. an H-alpha filter ) or shading parts are required in the telescope's beam path. The prominences telescope creates an artificial edge of the sun that is slightly larger than the solar disk and thus shields most of the solar radiation. Recently, small H-alpha telescopes have been offered on the market that are quite affordable for amateur astronomers .
Measurement
Astronomical measurement
The angle measurement across the sun edges is the oldest and still most accurate way to determine the Sun's diameter . If you know the distance to the sun (which varies from 147 to 152 million km every year) and if you have measured the apparent size of the solar disk (angle ), the diameter follows the sun
The angle is about 0.53 ° (31'28 "to 32'32") and the sun distance can z. B. can be calculated from Kepler's laws if the astronomical unit (AU = 149.598 million km) can be considered known. A century ago, however, they were only known to within three to four places, so that the “ scale ” of the entire solar system was incorrect by a few per mille. This was one of the reasons why astronomers prefer to give distances to planets and moons in AU rather than km.
A second task that can be solved by measuring the angle of the sun's edges is to investigate whether the sun has an exact spherical shape. Since it rotates slowly (depending on the latitude in 25–30 days), it should be slightly flattened. However, it has not been proven for a long time because the air turbulence (see phenomena 3 and 4 above) reduce the accuracy. The often assumed small variations in the size of the sun are also difficult to detect. B. the measurement of the Earth's moon , which appears almost the same size, is more accurate than that of the largest celestial body in our planetary system.
The edge of the sun is also used for other methods of astronomy, which include:
- The measurement of the contact times ( 1st to 4th contact ) during a passage through Mercury and Venus
- The analysis of solar eclipses and
- associated determination of the lunar rim profile
- Measurement of the location and rate of ascent of protuberances
- Measurement of the Doppler effect by spectral analysis at opposite edges of the sun, from which one can derive the exact speed of rotation
Geodetic measurement
In geodesy, measuring the direction to the sun is a simple means of establishing a survey network absolutely - i.e. H. orientate exactly to astronomical north . The two edges of the sun are measured with a theodolite by stopping their transit times on the vertical thread of the instrument. Times and directions are averaged (which corresponds to a fictitious measurement of the center of the sun) and then their azimuth is calculated. The spherical trigonometry gives formulas for this purpose, which include not only the time of observation, but also the geographical latitude and longitude as well as variables that depend on the date .
Similar calculations also have to be made when designing a sundial or when an architect needs the approximate sunshine duration for planning a settlement or a high-rise building .
The accuracy of a geodetic azimuth to the sun is about 0.001 ° if the measurements are taken carefully and repeated a few times. It is completely sufficient for the alignment of a small measuring network or the measurement of a plot of land , but the geodesist normally uses 2–3 measurement points in the area for this . Sometimes, however, they cannot be found or are destroyed - or the view is obstructed by vegetation , forest or tall buildings. Then the solar method is an economical substitute and of a similar quality to a GPS survey, which requires significantly higher investments.
The direction measurement of the edges of the sun has other useful applications, which, however, occur less often:
- Staking out or measuring isolated line structures (e.g. lines) in forest areas or in developing countries
- Control of slow movements and turns, e.g. B. in glaciology
- rapid determination of building directions, for example in archeology
- Location determination on expeditions (in addition to the direction, the elevation angle to the sun is measured here)
- with some space probes scanning the solar disk in order to determine the flight path from its direction and the too bright stars .
History
Even in prehistoric times, individual measurements with such high precision were likely that the apparent angle of the sun's disk already played a role. The accuracy with which the menhirs of Stonehenge were set up is sometimes so high that either the shadow direction of the edges of the sun or at least the center of the somewhat blurred shadow was taken into account. They even allow a back calculation of the ecliptic skew .
The fact that many artistic depictions of the sun show a disk also points to a deeper astronomical knowledge. The best known example of this is the Nebra Sky Disc from the Bronze Age .
In ancient Egypt and Mesopotamia, precise observation of the apparent path of the sun ( equinox , solstices , etc.) was a prerequisite for creating accurate calendar systems . The setting out of the pyramids - some of which only deviate from the south by a few minutes of arc - indicates a high level of measurement technology .
In this context, the hypothesis of Xenophanes should also be mentioned, who described the sun as a fiery cloud - in contrast to his contemporaries, who saw it as a supernatural appearance. Behind his (at that time highly hostile) knowledge there is likely to be a method of observing the solar disk and / or an experiment on the effects of solar radiation . The view of the sun as an object of physics was u. a. further developed by Anaxagoras , who called it a glowing stone. Later, however, mythical explanations prevailed again.
Thales of Miletus succeeded around 600 BC. Chr. An accurate prediction of a solar eclipse. The underlying Meton cycle could only be derived so reliably by the astronomers of Babylon through careful observation of the course of numerous eclipses.
Aristarchus of Samos tried around 200 BC To calculate the distance to the sun from special angle measurements at the time of the half moon phases . He must have dealt not only with averaging the edges of the moon , but also the sun. His idea that it represented the real world center was only taken up 1500 years later by Nikolaus von Kues , Regiomontanus and Nicolaus Copernicus .
It would be interesting to research when the corona that can be seen during solar eclipses was first related to the solar diameter . Since such observations are also possible with free eyes , this may have been long before the invention of the telescope . Even with pinhole cameras , the sun disk could be depicted centuries ago. The earliest observations of large sunspots handed down from China do not yet relate them to the edge of the sun. B. Galileo's drawings did from 1610 . The priority of such measurements also claimed Thomas Harriot and Johann Fabricius ; the latter interpreted the migration of the sunspots from the east to the west edge for the first time as a solar rotation .
literature
- Alfred Berroth , Walter Hofmann: Cosmic Geodesy . Braun, Karlsruhe 1960.
- Rudolf Kippenhahn : The star we live on. On the trail of the sun's secrets . Deutsche Verlagsanstalt, Stuttgart 1990, ISBN 3-421-02755-2 .
- Rudolf Sigl : Geodetic Astronomy . 4th edition, Wichmann, Karlsruhe 1991, ISBN 3-87907-190-X .
- Wolfgang Mattig: The sun . Beck, Munich 1995, ISBN 3-406-39001-3 .
