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Astro Midterm part 3
| Question | Answer |
|---|---|
| The zenith | the point directly overhead |
| The Nadir | the antipodal point directly opposite the zenith |
| The meridian | is the line that runs from the North to the South and through the zenith (bisects horizon into two hemispheres-east and west) |
| Altitude | elevation above the horizon. Or zenith angle |
| Zenith angle | is the angle from the zenith. 90 degrees - altitude |
| Azimuth | is the position along the horizon. Measured clockwise from North (N through E) 0 at N, 180 at S, 90 at E, 270 at W |
| Circumpolar stars | Always stay in view, never rise, never set, around the north star, depends on location on earth. At equator, no circumpolar stars, everything rises, everything sets |
| Altitude and Azimuth of an object will | change as a function of time as the sky rotates. Is not a time independent coordinate system |
| The celestial equator | the projection of the earth's equatorial plane onto the celestial sphere |
| Celestial poles | correspond to the rotation axis of the earth and are perpendicular to the celestial equator |
| Any object in the sky can be given fixed coordinates on the celestial sphere | don't change as the sky rotates, in the same way that longitude and latitude don't change as the sky rotates |
| Right Ascension | (RA or alpha) is angular position of the object along the celestial equator (measured clockwise around the celestial equator) equivalent to a longitude coordinate (E to W). MEASURED IN HOURS |
| Declination | (Dec or weird d) is the angular distance above or below the celestial equator-equivalent to latitude coordinate (N to S). MEASURED IN DEGREES |
| Hour angle | h. Is the angular distance from the meridian (measured clockwise; positive=west of meridian) HA changes with time allowing us to convert to horizontal coords. Right on meridian-h=0. 30 degrees from meridian=2 hours |
| Vernal Equinox | alpha (RA)=0 hours corresponds to this. it is one of the intersections of the ecliptic plane and the celestial equator. WHERE THE SUN CROSSES THE CELESTIAL EQUATOR IN THE SPRING |
| Ecliptic plane | a great circle/plane rotated 23 degrees 26 arc seconds from the celestial equator |
| Diurnal/daily motion | on a daily basis, everything (sun, moon, planets, stars) move from east to west |
| Direct motion | over time the sun, moon and planets move from west to east with the fixed stars |
| retrograde motion | moves backwards and does a loop |
| The ecliptic | a narrow path through the sky corresponding to the orbital plane of the planets around the sun. The constellations around it are known as the zodiacal constellations |
| ecliptic plane | is tilted 23.5 degrees with respect to the plane of the celestial equator |
| Equinoxes | occur when sun crosses the celestial equator. Vernal and Autumnal. Latin of equal night, day and night have equal length (12 hours) |
| Vernal Equinox date | March 21 |
| Autumnal equinox date | September 21 |
| Solstices | occur when the Sun is at highest or lowest point in the sky at solar noon (when it crosses the meridian). Winter and summer. Latin for the sun stands still |
| Winter Solstice date | December 21 |
| Summer Solstice date | June 21 |
| Earth is tilted | 23.5 degrees with respect to the Ecliptic plane, in summer pole points toward the sun. Sun travels higher in the sky an stays above the horizon longer |
| Earth in the northern hemisphere receives | more energy and direct sunlight as well. The opposite is true in the winter |
| Ptolemaic model | Planets moved on little circles (epicycles) centered on big circles (deferents). The deferents were centered on the earth so they could maintain their earth centric cosmology. Not great now |
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