HELIACAL

, something relating to the sun. Thus,

Heliacal Rising, of a star or planet, is when it rises with, or at the same time, as the sun. And Heliacal setting, the same as the setting with the sun.

Or, a star rises Hcliacally, when, after it has been in conjunction with the sun, and so invisible, it gets at such a distance from him as to be seen in the morning before the sun's rising. And it is said to set Heliacally, when it approaches so near the sun as to be hid by his beams. So that, in strictness, the Heliacal rising and setting are only an apparition and occultation.

HELICE Major and Minor; the same as Ursa Major and Minor.|

HELICOID Parabola, or the Parabolic Spiral, is a curve arising from the supposition that the common or Apollonian parabola is bent or twisted, till the axis come into the periphery of a circle, the ordinates still retaining their places and perpendicular positions with respect to the circle, all these lines still remaining in the same plane. Thus, the axis of a parabola being bent into the circumference BCDM, and the ordinates CF, DG, &c, still perpendicular to it, then the parabola itself, passing through the extremities of the ordinates, is twisted into the curve BFG, &c, called the Helicoid, or Parabolic Spiral.

Hence all the ordinates CF, DG, &c, tend to the <*>entre of the circle, being perpendicular to the circumference

Also, the equation of the curve remains the same as when it was a parabola; viz, putting x = any circular absciss BC, and y = CF the corresponding ordinate, then is , where p is the parameter of the parabola.

HELIOCENTRIC Place of a Planet, is the place in which a planet would appear to be when viewed from the sun; or the point of the ecliptic, in which a planet viewed from the sun would appear to be. And therefore the Heliocentric place coincides with the longitude of a planet viewed from the sun.

Heliocentric Latitude of a Planet, is the inclination of the line drawn between the centre of the sun and the centre of a planet, to the plane of the ecliptic. The greatest Heliocentric Latitude is equal to the inclination of the planet's orbit to the plane of the ecliptic.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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HEAT
HEAVEN
HEAVINESS
HEGIRA
HEIGHT
* HELIACAL
HELIOCOMETES
HELIOMETER
HELIOSCOPE
HELIOSTATA
HELIX