ASTR 341
Mid-Term Exam
Terms
undefined, object
copy deck
- circumpolar
- A star that never sets due to its proximity to either the North or South poles
- declination
- angular distance measured from the equator to a star along the star's hour cycle - designated as positive to the north of the equator, negative to the south, and is measured in degrees
- heliacal rising
- first appearance of a star after its invisibility due to conjunction with the sun
- Orion
- especially three stars in belt
- celestial equator
- prolongation of the earth's equator, or plane of rotation, onto the celestial sphere - 90 degrees distant from the celestial poles at all points
- synodic month or lunation
- 29.5 days - relative to sun
- celestial poles
- extension of the poles of rotation of the earth onto the celestial sphere
- superior conjunction
- configuration of a planet in which it is obscured by passage behind the sun
- saros cycle
- one of a number of cycles of similar seasonal eclipses that recurs after a period of about 18.03 years - combining synodic and draconic periods to give you an eclipse pattern over time
- Saturn
- a
- Gregorian calendar
- Pope Gregory 13th, 1582 - Had to get rid of 10 days in October (jumped from Oct 4th to 14th) to realign vernal equinox with March 21st - Also had ot make years more fitting = leap year every four years unless divisible by 100 - However, if it's divisible by 400 then it is a leap year (lose 3 days/400 years = .0075 days/year shortened from 365 days/year = 365.2425 days, closer but still not exact)
- sidereal month
- 27.3 days - relative to background stars
- penumbra
- portion of a shadow from which part of a light source is occulted by a body
- opposition
- configuration of a body when it is opposite the sun, that is, at elongation = 180 degrees
- lunar eclipse interval
- usually 6 lunations or 177 days, sometimes 5
- total eclipse
- Solar = occurs when the sun is completely obscured by the moon (earth is in moon's umbra) or Lunar, new moon phase or Lunar = when the moon is completely obscured by the earth (moon travels into earth's umbra), full moon phase
- autumnal equinox
- September 22 - point of intersection of the ecliptic and celestial equator where the sun passes from the Northern to the Southern Hemisphere
- retrograde motion of planet
- apparent short-term westward motion of a planet on the sky relative to the stars
- precession
- slow conical motion of the earth's axis of rotation about the poles of the ecliptic resulting in a motion of the celestial poles among the stars in a cycle of approximately 26,000 years
- Origin of weekday names *
- Sunday (sun), Monday (moon), Tuesday (mars), Wednesday (mercury), Thursday (jupiter), Friday (venus), Saturday (saturn) - Given names from romance languages (French, Latin) - Goes back to Babylonians and five naked-eye planets plus sun and moon
- Milky Way
- our galaxy - appears as a band of fuzzy light, like cloud - Ancient cultures saw this as a river to Underworld - goes all the way around the sky, most dramatic in Southern Hemisphere
- Tropic of Cancer
- parallel of latitude 23.5 degrees North (+)
- line of nodes
- 173 days (1/2 eclipse year - have to have full moon, so average is around 177 days) the points of the intersection of the orbit of one body with that of another - the ascending and descending nodes of the lunar orbit refer to the points of intersection of the orbit with the ecliptic, where the moon crosses the ecliptic passing to the north and to the south respectively
- winter solstice
- December 21 - point on the celestial sphere where the sun reaches its greatest distance south of the celestial equator
- superior planets
- Mars, Jupiter, Saturn, Uranus, and Neptune - orbits lie outside of that of the Earth - can be viewed at any angle relative to the sun
- regression of lunar orbit
- 18.61 years - westward (backward) movement of the nodes (of the lunar orbit) along the ecliptic
- synodic period
- interval between successive configurations of a body relative to the sun
- angular diameter
- defined in terms of the object's actual size and the angular size of the object as viewed from Earth
- occultation
- eclipse of a star or planet by the moon or another planet
- Venus synodic period
- 584 days - relative to sun
- sidereal period
- interval between successive passages of a body by a given star
- tropical year
- 365.24 days "seasonal year" - period of revoluation of the earth about the sun with respect to the vernal equinox
- Polaris
- at position of North Celestial Pole currently, approximately greater than 1,000 years ago it was not so special (due to precesion)
- zenith and nadir
- zenith = point directly overhead (opposite the direction of a plumb line), nadir = point underfoot, opposite the zenith
- altitude
- angular distance measured positive upward from the horizon to the star along the star's vertical circle
- umbra
- central, completely dark part of shadow
- elliptical orbit
- variable speed - as planets get closer to the sun they speed up, as they get farther away, they slow down
- annular eclipse
- Ooccurs when the Sun and Moon are exactly in line, but the apparent size of the Moon is smaller than that of the Sun (because it is further from the sun in its orbit). Hence the Sun appears as a very bright ring, or annulus, surrounding the outline of the Moon.
- draconic month
- 27.2 days - interval between successive passages of the moon by a given node of its orbit
- refraction
- objects will appear higher than they are, light from star bends in atmosphere, atmosphere slows light down
- Julian calendar
- Ceaser - Sosigenes in Alexandria = leap year (3 years of 365 days, 1 year of 366 days) and aligned with vernal equinox - In effect for hundreds of years, 365.25 days a year (leap year every four years), time of Easter = first Sunday after first full moon after vernal equinox, in comparison with tropical year if you go over 1,000 years, will be off by about 8 days
- summer solstice
- June 21 - point on the celestial sphere where the sun reaches its greatest distance north of the celestial equator
- Arctic circle
- +66.5 degrees
- extinction
- "absorption of light" - objects will appear fainter as light gets closer to the horizon - very with time, ex: volcano
- partial eclipse
- Solar = occurs when the moon only partially obscures the sun or Lunar = occurs when only a portion of the moon enter's into earth's umbra
- sidereal year
- 365.25 days - longer than tropical year/solar year - period of revolution of the earth about the sun relative to the background stars
- vernal equinox
- March 21 - point of intersection of the ecliptic and celestial equator where the sun passes from the Southern to the Northern hemisphere
- Pleiades
- very tight group of seven visible stars
- inclination of moon's orbit to ecliptic
- +/_ 5 degrees
- Antarctic circle
- -66.5 degrees
- supernova
- birth of new stars, especially important to Chinese
- Tropic of Capricorn
- parallel of latitude 23.5 degrees South (-)
- obliquity
- tilt of rotation axis - roughly constant, +/_ 23.5 degrees, varies by one degree over time
- lunar phases
- new, crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, crescent, full
- Metonic cycle
- 19 years - that period which returns the full moon to the same date in the calendar year - used to predict Easter Sunday - combines the synodic period and the year
- lunar standstill
- occur when the line of nodes is at its furthest and closest points to equatorq
- elongation
- Refers to the angle between the Sun and a planet, as viewed from Earth. When an inferior planet is visible after sunset, it is near its greatest eastern elongation. When an inferior planet is visible before sunrise, it is near its greatest western elongation.
- degree
- (in full, a degree of arc, arc degree, or arcdegree), usually denoted by ° (the degree symbol), is a measurement of plane angle, representing 1⁄360 of a full rotation
- inferior planets
- Mercury and Venus - orbits lie between the Earth and the Sun - appear to oscillate back and forth relative to the sun over a short angular distance
- inferior conjunction
- configuration of a planet in which it is obscured by passage infront of the sun
- commensurable periods
- property whereby a quantity can be related to another quantity by a ratio of two small whole numbers - for example, because 5 Venus years of 584 days equals 8 earth years of 365 days, we say that the two periods are commensurable in the ratio of 5 to 8
- Sirius
- brightest of the fixed stars, important for egyptians and time of helilical rising
- conjunction
- configuartion of celestial body when it lies at (or close to) the same position as another