Back to Examples

Finding the rising position of a star

To find the rising position of a star, simply place it on the eastern horizon (on the left side of the astrolabe) and read off the position using the azimuths. The azimuths are drawn every ten degrees. Those to the left of east are said to be "south of east" while those to the right are said to be "north of east." The same procedure can be used to calculate the setting position of a star--simply place it on the western horizon (on the right side of the astrolabe) and read off its position.

Example: What is the rising position of Spica? To answer this question, rotate the rete until Spica is on the eastern horizon (see below). Spica is about one and a half azimuths to the south of east or 15 degrees south of east. Remember, south is off towards the edge of the astrolabe while north is towards the middle. Now we know to look about 15 degrees to the south of due east to see Spica rise on a given night.