Chapter 2 Flashcards
Summer (June) solstice
Refers both to the point on the celestial sphere where the ecliptic is farthest North of the celestial equator and to the moment in time when the Sun appears at that point each year (around June 21st).
The moment when the Northern Hemisphere is tipped most directly toward the Sun and receives the most direct sunlight.
Winter (December) solstice
Occurs around Dec. 21st; the moment when the Northern Hemisphere receives the least direct sunlight.
Refers both to the point on the celestial sphere where the ecliptic is farthest south of the celestial equator and to the moment in time when the Sun appears at that point each year (around Dec. 21st).
Spring (March) equinox
Occurs around March 21st; The moment when the Northern Hemisphere goes from being tipped slightly away from the Sun to being tipped slightly toward the Sun.
Fall (September) equinox
Occurs around Sept. 22nd; The moment when the Northern Hemisphere first starts to be tipped away from the Sun
Altitude (above horizon)
The angular distance between the horizon and an object in the sky.
Angular size (or angular distance)
A measure of the angle formed by extending imaginary lines outward from our eyes to span an object (or the space between two objects).
Annular solar eclipse
A solar eclipse during which the Moon is directly in front of the Sun but its angular size is not large enough to fully block the Sun; thus, a ring (or annulus) of sunlight is still visible around the Moon’s disk.
Apparent retrograde motion
The apparent motion of a planet, as viewed from Earth, during the period of a few weeks or months when it moves westward relative to the stars in our sky.
Arcminute (or minute of arc)
1/60 of 1º
Arcsecond (or second of arc)
1/60 of an arcminute, or 1/3600 of 1º
Azimuth (usually called direction in this text)
Direction around the horizon from due North, measured clockwise in degrees. For example, the azimuth of due North is 0º, due East is 90º. due South is 180º, and due West is 270º.
Celestial Equator (CE)
The extension of Earth’s equator onto the celestial sphere.
Celestial sphere
The imaginary sphere on which objects in the sky appear to reside when observed from Earth.
Circumpolar star
A star that always remains above the horizon for a particular latitude.
Constellation
A region of the sky; 88 official constellations cover the celestial sphere.
Direction (in local sky)
ONe of the two coordinates (the other is altitude) needed to pinpoint an object in the local sky. It is the direction, such as North, South, East, or West, in which you must face to see the object. See also azimuth.
Ecliptic
The Sun’s apparent annual path among the constellations.
Eclipse seasons
Periods during which lunar and solar eclipses can occur because the nodes of the Moon’s orbit are aligned with Earth and Sun.
Eclipse
An event in which one astronomical object casts a shadow on another or crosses our line of sight to the other object.
First-quarter (phase)
The phase of the Moon that occurs one-quarter pf the way through each cycle of phases, in which precisely half of the visible face is illuminated by sunlight.
Gibbous (phase)
The phase of the Moon (or of a planet) in which more than half but less than all of the visible face of the moon is illuminated by sunlight.
Geocentric universe (ancient belief in)
The idea that Earth is the center of the entire universe.
Horizon
A boundary that divides what we can see from what we cannot see.
Latitude
The angular north-south distance between Earth’s equator and a location on Earth’s surface.
Local sky
The sky as viewed from a particular location on Earth (or another solid object). Objects in the local sky are pinpointed by the coordinates of altitude and direction (or azimuth).
Longitude
The angular east-west distance between the prime meridian (which passes through Greenwich) and a location on Earth’s surface.
Lunar eclipse
An event that occurs when the Moon passes through Earth’s shadow, which can occur only at full moon. A lunar eclipse may be total, partial, or penumbral.
Meridian
A half-circle extending from your horizon (altitude 0 degrees) due south, through your zenith, to your horizon due north.
Nodes (of Moon’s orbit)
The two points in the Moon’s orbit where it crosses the ecliptic plane.
North celestial pole (NCP)
The point on the celestial sphere directly above Earth’s North Pole.
Parallax
The apparent shifting of an object against the background, due to viewing it from different positions.
Partial lunar eclipse
A lunar eclipse during which the Moon becomes only partially covered by Earth’s umbral shadow.
Partial solar eclipse
A solar eclipse during which the Sun becomes only partially blocked by the disk of the Moon.
Penumbra
The lighter, outlying regions of a shadow.
Penumbral lunar eclipse
A lunar eclipse during which the Moon passes only within Earth’s penumbral shadow and does not fall within the umbra.
Phase (of matter)
The state determined by the way in which atoms or molecules are held together; the common phases are solid, liquid, and gas.
Phase (of the Moon or a planet)
The state determined by the portion of the visible face of the Moon (or of a planet) that is illuminated by sunlight. For the Moon, the phases cycle through new waxing crescent, first-quarter, waxing gibbous, full, waning gibbous, third-quarter, waning crescent, and back to new.
Precession
The gradual wobble of the axis of a rotating object around a vertical line.
Prime meridian
The meridian of longitude that passes through Greenwich, England; defined to be longitude 0 degrees.
Rotation
The spinning of an object around its axis.
Retrograde motion
Motion that is backward compared to the norm. For example, we see Mars in apparent retrograde motion during the periods of time when it moves westward, rather than the more common eastward, relative to the stars.
Synchrotron radiation
A type of radio emission that occurs when electrons moving at nearly the speed of light spiral around magnetic field lines.
Synchronous rotation
The rotation of an object that always shows the same face to an object that it is orbiting because its rotation period and orbital period are equal.
Solar eclipse
An event that occurs when the Moon’s shadow falls on Earth, which can occur only at new moon. A solar eclipse may be total, partial, or annular.
Saros Cycle
The period over which the basic pattern of eclipses repeats, which is about 18 years 11 1/3 years.
South celestial pole (SCP)
The point on the celestial sphere directly above Earth’s South Pole.
Stellar Parallax
The apparent shift in the position of a nearby star (relative to distant objects) that occurs as we view the star from different positions in Earth’s orbit of the Sun each year.
Totality (eclipse)
The portion of a total lunar eclipse during which the Moon is fully within Earth’s umbral shadow or a total solar eclipse during which the Sun’s disk is fully blocked by the Moon.
Total solar eclipse
A solar eclipse during which the Sun becomes fully blocked by the disk of the Moon.
Total lunar eclipse
A lunar eclipse is which the Moon becomes fully covered by Earth’s umbral shadow.
Third-quarter (phase)
The phase of the Moon that occurs three-quarters of the way through each cycle of phases, in which precisely half of the visible face is illuminated by sunlight.
Umbra
The dark central region of a shadow
Waning (phase)
The set of phases in which less and less of the visible face of the Moon is illuminated; the phases that come after full moon but before new moon.
Waxing (phases)
The set of phases in which more and more of the visible face of the Moon is becoming illuminated; the phases that come after new moon but before full moon.
Zenith
The point directly overhead, which has an altitude of 90 degrees.
Zodiac
The constellations on the celestial sphere through which the ecliptic passes.
