airglow - Light seen in the upper reaches of the Earth’s atmosphere (in the layer called the ionosphere). When high-energy radiation from the Sun collides with molecules in the ionosphere, the molecules release energy in the form of light

altitude - The angular elevation (height) of a celestial object above the horizon

aphelion - The point in the orbit of a body (such as a planet) where it is farthest from the Sun. Opposite of perihelion

apogee - The point in an object’s orbit farthest from the center of the body it is orbiting

asterism - A pattern of stars not considered an official constellation

asteroid - A rocky object orbiting the Sun; smaller than a planet, and larger than a meteoroid, they are considered planetoids. Found in the asteroid belt, between the orbits of Mars and Jupiter

Astronomical Unit (AU) - The mean distance from the Earth to the Sun. One AU equals approximately 93 million miles, or 149,597,870.691 km

aurora - A light-releasing phenonemon that occurs in the upper reaches of a planet’s atmosphere. Caused by the interaction of the atmosphere with energetic particles from the Sun. Electrons from the Sun follow the planet’s magnetic field lines toward its poles, where this phenomenon generally occurs

Autumnal Equinox - Midway between the Summer and Winter Solstices, the date when the ecliptic crosses the celestial equator. Usually September 23

axis - A straight line around which a body, such as the Earth or Moon, rotates

axis - The imaginary line running through the Earth from pole to pole, about which the planet spins

azimuth - The point directly over an observer’s head

caldera - A crater formed by an explosion or collapse of a volcanic vent

celestial equator - The Earth’s equator projected onto the imaginary celestial sphere

celestial meridian - Imaginary line running north to south through an observer’s zenith

celestial sphere - Imaginary sphere surrounding the Earth. Depicts alls celestial objects as if they were the same distance from the Earth

chromosphere - The lower level of the solar atmosphere, between the photosphere and corona

circumpolar - Circumpolar stars always remain above the horizon, appearing to circle the north celestial pole or south celestial pole

coma - Cloud of gas that surrounds a comet

comet - A ball of dust and ice, smaller than a planet, that orbits the Sun in a long, elliptical orbit

constellation - A pattern of stars. Eighty-eight constellations were made official in 1929 by the International Astronomical Union. The exact patterns may vary, but their boundaries have remained constant

convection - Circulation driven by temperature gradients (differences) in the presence of gravity. Cooler, denser material sinks while hotter, less dense material rises

Copernicaus, Nicolaus - (1473 - 1543) Polish astronomer who advanced the theory that our solar system is heliocentric--the Earth and other planets revolve around the Sun

crater - A hole in the ground created by the impact of an object such as a meteorite

declination - Angular distance of an object north or south of the celestial equator, measured in degrees

density - A measurement of how much mass is contained in a given volume. Measured in grams per cubic centimeter

direct sunlight - Sunlight that falls perpendicular or nearly perpendicular to the ground

due - Directly, exactly

ecliptic - (1) The imaginary plane containing the Earth’s orbit around the Sun. (2) The Sun’s apparent yearly path among the stars, as seen from Earth

ellipse - An oval

elliptical orbit - An oval-shaped orbit. Planets in our solar system follow elliptical orbits around the Sun, and some satellites are placed into elliptical orbits. A satellite or planet in an elliptical orbit is sometimes closer to the object it orbits

emit - To send out or give off, as in light or radio waves

energetic particles - Electrons and nuclei of atoms that are traveling extremely fast. They pose a threat to satellites, astronauts, and even Earth-based power and communication systems

equinox - The moment when the center of the Sun crosses the celestial equator, which occurs twice a year. The word literally means "equal night," referring to the equal hours of day and night all parts of the Earth receive on those two dates (around March 21 and September 23)

flare - An eruption of energy on the solar disk lasting minutes to hours

friction - The force that resists the rubbing together of two surfaces

galaxy - System of stars, gas, and dust held together by gravity. Galaxies can have from 1 million to 1 trillion stars, and their shapes can be including spiral, elliptical, or irregular. The Milky Way Galaxy, in which we live, is a spiral galaxy of over 200 billion stars

Galileo Galilei - (1564-1642) Italian astronomer and physicist credited with being the first to use telescopes to observe the heavens. He discovered the four largest moons orbiting Jupiter, proving that all bodies in the solar system were not in orbit around the Earth

greatest elongation - The largest separation between Venus or Mercury and the Sun, as seen from Earth

heliocentric - Sun-centered. It was once believed that the Earth was the center of the solar system, Copernicus was first to propose that the Sun was actually the center

horizon - Where the ground and sky seem to meet

Hubble Space Telescope - HST. An Earth-orbiting optical telescope designed to observe distant objects as well as those in our solar system

indirect sunlight - Sunlight that strikes the ground at an angle. The light and its heat are therefore spread out over a larger surface area than if it had struck the ground directly

inferior planets - Those planets with orbits between the Earth and Sun: Mercury and Venus

interstellar - The space between stars

Jovian planets - The gas giants: Jupiter, Saturn, Uranus, and Neptune. These are also the "outer planets." Kelvin - Kelvin (K). Temperature scale in which 0 K is absolute zero. Water melts at 273 K and boils at 373 K

kilogram - (kg)Unit of measure. 1 kg = 1,000 grams = 2.2 pounds Kuiper Belt - Region beyond the orbits of Neptune and Pluto where Kuiper Belt Objects (KBOs) reside until they are knocked into new orbits and become comets

latitude - Angular distance north or south of the equator, measured in degrees

lidar - Acronym for LIght Detection And Ranging. Similar to radar but using visible wavelengths of light which are bounced off of an object (such as moisture in the atmosphere) and detected

light-year - An astronomical unit of measure for the distance light travels in one year. Equal to 5,880,000,000,000 miles (5.9 trillion miles/9.5 trillion km)

lunar month - Synodic month. Period it takes the moon to go through all of its phases, i.e., from one full moon to the next full moon. The lunar month is 29 days, 12 hours, and 44 minutes long

macula - A dark spot. In astronomy, macula refers to the darkest part of a sunspot

magnitude - The degree of brightness of celestial objects, designated on a numerical scale. The brightest objects have low magnitudes, fainter objects have high magnitudes. The average human eye can observe objects as dim as 6th magnitude

magnitude - Used to describe the brightness of a celestial object. On the magnitude scale, the brightest objects have low magnitudes, the fainter objects have high magnitudes. The average human eye can observe objects as dim as 6th magnitude

meridian - (1) A great circle of the celestial sphere passing through its poles. (2) A great circle on the surface of the Earth, passing through the poles, numbered on maps to represent longitude

meteor shower - A number of meteors with approximately parallel trajectories or paths. The meteors belonging to one shower appear to come from the same place in the sky

meteorite - An extraterrestial particle that impacts the surface of a planet or moon

meteoroids - Extraterrestial particles, smaller than asteroids, that orbit the Sun

Milky Way - The spiral galaxy that contains our solar system

navigate - To steer a course, usually in a boat or airplane

north celestial pole - The point on the celestial sphere that is directly over the Earth’s North Pole

Oort Cloud - Theoretical (undiscovered) cloud of objects beyond the Kuiper Belt, believed to be the source of long-period comets

opposition - Position of a planet when it is on the opposite side of the Earth from the Sun, so that the Earth is between the Sun and that planet. Only the outer planets can be in this position

penumbra - 1. In a sunspot, the penumbra is the lighter outer region surrounding the cooler, darker umbra. 2. The partial shadow surrounding a perfect shadow (umbra), as during an eclipse

perhihelion - The point in the orbit of a body (such as a planet) where it is closest to the Sun. Opposite of aphelion

perigee - The point in an object’s orbit nearest to the center of the body it is orbiting

perihelion - The point in its orbit when a satellite (such as a planet) is closes to the Sun. (Opposite of aphelion.) perspective - The appearance to the eye of objects in respect to their relative distance and positions

perturb - Causing a planet or satellite to deviate from its "normal" orbital motion

phenomena - Plural form of phenomenon. (1) A rare or significant fact or event. (2) A fact or event of scientific interest

plane - A flat or level surface

plane of the ecliptic - A plane is a flat or level surface. The path of the Earth’s orbit is also called the ecliptic, and it is flat

planet - Object in orbit around a star that are bigger than an asteroid. There are nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto

planisphere - Star wheel. A flat representation of the celestial sphere

resolution - The amount of detail visible in an image

resonance - The periodic gravitational effects of one orbiting object on another

retrograde - Moving backward compared with the general motion of similar bodies. When the superior planets appear to reverse their normal direction

revolve - To orbit

rings - Bands of rocky and/or icy fragments revolving around planets such as Jupiter, Saturn, Uranus, and Neptune

rotate - To spin or turn around an axis

rotation - The time it takes for a body to make one complete turn on its axis. Earth takes 24 hours to make one rotation. Our Moon takes one month to make a single rotation

satellite - A small body that orbits a larger body. The Moon is a natural satellite of the Earth. The Earth also has many artificial satellites for communications, weather, and data collection

sidereal - Relating to the stars. A sidereal day is the length of time it takes the Earth to make one rotation in relation to the stars. The sidereal day is about 4 minutes shorter than a solar day because the Earth moves forward in its orbit while rotating on its axis

sidereal month - The average period of revolution (orbit) of the Moon around the Earth in relation to a fixed star. (Equal to 27 days, 7 hours, and 43 minutes

silicate - A compound containing silicon and oxygen

solar nebula - Clouds of gas and dust that condensed to form the solar system

solar system - A system made up of a star and the celestial bodies that orbit it

solar wind - Stream of particles from the Sun that fills the spaces between objects in our solar system

south celestial pole - The point on the celestial sphere that is directly over the Earth’s South Pole

stellar classification - Stars are given a designation (consisting of a letter and a number) that corresponds roughly to surface temperature. The classes are O, B, A, F, G, K, and M; O stars are the hottest, M the coolest. Our Sun is designated a G2 star

sublimation - When a solid passes directly into the vapor state without passing through the liquid state

Summer Solstice - Solstice means "Sun standing still." On the Summer Solstice, the Sun appears at its most northern position along the horizon at sunrise and sunset and reaches its highest midday altitude for the year. The North Pole is tilted most toward the Sun. Usually June 21

sunspot - A dark spot, generally circular, on the photosphere of the Sun. Sunspots appear dark because they are cooler than the surrounding photosphere and therefore emit less light. The darkest, central region of a sunspot is called the umbra, the lighter, outer region is called the penumbra

superior planets - Those planets having orbits beyond the orbit of the Earth: Mars, Saturn, Jupiter, Uranus, Neptune, and Pluto

terrestrial planets - Earth-like, rocky planets. The terrestrial planets in our solar system are Mercury, Venus, Earth, and Mars

turbulent - Irregular atmospheric motion, especially up-and-down currents

ultraviolet - Wavelength of light more energetic than visible light, is largely blocked by the Earth’s atmosphere

umbra - 1. The dark, central region of a sunspot. 2. The darkest, central part of a shadow cast by the Moon on the Earth during a solar eclipse, or the Earth on the Moon during a lunar eclipse

Universal Time (UT) - UT is used to refer to the time kept at longitude zero. This time is given in terms of a 24-hour clock and varies from local time depending on how far you are from longitude zero. There are actually two versions of UT: UT1 is based on the rotation angle of the Earth and is used for astronomical and navigational usage; Coordinated Universal Time (UTC) is based on atomic clocks and is used for most civilian purposes. UTC is adjusted periodically so that it never differs from UT1 by more than 0.9 second

Vernal Equinox - Midway between the Winter and Summer Solstices, the date when the ecliptic crosses the celestial equator. Usually March 21

white dwarf - Star with a high surface temperature, low intrinsic brightness, and a high density

Winter Solstice - Solstice means "Sun standing still." On the Winter Solstice, the Sun appears at its most southern position along the horizon at sunrise and sunset and reaches its lowest midday altitude for the year. The South Pole is tilted most toward the Sun. Usually December 21

young - In astronomy, used to describe a relatively recent surface feature on a planet. The surface of the Earth is considered young because it has changed significantly since the planet was formed; the cratered surface of Mercury is considered old

zenith - Point directly over an observer’s head