偕日升是一颗恒星（也可以是月球或行星）经过隐藏在地平线下一段时间或一定的周期之后，首度在拂晓时又出现在东方地平在线（即合之后），或正好在地平在线但隐藏在太阳的光芒之中的现象。在偕日升之后的每一天，这颗恒星出现的时间将一天比一天早一点，出现在天空中不被阳光屏蔽的时间也同样的日渐增长（太阳相对于恒星每日沿着黄道向东移动），过一段日子后，这颗恒星在黄昏时已移至地平线并西落，这时称为偕日落。一颗恒星在历经大约一年的时间后会再在东方天空偕日升。

并非所有的恒星都有偕日升：有许多恒星（与观测者在地球上的地理纬度有关）始终在地平线之上，永远都能在阳光淹没前于黎明的曙光中看到（即在恒显圈内），同样也有许多恒星始终地平线下看不见的（即在恒隐圈内）。

The heliacal rising (/hɪˈlaɪəkəl/, hi-LY-ə-kəl) of a star occurs annually 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.

Relative to the stars, the sun appears to drift eastward about one degree per day along a path called the ecliptic. While the sun is moving past a given star, the star cannot be seen because it is only above the horizon during the day. The heliacal rising occurs when the sun has moved far enough past the star that the star rises and becomes visible before the sun rises in the morning. Each day after the heliacal rising, the star will rise slightly earlier and remain visible for longer before the light from the rising sun overwhelms it. Over the following days the star will move further and further westward (about one degree per day) relative to the sun, until eventually it is no longer visible in the sky at sunrise because it has already set below the western horizon. This is called the cosmical setting. The same star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising. Because the heliacal rising depends on the observation of the object, its exact timing can be dependent on weather conditions.