The ascendant of a given geographic location at a given point in time is the point on the zodiac which coincides with â€” i.e., is ascending over â€” the eastern horizon as viewed from that place and time. The Greek term from which the word ascendant derives from is hÅroskopos, which means 'hour-marker' because it marks the hour of birth.
The astrological ascendant, or rising sign, is the zodiacal sign that was ascending on the Eastern horizon at the moment of birth. For example, if an individual was born exactly at the moment of sunrise and the Sun was in Leo at that moment, the ascendant would be in the same sign as the Sun-sign, so the person would have Leo "rising" (or Ascendant in Leo) in addition to having the Sun in Leo.
Briefly, the ascendant is the precise degree (01 of 360) of the zodiac rising over the eastern horizon of the birthplace at the moment of birth. There are exactly 30 degrees per astrological sign, and because 12 multiplied by 30 equals 360 which is the span of the ecliptic, all twelve signs rise and fall during the course of one 24-hour day.
Because all twelve of the zodiac signs rise and set during one 24-hour day and about one degree of the ecliptic rises about every four minutes, each sign takes approximately two hours to rise and set depending on the time of year; thus, because it takes the earth about 24 hours to do a full 360 degree turn on its axis, all twelve signs rise and set each day. However, each sign does not take the same amount of time to rise, and there is a substantial amount of importance attributed to the actual ascension times of the signs in Hellenistic and Medieval astrology.
Due to its swift nature in moving approximately one degree every four minutes, an exact birth time is vital to calculating the ascendant with accuracy.
When looking at a modern western natal chart, the ascendant is located on the left hand side of the chart, usually on or near the cusp of the first house of the astrological chart â€” at the nine o'clock position. In most quadrant systems of house division, this exact rising degree forms the first house cusp of the chart, although in whole sign houses the entire sign that the ascendant is found in becomes the first house.
The ascendant is said to be of great significance with regards to the character, life and the body of the individual. The planet that rules the astrological sign rising in the East at the moment of birth is often called the "chart ruler" in modern western astrology. All traditions of horoscopic astrology place an extra amount of emphasis upon the planet that rules the sign on the ascendant.
The ascendant is of great significance in most schools of astrology due to the fact that it serves as the filter through which everything in a natal chart- including the Sun and Moon- is expressed. Thus, many astrologers believe that the rising sign exerts an influence equal to or more powerful than that of the Sun and Moon signs.