Each day after the heliacal rising, the star will rise slightly earlier and remain visible for longer before the light from the rising sun makes it disappear ( the sun appears to drift eastward relative to the stars by about one degree a day along a path called the ecliptic ).

It is still possible to estimate when Thutmose II's reign would have begun by means of a heliacal rise of Sothis in Amenhotep I's reign, which would give him a reign from 1493 BC to 1479 BC, although uncertainty about how to interpret the rise also permits a date from 1513 BC to 1499 BC, and uncertainty about how long Thutmose I ruled could also potentially place his reign several years earlier still.

In the ninth year of Amenhotep I, a heliacal rise of Sothis was observed on the ninth day of the third month of summer.

Three specific observations of the heliacal rise of Sirius are extremely important for Egyptian chronology.

Determining the date of a heliacal rise of Sothis has been shown to be difficult, especially considering the need to know the exact latitude of the observation.

Another problem is that because the Egyptian calendar loses one day every four years, a heliacal rise will take place on the same day for four years in a row, and any observation of that rise can date to any of those four years, making the observation not extremely precise.

The tablet recorded the rise times of Venus and its first and last visibility on the horizon before or after sunrise and sunset ( the heliacal risings and settings of Venus ) in the form of lunar dates.

The reason given was that the rise of Sothis advances to another day in every 4 years, so that attaching the beginning of the year to the heliacal rising of the star Sirius would keep the calendar synchronized with the seasons.

Sopdu's name, meaning with Sopd, derives from this heat arriving shortly after the star Sirius has its heliacal rising, and thus being seen as coming with Sopdet, Sopdet being the deification of Sirius ( Sopd is the masculine form of Sopdet, t being the feminine determinant ).

This significance led the Egyptians to base their calendar on the heliacal rising of Sirius.

The lunar calendar began with the heliacal rising of Sirius, which during the time of the ancient Egyptians occurred around September 20 ( according to the Julian calendar — in Egypt the Sothic year happens to be of the same length as the Julian ); the four months of their lunar calendar are roughly equivalent to the period from the rising of Sirius to the middle of November.

The months of the lunar calendar are roughly equivalent to the period from early May to the heliacal rising of Sirius around the beginning of September.

The ancient Egyptians based their calendar on the heliacal rising of Sirius and devised a method of telling the time at night based on the heliacal risings of 36 stars called decan stars ( one for each 10 ° segment of the 360 ° circle of the zodiac / calendar ).

It has been suggested that this may be related to the fact that the Egyptians regulated their year by the heliacal rising of the star Sirius, a phenomenon which occurs in the morning just before dawn.

A tablet from the reign of First Dynasty King Djer ( c. 3000 BC ) was conjectured by early Egyptologists to indicate that the Egyptians had already established a link between the heliacal rising of Sirius ( Egyptian Sopdet, Greek Σείριος Seirios ) and the beginning of the year.

The first inundation according to the calendar was observed in Egypt's first capital, Memphis, at the same time as the heliacal rising of Sirius.

According to Roman writer Censorinus, the Egyptian New Year's Day fell on July 20 in the Julian Calendar in 139 CE, which was a heliacal rising of Sirius in Egypt.

For example, the ancient Egyptians used the heliacal rising of Sirius to predict the flooding of the Nile.

The Dog Days originally were the days when Sirius rose just before or at the same time as sunrise ( heliacal rising ), which is no longer true, owing to precession of the equinoxes.

The Old Farmer's Almanac lists the traditional period of the Dog Days as the 40 days beginning July 3rd and ending August 11th, coinciding with the ancient heliacal ( at sunrise ) rising of the Dog Star, Sirius.

During a Sothic cycle, the 365-day year loses enough time that the start of the year once again coincides with the heliacal rising of the star Sirius ( the Latinized name for Greek Σείριος, a star called Sopdet by the Egyptians, in Greek transcribed as Sothis ; a single year between heliacal risings of Sothis is a Sothic year ).

One also needs to know the place of observation, since the latitude of the observation changes the day when the heliacal rising of Sirius occurs, and mislocating an observation can potentially change the resulting chronology by several decades.

In astrology, noted for making predictions and having written the Kitab bulugh al-umniyya (" On the Attainment of Desire "), a work concerning the heliacal risings of Sirius, and on predictions concerning what day of the week the Coptic year will start on.

* 1460 years – length of the Sothic cycle ; the time it took the ancient Egyptian calendar to realign with the heliacal rising of Sirius

Because heliacal rising occurs in the east, and the sun's heat begins there daily, Sopdu was referred to as Lord of the East, and had his greatest cult centre at the easternmost nome of Lower Egypt, which was named Per-Sopdu, meaning place of Sopdu.

A 584-day Venus cycle was also maintained, which tracked the heliacal risings of Venus as the morning and evening stars.

A heliacal rising of Sothis was recorded in the reign of Thutmose's predecessor, Amenhotep I, which has been dated to 1517 BC, assuming the observation was made at Thebes.

The rising of a constellation just before sunrise ( its heliacal rising ) was considered the last hour of the night.

Chinese astronomy was equatorial, centered as it was on close observation of circumpolar stars, and was based on different principles from those prevailing in traditional Western astronomy, where heliacal risings and settings of zodiac constellations formed the basic ecliptic framework.

Cleomedes states: The band of the Zodiac ( zōdiakos kuklos, " zodiacal circle ") is at an oblique angle ( loksos ) because it is positioned between the tropical circles and equinoctial circle touching each of the tropical circles at one point … This Zodiac has a determinable width ( set at 8 ° today ) … that is why it is described by three circles: the central one is called " heliacal " ( hēliakos, " of the sun ").

The MUL. APIN, contains catalogues of stars and constellations as well as schemes for predicting heliacal risings and the settings of the planets, lengths of daylight measured by a water-clock, gnomon, shadows, and intercalations.

The Egyptians may have used a luni-solar calendar at an earlier date, with the intercalation of an extra month regulated either by the heliacal rising of Sothis or by the inundation of the fields by the Nile.

The term heliacal circle is used for the ecliptic, which is in the center of the zodiacal circle, conceived as a band including the noted constellations named on mythical themes.

Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.

Some stars, when viewed from a particular latitude on Earth, will not have a heliacal rising or setting.

Every summer, at the start of the Egyptian year, Sothis ' heliacal rising, in which the star rose above the horizon just before the sun itself, heralded the start of the Nile inundation, which watered and fertilized Egypt's farmland.

To the Māori of New Zealand, the Pleiades are called Matariki and their heliacal rising signifies the beginning of the new year ( around June ).

By the third millennium BCE, widespread civilizations had developed sophisticated awareness of celestial cycles, and are believed to have consciously oriented their temples to create alignment with the heliacal risings of the stars.

Calendars that are not solar calendars include the Islamic calendar, which is a purely lunar calendar and calendars synchronized to the synodic period of Venus or to the heliacal risings of stars.

( See heliacal rising.

The heliacal rising (, ) of a star ( or other body such as the moon, a planet or a constellation ) occurs when it first becomes visible above the eastern horizon for a brief moment just before sunrise, after a period of time when it had not been visible.

The same star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising.

The Sumerians, the Babylonians, and the ancient Greeks also used the heliacal risings of various stars for the timing of agricultural activities.

For most Earth observers, the heliacal rising of Regulus occurs in the first week of September.

Every 8 years, Venus passes Regulus around the time of the star's heliacal rising, most recently in 2006.