By studying the relative aspects of the current planetary positions and the aspects in ones' natal chart certain astrological observations can be made.
Aspects can be categorized in two groups:
Dynamic or Challenging Aspects
This refers to the square (90°), opposition (180°), quincunx (150 the conjunction (0°) and the semi-sextile (30°). These are also known as hard aspects and denote certain harsh or hard periods in our life.
Harmonious or Flowing Aspects
This refers to the trine (120°), sextile (60°), and some of the conjunctions (0°) (depending of the planets involved) and semi-sextiles (30°). These aspects are generally benefic.
Do you know that the planetary aspects are measured using a cosmolabe??. This cosmolabe was first made by Jacques Besson in 1567.