annular solar eclipse: a solar eclipse in which the entire Moon blocks the Sun, but the entire Sun is not blocked by the Moon because, at the time, its angular width is greater.
Apollo missions: NASA’s human space flight programme, which accomplished the first human landings on the Moon from 1969 to 1972.
chromosphere: the diffuse atmospheric layer just above the Sun’s photosphere where the temperature rises from 5777 K to 20,000 K.
corona: the Sun’s thin atmospheric layer which extends millions of kilometres above its chromosphere and reaches temperatures in excess of one million Kelvin.
earthshine: sunlight reflected off the Earth, which we see reflected back at us at twilight as it illuminates the dark side of the Moon. The general term involving other planets and their moons is called planetshine.
earthshine hypothesis: a proposed idea that the difference in crustal thickness between the Moon’s near and far sides may have been influenced by intense irradiation from a hot, young Earth early in lunar history.
eclipse season: one of two times in an eclipse year when the line of nodes aligns with the Sun and eclipses occur at new and full moons.
eclipse year: the time interval that it takes the Sun to circle the ecliptic and return to a node in the Moon’s orbit, which is less than a year due to the precession of the Moon’s orbit.
ESA (European Space Agency): an intergovernmental aerospace agency comprised of 22 member states.
gravity assist: the technique in which a spacecraft uses a planet’s gravity to slingshot itself into a different trajectory.
JAXA (Japanese Aerospace eXploration Agency): The Japanese national aerospace agency.
large-impact hypothesis: the currently accepted hypothesis for the formation of the Moon from a giant grazing impact.
late heavy bombardment: a period of time evidenced by rock samples from NASA’s Apollo missions during which there was a spike of meteorite activity that heavily bombarded the Moon, creating its heavily cratered highlands and causing the lava flows that left behind the maria.
line of nodes: the intersection between the Earth’s and Moon’s orbital planes; in other words, the line connecting the two nodes at which the Moon’s path crosses the ecliptic.
lunar eclipse: an alignment of the Sun, Earth, and Moon, in which the Moon passes through the Earth’s shadow.
lunar phases (new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent, new moon): the cycle of illumination by the Sun of the Moon’s near side while it orbits the Earth.
maria (singular, mare): smooth, dark “seas” on the surface of the Moon thought to be the sites of ancient lava flows caused by giant impact craters.
Mariner 10 mission: NASA’s 1973-1975 unmanned mission to Mercury; also the first interplanetary mission to use gravity assist and the first spacecraft to visit two other Solar System planets.
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission: NASA’s 2004-2015 unmanned mission to Mercury, which orbited the planet for more than four years between 2011 and 2015.
Moon illusion: an illusion caused by the human perception of the sky, causing the Moon to appear larger near the horizon than when it is at higher altitudes.
NASA (National Aeronautics and Space Administration): the United States national aerospace agency.
nodes: the two points at which the Moon’s path crosses the ecliptic.
partial lunar eclipse: a lunar eclipse in which the Moon passes only partway into the Earth’s umbra.
partial solar eclipse: a solar eclipse viewed from a location on the Earth that is in the Moon’s penumbra.
penumbra: the partially shaded outer region of a shadow, from which part of a light source is visible.
penumbral lunar eclipse: a lunar eclipse in which the Moon only passes through the Earth’s penumbra.
photosphere: the opaque layer of the Sun from which its light appears to be emitted.
planetshine: see earthshine.
prominences: streams of gas ejected from the Sun’s photosphere which are often visible during a total solar eclipse.
sidereal period: the period in which the Moon completes its orbit around the Earth.
solar eclipse: an alignment of the Sun, Moon, and Earth, in which the Moon passes between the Earth and Sun, casting its shadow on part of the Earth.
solar sailing: a technique used during NASA’s MESSENGER mission which used the spacecraft’s solar shield as a solar radiation sail in order to conserve fuel.
sputtering: a process whereby energetic particles hitting a solid object cause other particles to be ejected.
synchronous rotation: the condition in which an orbiting body keeps the same face toward its partner (1:1 resonance; e.g., the Moon). Mercury instead is in a 3:2 spin–orbit resonance.
synodic period: the amount of time it takes the Moon to go through a complete cycle phases, which is less than its sidereal period due to Earth’s orbit causing the Sun to shift along the ecliptic.
terminator: the line on the Moon’s visible surface separating its day from night.
terrestrial planets: the four rocky inner planets in our Solar System—Mercury, Venus, Earth, and Mars.
tidally locked: an object is said to be tidally locked to another when tidal forces cause its orientation with respect to the other object to remain fixed.
total lunar eclipse: a lunar eclipse in which the Moon passes entirely into the Earth’s umbra.
total solar eclipse: a solar eclipse viewed from a location on the Earth that is in the Moon’s umbra.
transit: the phenomenon in which an inner planet passes directly between the Earth and Sun, visible as a small silhouette. For Mercury, transits occur roughly 13–14 times per century; for Venus, they occur only in special paired sequences, with the next after 2117.
umbra: the fully shaded inner region of a shadow, from which a light source is totally obscured from view.
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