Astro 001 Glossary

Astronomical Unit (A.U.) – average distance of the Earth from the Sun over the

course of a year; equivalently, the semi-major axis of Earth’s orbit

blueshift – the shifting of light to higher (bluer) frequencies due to motion of a

source towards an observer

circumpolar stars – stars that are never seen to set from a given location

conjunction – alignment of two or more bodies as seen from Earth; when only

one planet is mentioned, alignment of that planet with the Sun

deferent – primary circle of motion for the planets, Sun, and Moon in the

geocentric systems of Aristotle, Ptolemy, and others

ecliptic or ecliptic plane – the circle on the celestial sphere defined by the

motion of the Sun with respect to the fixed stars over the course of a year;

equivalently, the plane through the Solar System defined by Earth’s orbit

epicycle – secondary (additional) “circle upon a circle” of motion for the planets,

Sun, and Moon in geocentric systems of Aristotle, Ptolemy, and others

geocentric – Earth-centered (Sun revolving around the Earth)

heliocentric – Sun-centered (Earth revolving around the Sun)

inferior conjunction – when a planet is seen to be in conjunction with the Sun,

and on the near side of the Sun, as viewed from Earth

opposition – when one body is seen to be in the opposite direction from

another body, when viewed from Earth; when only one planet is mentioned,

when that planet is in the opposite direction from the Sun

parallax – the apparent motion of nearby objects with respect to more distant

objects, due to motion of the observer

pole star – when a particularly bright star is located close to the North or South

celestial pole, it may be called a pole star

prograde motion – when a planet moves from West to East with respect to the

fixed stars, as viewed from Earth

quadrature – when a planet (or the Moon) is seen to be at a 90-degree angle

with respect to the Sun, as viewed from Earth

redshift – the shifting of light to lower (redder) frequencies due to motion of a

source away from an observer, or to other physical effects

retrograde motion – when a planet moves from East to West with respect to

the fixed stars, as viewed from Earth

semi-major axis – half the length of the major (longer) axis of an ellipse, usually

denoted as “a”. Note that the semi-major axis of a circle is its radius.

sidereal month – the duration of the Moon’s orbit around the Earth, measured

with respect to the fixed stars

Solar mass – the mass of the Sun

superior conjunction – when a planet is seen to be in conjunction with the Sun,

and on the far side of the Sun, as viewed from Earth

synodic month – the duration of the month, as measured by the Moon’s phases

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Astro 001 Glossary

Astronomical Unit (A.U.) – average distance of the Earth from the Sun over the course of a year; equivalently, the semi-major axis of Earth’s orbit blueshift – the shifting of light to higher (bluer) frequencies due to motion of a source towards an observer circumpolar stars – stars that are never seen to set from a given location conjunction – alignment of two or more bodies as seen from Earth; when only one planet is mentioned, alignment of that planet with the Sun deferent – primary circle of motion for the planets, Sun, and Moon in the geocentric systems of Aristotle, Ptolemy, and others ecliptic or ecliptic plane – the circle on the celestial sphere defined by the motion of the Sun with respect to the fixed stars over the course of a year; equivalently, the plane through the Solar System defined by Earth’s orbit epicycle – secondary (additional) “circle upon a circle” of motion for the planets, Sun, and Moon in geocentric systems of Aristotle, Ptolemy, and others geocentric – Earth-centered (Sun revolving around the Earth) heliocentric – Sun-centered (Earth revolving around the Sun) inferior conjunction – when a planet is seen to be in conjunction with the Sun, and on the near side of the Sun, as viewed from Earth opposition – when one body is seen to be in the opposite direction from another body, when viewed from Earth; when only one planet is mentioned, when that planet is in the opposite direction from the Sun parallax – the apparent motion of nearby objects with respect to more distant objects, due to motion of the observer pole star – when a particularly bright star is located close to the North or South celestial pole, it may be called a pole star prograde motion – when a planet moves from West to East with respect to the fixed stars, as viewed from Earth quadrature – when a planet (or the Moon) is seen to be at a 90-degree angle with respect to the Sun, as viewed from Earth redshift – the shifting of light to lower (redder) frequencies due to motion of a source away from an observer, or to other physical effects retrograde motion – when a planet moves from East to West with respect to the fixed stars, as viewed from Earth semi-major axis – half the length of the major (longer) axis of an ellipse, usually denoted as “ a ”. Note that the semi-major axis of a circle is its radius. sidereal month – the duration of the Moon’s orbit around the Earth, measured with respect to the fixed stars Solar mass – the mass of the Sun superior conjunction – when a planet is seen to be in conjunction with the Sun, and on the far side of the Sun, as viewed from Earth synodic month – the duration of the month, as measured by the Moon’s phases

Astro 001 Laws & Equations

Kepler’s Laws:

The planets move in ellipses with the Sun at one focus
The speed of each planet’s motion varies during the orbit such that equal areas are swept out in equal times by an imaginary line connecting the planet and the Sun
The orbital period measured in years ( P ) and the semi-major axis measured in A.U. ( a ) of each planet satisfies the relation: P^2 = a^3 Newton’s Laws of Motion:
Objects at rest remain at rest, and objects in motion remain in motion, unless subject to an external force
Force is proportional to mass times acceleration, F = m a
Every force has an equal and opposite counter-force Newton’s Law of Gravity: The magnitude of the gravitational force between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G m M / r^2 Newton’s Revision of Kepler’s Third Law: (m + M) P^2 = a^3. Here M is the mass of the Sun and m the mass of the planet, measured in solar masses The Nature of Light:

The speed of light ( c ) is a constant
Frequency and wavelength are inversely proportional: ν = c / λ
Energy per photon is proportional to frequency: E = h ν
Flux decreases as the square of distance because it spreads out over the surface of a sphere: F = L / (4 π r^2 ) Doppler shift: The change in frequency (or wavelength) of light is roughly proportional to the speed of relative motion divided by the speed of light. Simple version: Δν/ν = Δλ/λ = v / c. More complex (accurate) version: Δν/ν = Δλ/λ = ( v / c ) / √(1 + v^2 / c^2 ). Note that +/– signs are ignored in these equations. Blackbody Radiation Equations:

The peak frequency of a blackbody is proportional to its temperature, νpeak = k T
The total emission of a blackbody is proportional to the fourth power of the temperature, L = k T^4

Astronomy Glossary and Laws: Terms, Kepler's Laws, Newton's Laws, and The Nature of Light, Study notes of Astronomy

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Astro 001 Glossary

Astro 001 Laws & Equations