For the observation of the stars we have to consider the motion of the Sun. As obvious when the Sun is in conjunction or close to star or planet, it makes them invisible. It must be remembered that due to the revolution of the Earth around the Sun, the Sun for an observer on the Earth appears to move among the stars, to be considered fixed for their long distance, from west to the east. The projection of this motion on the celestial sphere is called Ecliptic, as mentioned in previous paragraph.
The Sun, projected onto the celestial sphere has an apparent motion around the Earth (i.e., the sun appears to move relative to the fixed stars). The Sun moves eastward with respect to a fixed star of about one degree every day. In terms of time we can say that a fixed star anticipates its passage through the same point of the sky about four minutes a day with respect to the Sun or similarly we can say that the Sun rises every day a little later, 4 minutes, then the stars. So, if a constellation in a given year rises and sets with the sun is not visible, but after six months (twelve hours) it will rise before the Sun and will be visible throughout the night.
This is true for all the stars and planets near the Ecliptic.
The same occurs with the planets, by which we speak about Maximum elongation, i.e. East elongation when the planet follows the sunset, West elongation when it precedes the sunrise.
Based on these apparent motions of the stars relative to the Sun, there are four heliacal phenomena: the heliacal rising and sunset, the acronical sunrise and sunset.
1) The heliacal rising of a star is the first day of visibility of the star before the sunrise. As the Sun rises on the eastern horizon, a star, to be visible, must fulfill two conditions to avoid being overwhelmed by sunlight. In height or latitude, the star should find a few degrees above the local horizon, while the Sun is still a few degrees below it (the sky is already illuminated by the light of the next rising Sun). This is called “arcus visionis” minimum height (negative height that the Sun must have to allow a star to be visible at sunrise or sunset). It is measured taking into account the magnitude of the star or planet. Second condition is that the star must be shifted a few degrees in longitude, then to the west, so it results at the minimum degree of elongation west of the Sun.
Met these minimum requirements, we can look for the first time at the star before the sunrise after a long period it could not be observed. The visibility will be very short: soon after the light of the Sun below the horizon, will make the star invisible, but it is the first time that the star is visible in the year. From that day the Sun in its apparent motion along the Ecliptic, moves eastward one degree per day (see paragraph is Ecliptic) and increases its distance from the star, resulting in a delayed rising. Over the following days the star will move further and further westward (one degree per day, increasing its west elongation) from the rising Sun and it will be longer and longer visible, up to all night before the sunrise.
2) The heliacal set of a star is the last day of visibility of the star when it goes below the horizon after sunset. Both Sun and the star are therefore on the western horizon, where the Sun goes below horizon just before the star, which remains visible for a very short time. Due to its apparent motion, the Sun sets every day before the star and the angular distance between the Sun and star decreases day by day as the Sun is always moving towards the east (towards the star) by one degree per day. So there will be a day when the distance between the Sun and star is so little that immediately after the sunset, the star will be visible only for few moments and in following days the star will no longer be visible. The Sun will go down at the same time and then even earlier, but its brightness will have prevented the display of the star. Thus, the heliacal setting of a star is the situation in which the star can be seen in the western sky after sunset for the last time during the year.
3) The achronical rising of a star is the first day of visibility of a star when the star rises in the east as soon as the Sun goes below the horizon. As the sky becomes dark after the sunset, then the star becomes visible.
4) The achronical set of a star is the last day of visibility of the star because the stars go below horizon and the Sun rises, overwhelming the star with its light and making it no more visible. To summarize, a planet or a star are visible throughout the year in the period from the heliacal rising to the heliacal sunset, and the opposite for the period of invisibility.
Not all the stars follow these phenomena.
The circumpolar stars are visible all year round in the sky.
The heliacal events were commonly observed by ancient populations, while the achronic events were not followed because of the higher error in their observation and because the heliacal phenomena can be better predicted.
The last ones were therefore used for the creation of the first calendars in order to schedule the major holidays, the ritual events including the several agricultural and pastoral activities, the dates for hunting and fishing, navigation and travel. For example, Hesiod, a Greek poet from the eighth and seventh century BC in his famous work "The Works and Days", mentions several times the heliacal phenomena connected to human activities and the Egyptians regarded the heliacal rising of Sirius as a reference for the beginning of the Nile flood, so fundamental for agriculture, but further examples can be found among all ancient civilizations in China, India and Babylonia.