Planisphere
As planisphere which is central projection of the celestial sphere refers to a layer.
The projection usually takes place from one of the two celestial poles , whereby the area around the projection center cannot in principle be reproduced. If, as in the astrolabe, the stereographic projection of the sky is applied to the so-called rete , one is content with fully grasping the ecliptic circle. When projecting from the South Pole, the northern sky is recorded up to the southern tropic , from the North Pole the southern sky up to the northern tropic.
Some modern star maps use azimuthal equidistant projection , which is not a true projection. With it, it is possible to map up to the opposite pole as a circle on the outer edge, but this is a distortion that is unrealistic. The image is therefore only extended just beyond the ecliptic. The advantage is that the declination circles appear evenly spaced around the center of rotation. In contrast to the stereographic projection, the images of the horizon and the ecliptic are not circles.
history
Hipparchos discovered that, in a stereographic projection conformal image of the sky is possible, but has with increasing distance from the pole always larger area error.
The construction of planispheres is z. B. described by Ptolemy . Although no ancient planispheres have survived, specimens from the 8th to 10th centuries are probably direct copies of ancient models.
The world map made by Paolo dal Pozzo Toscanelli in 1457 and the world map acquired by Alberto Cantino in 1502 are also called the planisphere.
The flat astrolabe, known since the Middle Ages, serves not only as a measuring instrument, but also to represent the rotating starry sky. Its rete contains a few selected stars and is just as rotatable as the disc of a modern star map.
Rotatable star map
The part of a rotatable star map visible through the window is the starry sky that can be observed from a certain geographical latitude over the course of the year. The edge of the window is the image of the horizon. Most of the maps intended for Germany are designed for the latitude 50 ° North, on which Frankfurt am Main is roughly located. Some manufacturers offer a range between 40 ° south and 60 ° north in steps of 10 ° for the northern hemisphere. Only the window is different, the pane with the stars can be used repeatedly.
Construction with two panes
A simple star map consists of only two panes, with the smaller cover pane containing the window. Sometimes a rotating pointer is also attached as an adjustable memory aid.
Star disk
The sky is depicted on the star disk (made of cardboard or plastic): constellations , individual stars up to the 4th or 5th magnitude and some larger celestial objects such as star clusters and nebulae . Everything is there that can be observed at a certain latitude over the course of a year. For a star map for 50 ° north latitude, this is the northern sky and a strip up to 40 ° south latitude (+ 50 ° −90 ° = −40 °). The ecliptic circle is highlighted above the circles of latitude and longitude lines .
Several concentric scales can be attached to the edge:
- an annual calendar (12 months, 365 days) as the essential scale for setting the moment of observation,
- a scale that differs from the above annual calendar at times (maximum about 4 °), which takes into account the equation of time for observing the sun, the scale for the true sun ,
- a right ascension scale (in ° for the right ascension angle, in hours for the sidereal time ).
Cover disk
Only the currently visible part of the sky appears in your window. The nearly circular window edge represents the horizon. The covered part of the sky is in reality below the horizon. The edge of the window is labeled with the four cardinal points. The south curve (the upper edge in the upper picture) is longer than the north curve (the lower edge in the upper picture). The four cardinal points have the reverse order as usual. The reason for this is that the gaze is directed to the sky, not to the earth as in normal use:
“Imagine you are lying on the ground on a starry night and looking
straight up at the sky. Your feet point to the south. Then
right is west and left is east, exactly as it is shown on the sky map. "
The edge of the star disk with its scales protrudes from under the cover disk. The time scale with 24 hours on the cover plate corresponds to these scales.
Pointer
There is a declination scale on it . If the position (right ascension and declination) of an object is known, the rod can be rotated to the right ascension. The object is at the intersection with the declination.
Construction with three panes
Using three disks creates a more stable star map. The star washer is located in a housing consisting of a cover and washer with slots at the edge from which it protrudes so that it can be touched for turning.
use
Setting the moment of observation
As seen from Earth, the starry sky makes 1 ⁄ 365 more than one turn every 24 hours (a sidereal day only lasts about 23 h 56 m 4 s ). The view of the starry sky is therefore not only dependent on the time of day, but also on the season, which requires the corresponding setting of the star map: The cover and star disks are rotated against each other so that the time and date coincide. The pointer can be used as a setting aid.
Daylight saving time requires a correction of one hour. The longitude difference to 15 ° East (reference degree for the CET ) has to be considered with a shift of four minutes per degree. In the “Rotatable Kosmos Star Map” ( Kosmos-Verlag ), mean local time in Frankfurt am Main (8 ° 41 'East) has already been selected for the time scale . Example: Munich (11 ° East), July 10th, 10:00 p.m.
CET is 9:00 p.m. (22−1 = 21). The local time in Munich is 16 minutes earlier ((15−11) 4 = 16). The time to be set is 8:44 pm.
observation
The set star map coincides with the current sky if you hold it up in front of you so that the North Star is exactly behind the center of the map. The solid angle shown is much larger than the human angle of vision (it is more like a fisheye ). To view the outer parts of the currently visible sky, it is necessary to turn your head in a different direction. Conveniently, you have the star map lying horizontally in front of you and look at a piece of sky between the horizon and the zenith, and you cannot avoid turning your head up and down. You have that window edge of the map on your side that belongs to the horizon part in whose direction you have positioned yourself.
Readings
For example, the time of sunset is to be read on July 10th in Munich. The pointer is placed on July 10th on the true sun scale. The pointer, western horizon and ecliptic are brought into line and the time is read: 7:58 p.m. Now the time corrections must be taken into account with the opposite sign . The sun sets an hour and 16 minutes later after CEST, which means 9:14 p.m.
See also
Web links
- Star map for download as a Postscript file
- Star map for download as a pdf file
- Usage information
- Rotatable star map with current positions of the sun, moon and planets (JavaScript)
Individual evidence
- ↑ "2.2 Why are east and west reversed on the sky map?" ( Memento of the original from March 29, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , astroviewer.de, accessed on January 11, 2012
