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Flashcards in Astronomy First-Second Test Deck (84)
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tropical year

365.2422 mean solar days


calendar year

365 days


how to reconcile tropical and calendar years

one leap year every four years, add .25 days/yr, common years= centuries not divisible by 400 have no leap year, -.0075 days/year. result in avg year of 365.2425, good for 3 days in 10k years.


apparent motion of things in sky.

everything transits once per day, stars transit 4 mins earlier each day (resets every year). Moon and planets not so simple.



crossing of celestial meridian (due N to due S) at top sky


earth's rotational axis and orbital plane

not parallel- this is reason for season. earth's axis tilted 23.5° compared to orbital plane.



day/night line on earth, perpendicular to sun's rays. perpendicular to equator on equinox, 23.5° off on solstices.



highest point of sky


tropics of cancer and capricorn

highest latitude where sun reaches zenith. Cancer in N hemisphere, capricorn in south



outside of ring of celestial body seen from space, ie circle


reasons for season

during winter same sunlight is spread over wider area: think square held up at different angles.
also: length of day, terminator moves as earth moves around sun, longer and shorter days means more and less sun's heat on earth.

MAYBE JUST: During winter, that part of earth is tilted away from the sun and is less exposed to sunlight than in summer.


NOT reason for seasons

closeness to sun during earth's elliptical orbit. sun's apparent size only varies by 2%, not a factor


sun movement relative to stars

west, covers different stars every day.


celestial sphere

projection of earth's coordinates out into space, infinite radius (pretend stars are infinitely far away). same rotation and poles and equator and axis as us. earth's equator and poles become celestial equator and celestial poles


cut off polar 30° of earth and then cut vertically at jun 21

plot positions of sun seen from equator... it forms a SINE WAVE



when the sun crosses celestial equator. time and a place


stellar coordinates

declination and right ascension



dec-lower case delta. measured in degrees. degrees north or south of celestial equator, like latitude


right ascension

RA- lower case alpha. measured in hours/minutes/seconds, NOT degrees. hours east of vernal equinox.


sun above arctic circle

jun 21, 23.5°, sun angle above horizon. sept 21 (equinox) halfway up, dec 21 twilight all day (goes 23.5° down)


seeing celestial poles from Earth

standing on a tall mountain on equator, looking up or down curvature of earth: long exposures can see the dot in the middle of the celestial poles


zenith diagram

angle from equator to where observing is standing on 2d circle representing earth is latitude. latitude that keeps going out into space is zenith, highest point in sky. Horizon is tangent to "limb" of earth. Point has its own celestial equator, jun 21 sunlight angle is 23.5° up, dec 21 is down.


precession of earth

rotates on axis in addition to N|S rotation. Period of ~26,000 years. NOT cause of seasons: that tilt stays basically the same throughout a year, what changes is how tilt is facing the sun. this tilt means that in 13k years earth's tilt will be 44°.


star epochs

precession of earth means that star charts need to be constantly corrected. 2010.0 means chart is correct for jan of 2010. 2013.7 for now, probably, is done by tenths. have to precess coordinates.


reason for precession of earth

gravity of sun and moon combined. nutation is the difference between the sun's part and the moon's part. nutation 9arcsecs, 19yrs


cause of phases of moon

NOT earth blocking it: what side of moon is sun hitting, what side is earth looking at? It's all about the angles.


phases of the moon

new, crescent, quarter, gibbous, full. waxing first then wanes backwards


moon's orbit

e=.55. has apogee and perigee, distance changes during orbit. not actually bigger on horizon than in sky-optical illusion caused by small ground objects


synodic lunar month

from one phase to repeat of phase, usually quarter is best (terminator straightness can be seen)


sidereal lunar month

time between the moon occulting a certain star. synodic longer by two days because earth's position around the sun changes by one month of orbiting, so moon has to go a bit further.