ASTR 101 Lecture 3 Outline of Last Lecture I. The Milky WayA. What is it?II. Define Zenith, Horizon, and Angular SizeIII. FACTSOutline of Current Lecture I. Days and SeasonsA. Summer SolsticeB. Winter SolsticeC. Spring EquinoxD. Fall EquinoxII. Phases of the MoonIII. EclipsesCurrent LectureDays and Seasons- As earth orbits the sun, the sun appears to move eastward.- At midnight the stars on our median are opposite the sun.Solar day = 24 hours- Sidereal day (earth’s rotation period) = 23 hours and 56 minutesFrom earth:- 2000 stars and Milky Way can be seen- We can specify positions by altitude (above horizon) and direction (along horizon)- Stars rise and set because of earth’s rotationSeasons depend on how earth’s axis affects the directness of sunlight. It depends on tilt, not distance from the sun.- Axis tilt changes directness of sunlight over the year.- Suns altitude changes with seasons.- Higher altitude means more direct light (summer solstice)- Earth’s axis points the same way all year round, its position relative to the sun changes.- Without the axis tilt, there would be no seasons.- The larger the tilt the stronger the weather patterns are.Variation of Earth-Sun distance is small, 3%, so it’s overshadowed by the tilt.We define 4 special points:1. Summer solstice – highest path, rise and set at extreme north of due east2. Winter solstice – lowest path, rise and set at extreme south of due east3. Spring (vernal) equinox – sun rises at exactly due east and sets at exactly due west.4. Fall (autumnal) equinox – “ “Seasonal changes are more extreme at higher latitudes.Although the axis seems fixed, it actually progresses over about 26,000 years.- Polaris won’t be the north star forever- Positions of equinoxes shift around orbit; the spring used to be Aries and now it is Pisces.Phases of the Moon- Half of the moon is illuminated by the sun and half of it is dark.- We see a changing combo of the bright and dark faces as the moon orbits the earth.- We only see one side of the moon.o Synchronous rotation – the moon rotates exactly once with each orbit.Eclipses- The earth and moon cast shadows- When either passes through the other’s shadow, we have an eclipse.o Lunar eclipse – only on a full moon. Can be penumbral, partial, or total.- The moon’s orbit is tilted 5 degrees to the elliptic plane.- So we have about 2 eclipse seasons, with a lunar at a full moon and a solar at a new
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