chapter 2 (pt. 2) Flashcards
Earth-viewed celestial cycle and time
Sun Cycle
rotation, ecliptic
season
revolution, vernal/autumnal equinox, summer/winter solstice, perihelion, aphelion
Moon Cycle
phase (waxing or waning of full, gibbous, quarter, crescent, new)
eclipses
solar eclipse, penumbra, umbra, annular eclipse, lunar eclipse, Saros cycle
rare, the occurrence can be mathematically calculated, lunar is more common than solar
time keeping
solar day, sidereal day/month/year, synodic month, tropical year (solar year), apparent solar time
constellation
sky region and/or a star pattern
88 chosen by International Astronomical Union (IAU)
asterism
star group (Big Dipper) not primary constellation, may cross regions (Great Square of Pegasus)
apparent visual magnitude
measure of brightness (seen with human eye) (Sun ~.27, Sirius -1.47)
Hubble sees magnitude ~30 (faint)
flux
measure of light energy reaching a m^2 in one second
celestial sphere
ancient belief the sky is a ceiling (sphere) surrounding Earth with attached objects, now a scientific model to describe the observed sky
scientific model
way to think about nature, not necessarily true
star trail
bright line or curve left by star on untracked time-lapse image of the night sky
circumpolar constellation
never rise or set, circling a polar star for latitudes away from the equator cardinal directions (north point, south point, east point, west point)
zenith
centre of sky above (opposite is nadir on hidden half)
precession
slow circular sweep in orientation of Earth axis of rotation (like a toy top spins)
Messier object
Catalogued by Charles Messier, 1760s
103 non-comet objects (all binocular viewable)
declination (dec)
latitude in degrees, arc min., arc sec. of north (+), or south (-), of celestial equator - equator (dec=0º), north celestial pole (dec = +90º)
right ascension (RA)
extension of longitude, in hours (h), min. (m), sec. (s) within 24 h - longitudunal line of the spring equinox (RA = 0º)
north celestial pole
apparent pivot point, opposite south celestial pole, divided by celestial equator
angular distances
measured in units of degree, arc minute (1/60th degree), arc second (1/60th minute - from horizon to pole is the latitude of observer (the base principle of celestial navigation)
Ursa Major
the Great Bear - part of the big Dipper (not a constellation) a pointer for Polaris
Canis Major
the Great Dog
Sirius
Dog Star, 8.6 ly
brightest star in the sky
Ursa Minor
the Little Bear
Polaris
the North Star (Alpha Ursa Minoris)
axis
imaginary line through centre of rotation of planet (Earth tipped 23.5º, fixed as it revolves)
rotation
motion around an axis (Earth produces day and night) [24 h, 1700km/h]
ecliptic
Earth’s orbit projected on the celestial sphere
evening star
planet in the sunset sky, close to Sun over horizon
morning star
in dawn sky
seasons
change in the amount of solar energy in Northern and Southern hemispheres
revolution (orbital motion)
about a point outside a body (earth almost a perfect circle, 30 km/s)
aphelion
Jul. 5 ~1.7% further from Sun
perihelion
Jan. 3 ~1.7% closer, and seasonal cycle
vernal equinox
March 20* spring begins (date varies)
summer solstice
June 22* summer begins (date varies)
autumnal equinox
September 22* fall begins (date varies)
winter solstice
December 22* winter begins (date varies)
synodic
meeting of the Sun and the Moon at new Moon
month
with a phase cycle of the Moon cycle
new Moon
dark, see no moon at all
full Moon
fully lit
solar eclipse
Moon passes between the Earth and Sun, always at new Moon
blocking part of the Sun and causes penumbra and umbra
penumbra
partial shadow
umbra
in total shadow
annular eclipse
totality when the annulus (“ring”) of the Sun’s atmosphere is visible around Moon
lunar eclipse
Moon moves through the shadow of the Earth, always at a new Moon, up to 1h 45min
Saros cycle
18y 11 1/3 day pattern of solar and lunar eclipses, due to Moon tip of 5º to the ecliptic
solar day
average length of time between successive passes of Sun across local meridian, 23 hr. 52 min.
sidereal day
time of successive passes for a star across the local meridian, 23 hr. 56 min.
sidereal month
time for Moon to orbit Earth once relative to a star, 27.32 days
sidereal year
time for a complete orbit of Earth around the Sun relative to the stars, 365.26 days
synodic month
time for complete cycle of lunar phases, 29.52 days
tropical year (solar year)
time between successive spring (or autumn) equinoxes, 365.25 days
apparent solar time
used most Sun on the local meridian noon ante (before, a.m.) post (after, p.m.)
