When taking a sight by sextant, the important thing is the angle between the position lines.

So if we are taking the Sun sight, we need two position lines separated by as close as possible to 90 degrees. This is done by taking first sight in the morning and calculating it by Long by Chron method. Then take the second sight at the Noon and calculating it by MERPAS method.

Long by Chron will give us position line running in N-S direction (as Obs long) and MERPAS will give us position line in E-W direction (as obs latitude).

While taking star sight (that includes planets and moon), we need to choose 3 stars which are at least 45 degrees apart from each other. This ensures that the position lines that we will get by star sight are not running parallel to each other.