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Basics of Celestial Motion

Glossary
References: [Arny 2002, Wikipedia]

Each entry in this glossary consists of a simple text definition of various astronomy terms used throughout the website as well as links to the Basics of Celestial Motion website content that references the term. Entries shown in blue correspond to the names of important figures in astronomy. (Keep in mind that Wikipedia is a good place to start when researching new terms or names in more depth.)

A | BCD | E | FGHIJK | KL | MNO | PQR | STUV | WXYZ
A [index]

altitude the height of the Sun above the horizon measured in degrees. Discussed in The Seasons - Sunrise and Sunset (The Earth and the Sun). Tabulated in U.S. Naval Observatory Sun or Moon Altitude/Azimuth Table online tool.

angular or apparent size a measure of how large a celestial body looks to an observer on Earth, which may be computed using the true diameter of the body and its distance from Earth:

Angular Size to Apparent Size Equation

The Moon's Angular size of 0.52 degree is approximately equal to the Sun's angular size of 0.53, such that when the two are aligned the result is a solar eclipse. Discussed in The Moon's Angular Size and Solar Eclipses (The Earth and the Moon) as well as Apparent or Angular Sizes of Planets (The Major Planets).

annular solar eclipse occurs when the New Moon passes through an orbital node at a sufficiently large distance from the Earth. In this case the Moon's angular or apparent size is not large enough to block the Sun's disk completely, so that observer standing in the Moon's antumbra can still view the "outer ring" of the Sun's disk. Discussed in The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

antumbra the area of the Moon's shadow from which an annular eclipse may be viewed. Sometimes called the "negative shadow", it is the portion of a celestial body's shadow that lies behind the umbra and is flanked on either side by the pentumbra . Discussed and diagrammed in The Earth's Shadow and Lunar Eclipses and The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

aphelion the point along the elliptic orbital path at which the Earth or any planet is at its farthest distance from the Sun. Equals to 152.5 million kilometers for the Earth. Discussed in The Earth's Orbit- The First Great Misconception (The Earth and the Moon).

apogee the point along the elliptic orbital path at which the Moon or any Earth-orbiting satellite is at its furthest distance from the Earth. Equals 406,700 kilometers for the Moon. Discussed in The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

asteroid a small rocky celestial body orbiting the Sun and ranging in diameter from a few meters to hundreds of kilometers. Discussed in The Ordinal Solar System (The Major Planets).

asteroid belt the region between Mars and Jupiter containing most of the asteriods in the Solar System Discussed in The Ordinal Solar System (The Major Planets).

astronomical unit (AU) a standard unit of measure used in astronomy equal to 149,597,870.69 kilometers which is the length of the semi-major axis of the Earth's elliptical orbit around the Sun. Discussed in the The Ordinal Solar System (The Major Planets).

axis of rotation the imaginary line running through the center of the Earth from the North to South poles around which the Earth spins. Is titled 23.5 degrees with respect to a line perpendicular to the ecliptic plane of the Earth's orbit around the Sun. Discussed in The Seasons - Summer and Winter (The Earth and the Sun).

azimuth measure of direction along the horizon. North equals 0 degrees azimuth, East equals 90 degrees azimuth, South is 180 degrees azimuth and West is 270 degrees azimuth. Discussed in The Seasons - Sunrise and Sunset (The Earth and the Sun). Tabulated in U.S. Naval Observatory Sun or Moon Altitude/Azimuth Table online tool.

B [index]

Brahe, Tycho Danish astronomer (1546-1601). Credited as being the greatest "naked-eye" astronomer in history, and also one of the last astronomers to hold to the geocentric model of the universe. His meticulous measurements of the position of the planet Mars were used by his assistant Johannes Kepler to propose three laws of planetary motion which form the basis of the heliocentric model of the universe. Discussed in The Earth's Orbit-The First Great Misconception (The Earth and the Sun).

celestial body any naturally-occuring object found in the Universe or "space". Also called a "heavenly body"

C

comet a small body that orbits the Sun which consists of a tiny, icy core and a "tail" of gas and dust. The tail forms only when the comet is near the Sun.

Copernicus, Nicolas (1473-1543) Polish physician and lawyer who first to propose a heliocentric view of the universe in which the Earth and other celestial bodies orbited around the Sun. Discussed in The Earth's Orbit-The First Great Misconception (The Earth and the Sun).

D

E [index]

eccentricity a measure of the "elongation" or "ovalness" of an ellipse with values falling between 0 and 1. If e=0 the ellipse is equivalent to a circle, and as e approaches 1 the ellipse becomes more and more elongated along its semi-major axis. Discussed in Kepler's Three Laws of Planetary Motion (The Earth and the Sun).

eclipse the blockage of sunlight caused as one celestial body passes between the Sun and a second celestial body. A lunar eclipse occurs when the Earth block's sunlight reaching the Moon, and a solar eclipse occurs when the Moon blocks sunlight reaching the Earth. Discussed in The Earth's Shadow and Lunar Eclipses and The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

ecliptic plane the plane of the Earth's elliptical orbit around the Sun. The Earth's axis of rotation is tilted 23.5 degrees with respect to a line perpendicular to the ecliptic plane. The Moon's orbit around the Earth is tilted 5 degrees with respect to the ecliptic plane. Discussed in The Seasons - Summer and Winter (The Earth and the Sun) and The Earth's Shadow and Lunar Eclipses (The Earth and the Moon).

ellipse, elliptical a conic shape defined by two foci or equivalently a semi-major axis and a semi-minor axis and an eccentricity. Defines the shape of each celestial body's orbit around the Sun, as well as the orbit of one celestial body around another. First postulated as the shape of a planetary orbit by Johannes Kepler. Discussed in Kepler's Three Laws of Planetary Motion (The Earth and the Sun).

equator imaginary line bisecting the Earth symmetrically into the Northern and Southern Hemispheres. Is the zero or reference point for measures of latitude.

equinox the two times of the year when the Sun's rises and sets at exactly the same time at every point on Earth, so that the amount of day and night are the same everywhere. More popularly known as the two days of each year when the amount of daytime and nighttime are equal. Generally occurs around March 21 ( vernal equinox) and September 23 ( autumnal equinox) each year. Discussed in The Seasons - Solstices and Equinoxes (The Earth and the Sun).

F [index]

far side of the Moon the part of the Moon that always faces away from the Earth. The Moon rotates on its own axis at about the same rate that it orbits the Earth with the result that only one side of the Moon (actually 59%) is ever visible from the Earth. Sometimes called the "dark side of the Moon". Discussed in Relative Positions of the Earth, Moon and Sun (The Earth and the Moon).

foci, focus the two points located symmetrically along the semi-major axis of an ellipse such that the sum of the distances from the two foci to any point on the ellipse is the same. In a planet's orbit the Sun is located at one focus of the elliptical path. Discussed in Kepler's Three Laws of Planetary Motion (The Earth and the Sun).

G

Galilei, Galileo (1564-1642) Italian scientist who championed a heliocentric view of the universe, and was famously tried by the Holy Inquisition and forced to recant his theories. Discussed in The Earth's Orbit-The First Great Misconception (The Earth and the Sun).

geocentric defines a model of celestial motion in which the Earth is at the center of the solar system and all other bodies orbit around it. The model perpetuated by "ancient or classical" astronomers like Claudius Ptolemy who also advocated circular orbits for all bodies in the solar system. Discussed in The Earth's Orbit - The First Great Misconception (The Earth and the Sun).

H

heliocentric defines a model of celestial motion in which the Sun is at the center of the solar system and all other bodies orbit around it. An important shift in science's understanding of astronomy which was first advocated by Copernicus and supported by Kepler and Galileo. Discussed in The Earth's Orbit - The First Great Misconception (The Earth and the Sun).

I

J

K [index]

Kepler, Johannes (1571-1630) German mathematician and assistant to Tycho Brahe who took Brahe's observations and used them to derive his three laws of planetary motion which characterize a heliocentric model of the universe in which celestial bodies move around the Sun in elliptical orbits. Discussed in The Earth's Orbit - The First Great Misconception (The Earth and the Sun).

Kepler's Three Laws of Planetary Motion three postulates of planetary motion derived by Johannes Kepler based on the meticulous measurements of Tycho Brahe. Discussed in The Earth's Orbit - The First Great Misconception (The Earth and the Sun). These three laws state that:

  1. Planets move in elliptical paths around the Sun with the Sun at one focus.
  2. An imaginary line joining each planet and the Sun will sweep over equal areas in equal amounts of time.
  3. The square of the orbital period of a planet is directly proportional to the cube of its distance from the Sun.

L

latitude imaginary lines parallel to the equator measured in degrees offset from the equator of each point on the Earth. The North pole lies at 90 degrees North latitude and the South pole lies at 90 degrees South latitude.

longitude imaginary lines running from the North to South poles that are measured in degrees offset from the prime meridian which runs through Greenwich England. The Western Hemisphere lies to the west of the prime meridian and extends out to 180 degrees West longitude. The Eastern Hemipshere lies to the east of the prime meridian and extends out to 180 degrees East longitude.

lunar eclipse occurs when the Full Moon passes through an orbital node aligning the Earth between the Sun and Moon causing the Earth's shadow to block direct sunlight to the Moon. Some sunlight is refracted around the Earth's atmosphere, which absorbs the blue wavelength, so that light that reaches the Moon makes it appear a dull, red coppery color. Discussed in The Earth's Shadow and Lunar Eclipses (The Earth and the Moon).

lunar phases see phases of the Moon

M [index]

major planet or planet large celestial bodies that orbit the Sun are considered to be "real" planets by the International Astronomical Union (IAU) as per a given set of criteria (see Wikipedia: Planet). Includes Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Discussed in The Ordinal Solar System (The Major Planets).

minor planet celestial bodies that orbit the Sun but are too small to qualify as major planets. Generally includes comets and asteroids.

N

Newton, Sir Isaac (1642-1727) English physicist and mathematician who expanded on Kepler's laws of planetary motion. Often credit as being the greatest scientist of all time, Newton derived much of modern physics and astrophysics as well as the universal law of gravitation which describes the force between two celestial bodies and forms the basis for all modern rocket science. Discussed in The Earth's Orbit - The First Great Misconception (The Earth and the Sun).

O

orbital nodes the two points in the Moon's orbit around the Earth at which it passes through the ecliptic plane of the Earth's orbit around the Sun. The Moon's orbit is actually titled 5 degrees from the Earth's orbital plane. If the Moon is in New Moon or Full Moon stages as it passes through an orbital node, the result is a solar eclipse or a lunar eclipse, respectively. Discussed in The Earth's Shadow and Lunar Eclipses (The Earth and the Moon).

P [index]

partial solar eclipse occurs when the New Moon passes through an orbital node aligning the Moon between the Earth and the Sun, and is observed as from within the Moon's penumbra as a partial blocking of the Sun's light to the Earth. Discussed in The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

penumbra the portion of a celestial body's shadow that is partially lit by the Sun's light. It flanks the umbra and antumbra, and is that portion of the Moon's shadow from which a partial solar eclipse is visible. Discussed and diagrammed in THe Earth's Shadow and Lunar Eclipses and The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

perigee the point along the elliptic orbital path at which the Moon or any Earth-orbiting satellite is at its closest distance from the Earth. Equals 356,400 kilometers for the Moon. Discussed in The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

phases of the moon, lunar phases the eight stages of the Moon's appearance as sunlight illuminates some portion of its Earth-facing side as it orbits the Earth. The eight stages are the New moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent. Discussed in Phases of the Moon - A Popular Misconception (The Earth and the Moon).

planet see major planet

Ptolemy, Claudius (A.D. 150) Ancient Egyptian astronomer who championed a geocentric view of the universe in which all celestial bodies moved around the Earth in perfect circular orbits. Discussed in The Earth's Orbit - The First Great Misconception (The Earth and the Sun).

Q

R

S [index]

semi-major axis the longer expanse of an ellipse along which the two foci lie. Is perpendicular to the semi-minor axis. Discussed in The Ellipse (The Earth and the Sun).

semi-minor axis the shorter expanse of an ellipse that is perpendicular to the semi-major axis. Discussed in The Ellipse (The Earth and the Sun).

solar eclipse occurs when the New Moon passes through an orbital node aligning the Moon between the Sun and the Earth, causing the Moon to block some or all of the Sun's light to the Earth. This phenomenon is made possible because the Moon's angular or apparent size (0.52 degree) is roughly equal to the Sun's angular or apparent size (0.53 degree). The eclipse is viewable as a total solar eclipse from the Moon's umbra, as a partial solar eclipse from the Moon's penumbra and as an annular eclipse from the Moon's antumbra. Discussed in The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

solstice the two days of each year when the Sun reaches its northern or southern most rising position in the sky and then "stops" and begins its gradual move in the opposite direction. Defines the beginning of summer and winter, and is derived from the Latin terms "sol" meaning Sun and "sistere" meaning "stand still". Discussed in The Seasons - Solstices and Equinoxes (The Earth and the Sun).

T

total solar eclipse occurs when the New Moon passes through an orbital node aligning the Moon between the Sun and the Earth, causing the Moon to block some or all of the Sun's light to the Earth. This phenomenon is made possible because the Moon's angular or apparent size (0.52 degree) is roughly equal to the Sun's angular or apparent size (0.53 degree). The eclipse is viewable as a total solar eclipse from the Moon's umbra. Discussed in The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

U

umbra the portion of a celestial body's shadow that lies in complete darkness because no sunlight is present. Is also the portion of the Moon's shadow in which a total solar eclipse is viewable. Discussed in The Moon's Angular Size and Solar Eclipses (The Earth and the Moon).

universal law of gravitation Fundamental law of physics derived by Sir Isaac Newton which states that the force F exerted on a body of mass m by a larger body of mass M is inversely proportional to the square of the distance d between them scaled by the universal constant of gravity G:

V

W [index]

waning phases the phases of the Moon during which the portion of the Earth-facing side that is illuminated by the Sun is increasing. Discussed in Phases of the Moon - A Popular Misconception (The Earth and the Moon).

waxing phases the phases of the Moon during which the portion of the Earth-facing side that is illuminated by the Sun is decreasing. Discussed in Phases of the Moon - A Popular Misconception (The Earth and the Moon).

X

Y

Z


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