HELENIUM, in botany, a genus of the Syngenesia Polygamia Supergua class and order. Natural order of Composite; Discoidex. Corymbiferx, Jussieu. There are two species. These plants are na tives of America, where they grow wild, in great plenty, in the woods and other shady places where the ground is moist.
ITELlACAL, in astronomy, a term ap plied to the rising or setting of the stars, or, more strictly speaking, to their emer sion out of and immersion into the rays and superior splendour of the sun. A star is said to rise heliacally, when, after having been in conjunction with the sun, and on that account invisible, it comes to be at such a distance from him, as to be seen in the morning before sun-rising, the sun, by his apparent motion, receding from the star towards the east ; on the contrary, the heliacal setting is when the sun approaches so near a star, as to hide it with his beams, which prevent the faint er light of the star from being perceiv ed, so that the terms apparition and oc cultation would be more proper than ri sing and setting.
All the fixed stars in the zodiac, and also the superior planets, Mars, Jupiter, and Saturn, rise heliacally in the morning, a little before sun-rising, and a few days after they have set cosmically. Again,
they set heliacally in the evening, a little before their achronycal setting. But the moon, whose motion eastward is always quicker than the apparent motion of the sun, rises heliacally in the evening, after the new moon ; and sets heliacally in the morning, when old and approaching to a conjunction with the sun.
The inferior planets, Venus and Mer cury, which sometimes seem to go west ward from the sun, and sometimes again have a quicker motion eastward, rise heliacally in the morning, when they are retrograde ; but when direct in their motions, they rise heliacally in the even ing. The beliacal rising or setting of the moon happens when she is seventeen de grees distant from the sun : but for the other planets, twenty degrees are requir ed ; and for the fixed stars, more or less, according to their magnitude.