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Lecture 8

AS101 Lecture 8

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Department
Astronomy
Course
AS101
Professor
Patrick Mc Graw
Semester
Winter

Description
Lecture 8 2/2/2013 12:28:00 PM The Wanderers Homework:  Stars closer to the NPC than the horizon are circumpolar—they never get below the horizon.  Close to the NCP means declination close enough to +90  How close is close enough? Bigger than 90 – (your latitude). For latitude 43N, that means between 47 and 90.  How about the stars we cant see from here?  They are close to the SCP so they never get above the horizon.  Their locations on the celestial sphere are exactly on the opposite side of the CE from the ones that are north circumpolar.  So, declinations between -47 and -90.  The sun: o Think of the sun just like any other star. At the summer solstice, it is 23.5˚ north of the CE, i.e., at declination +23.5 o So suppose your latitude was 90-23.5 = 66.5˚N o Then anything about +23.5 is circumpolar, so at the summer solstice, the sun becomes a circumpolar star. The Seasons, Revisited:  How high does the sun get in the summer? In the winter?  Suppose we’re at 45˚N latitude, and lets think about where the sun is when it crosses the meridian.  Our Zenith is 45˚ north of the celestial equator, and the sun is 23.5˚ north, so the sun is 45-23.5=21.5˚ away from the zenith.  Where is that in relation to the S horizon?  90-21.5=68.5˚, so that is the suns altitude.  How high does the sun get in winter?  Suppose we’re at 45˚ latitude, and lets think about where the sun is when it crosses the meridian.  Our zenith if 45˚ north of the celestial equator, and the sun is 23.5˚ south, so the sun is 45+23.5=66.5˚ away from the zenith.  Where is that in relation to the S horizon?  90-66.5= 23.5˚, so that is the suns altitude. Some special latitudes:  tropic of Cancer: o 23.5˚N  Tropic of Capricorn: o 23.5˚S  between these two latitudes, the sun reaches the zenith on at least one day per year.  Arctic Circle: o 66.5˚N (this is 90-23.5)  Antarctic Circle: o 66.5˚S  north of the arctic circle (or south of the Antarctic circle), the sun is above the horizon all day for at least one day of the year. Moon Phases, recap:  the moon’s orbit around the earth makes it appear to move eastward compared to the sun.  so if you see the moon on night at sunset, then the next night at sunset it will be farther eat, farther away from the western horizon.  The farther the moon’s angular distance is from the sun, the more of its lit side you can see. Eclipse recap:  Lunar eclipses only happen at full moon  Solar eclipses
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