1) the fact Earth - Sun distance changes
2) the rotation of the Sun
3) the rotation axis of the Earth being inclined by 23.4° to the ecliptic
4) the rotation axis of the Sun being inclined by 7.25° to the ecliptic
This can be calculated once we have
1) the apparent diameter of the Sun
2) the heliographic latitude B0 of the centre of the Sundisk
3) the heliographic longitude L0 of the centre of the Sundisk
4) the position angle P of the north end of the axis of rotation, when positive it is east of the north point
In a later blog I will post more details about this calculation. Below is the Sun (with Sunspot) including the position angle. The software used was HelioViewer from Peter Meadows.
North is defined by taking two pictures with tracking modus off when using an equatorial set up. By drawing a line between the two sunspots and turning this horinzontal, we find north - south of the Sun.
The "north" positioned Sun (BMP format) is uploaded within HelioViewer. Once your settings (position, time) are entered, the software calculates all coordinates and makes a final drawing of the Sun with Heliographic coordinates.
