Chapter 2 Flashcards
Heliocentric
sun-centered model for the universe
Magnitude System
system devised by Hipparchus for describing brightness of stars
Prograde motion
eastward motion
Geocentric
Earth centered model for the universe
Epicycle
the circle path a planet travels around
Deferent
the larger circle which the epicycle moves at a constant speed around
Parallax
the shift in apparent position of an object when seen from two different locations
geocentric parallax
the shift in apparent position of a relatively nearby object
Heliocentric parallax
the shift in apparent position of a relatively nearby star when seen from two different points on the Earth’s orbit
Inferior Planets
those with orbits smaller than the Earth’s orbit (Mercury, Venus)
Superior Planets
those with orbits larger than the Earth’s orbit.
Opposition
occurs when the Earth lies between the Sun and the superior planet. The Sun and planet are 180 degrees apart
Conjunction
occurs when the Sun and the superior planet are 0 degrees apart as seen from the Earth
Quadrature
occurs when the Sun and the superior planet are 90 degrees apart as seen from the Earth, can be either eastern or western
Inferior conjunction
occurs when the inferior planet lies between the Earth and the Sun
Superior conjunction
occurs when the Sun lies between the Earth and the inferior planet
elongation
the angle in which the planet is separated from the Sun on the celestial sphere
Synodic period
time interval which can be found by measuring the time elapsed between either successive superior conjunctions or successive inferior conjunctions
Sidereal period
the time it takes the planet to complete one full circuit of the sky relative to the fixed stars.
Greatest elongation
the maximum angular separation between the planet and Sun as seen from the Earth
Astronomical Unit
average distance from the Earth to the Sun
Kepler’s first law
Planets travel on elliptical orbits with the Sun at one focus
major axis
the longest distance across the ellipse
minor axis
the shortest distance across the ellipse
eccentricity
the distance between the foci divided by the length of the major axis, if the foci coincide then the ellipse is a circle, e = 0
Kepler’s second law
A line drawn from the Sun to a planet sweeps out equal areas in equal time intervals
perihelion
the point on a planets orbit closest to the Sun, (moves rapidly)
aphelion
the point farthest from the Sun, (least rapidly)
Kepler’s third law
The squares of the sidereal orbital periods of the planets are proportional to the cubes of the semi-major axis of their orbits.
aberration of starlight
an effect that causes the apparent positions of stars on the celestial sphere to be deflected in the direction of the observer’s motion.
parsec
the distance at which a star has a parallax of exactly 1 arcsecond
