Terms and Definitions

Hall, Asaph 1829-1907: American astronomer. Hall discovered the two moons of Mars, Deimos and Phobos.
Halley, Edmund 1656-1742: English astronomer. Halley applied Newton's laws of motion to predict correctly the period of the comet which now bears his name.
H-alpha: A narrow wavelength of red light which is emitted and absorbed by the element hydrogen; this wavelength is often used to study the Sun.
heliocentric: Sun centered; see Copernicus, Kepler, Galileo.
heliopause: The point at which the solar wind meets the interstellar medium or solar wind from other stars.
heliosphere: The space within the broundary of the heliopause containing the Sun and solar system.
hemisphere: A half of the celestial sphere that is divided into two halves by either the horizon, the celestial equator, or the ecliptic.
Herschel, Sir William 1738-1822: British astronomer. Herschel discovered Uranus and cataloged more than 800 double stars and 2,500 nebulae.
high-pressure mineral phase: In this phase, mineral forms that are stable only at the extremely high pressures typical of Earth's deep interior but not its surface. Such pressures are generated instantaneously during meteorite impact. Stishovite is the high-pressure polymorph of quartz, a common crustal mineral.
hot spot: Center of persistent volcanism, thought to be the surface expression of a rising hot plume in Earth's mantle.
hummocky: Uneven, lumpy terrain.

Dutch physicist and astronomer. Huygens first described the nature of Saturn's rings (1655) and discovered its moon Titan. He also pioneered the use of the pendulum in clocks.