Heliacal Rising

The Heliacal Rising is the moment a Star or Planet becomes visible for the first time after a period of absence, just before Sunrise above the Eastern Horizon.
The opposite occurs at the Heliacal setting of a Star or Planet, when it no longer becomes visible before the Sunrise.

Following a period when a star or Planet has not been visible, its first appearance on the eastern horizon just before sunrise is known as its Heliacal Rising.

From this moment onwards, the star will rise earlier and stay visible for longer before sunrise.
The star then rises approximately one degree further to the west each day, until it is no longer visible, because it has sunk below the horizon. This is known as its Heliacal setting.

One year later, the same star will rise again in exactly the same position, but the day and time may vary, because its observation will depend on the weather conditions.

Prior to the Heliacal Rising of a Star or Planet, a conjunction with the Sun occurs.
The type of conjunction with the Sun may be either a Syzygy, Eclipse, Transit or Occultation.

When a Planet or Star rises in the East during Sunset, this is known as an Acronychal Rising.
This results in an Opposition to the Sun, leading also to a Syzygy or Eclipse.

By 2,100 BCE, the Ancient Egyptians created a Sidereal Star Clock, as a method of telling the time at night.
This was based on the Heliacal Rising of 36 groups of star constellations, known as the Decan Stars, one for every 10° in the 360° of the Zodiac.
Each 10° represented one Decanal ‘hour’ in the Sidereal Star Clock.
As the Earth rotates through 10°, one of the 36 Decans will rise Heliacally in a fixed sequence, and the time in hours and minutes can be calculated.

Every ten days there is a Heliacal Rising of a new Decan Constellation.
The Ancient Egyptians started the year with the Heliacal Rising of the Decan Sirius, followed by a new Decan starting every ten days.
Three Decan Constellations were grouped into each of the twelve divisions of the Zodiac.
Groups of ten day Periods divided the Ancient Egyptian Year.
This Calendar based on ten day divisions is described in the Book of Nut.

This is defined as the Hour angle of the Vernal Equinox at a particular geographical point.
Stars and Planets are located in the night sky using Right Ascension and Declination relative to the Celestial Equator.
This is the night sky equivalent of Latitude and Longitude on the Earth’s surface.