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Lecture 2

PHYS 1600 Week 2 Notes.docx

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University of Guelph
PHYS 1600
Mike Massa

Reading 1-4  Rotation is a planets movement around an axis o The counter clockwise direction of earth’s rotation is called Prograde Motion and is the dominant motion in the solar system. o When something rotates or revolves in the opposite direction it is called Retrograde Motion  Revolution is a planets movement around another astronomical object(orbit) o Earth takes approximately 365 ¼ days to complete one full revolution around the sun  Diurnal Motion is a planet/object’s daily motion  Sidereal Period is the length of any cycle of motion that is measured with respect to stars (ex. earths orbit around the sun)  Zenith is the point directly overhead anywhere on Earth; this is where no stars rise or set  Stars and constellations that never go below the horizon are circumpolar  At the equator, all stars seem to rise straight up, and set straight down  None of the stars seen from the equator are circumpolar Reading 1-6  Solar Day is ideally the interval of time between when the sun is highest in the sky on one day until the time it is highest in the sky on the next day o The length of the solar day varies throughout the year  This is because Earth’s orbit around the sun is not perfectly circular  Therefore the average interval is 24 hours, which is what we determined the time that we use on our clocks  This is called the MEAN or AVERAGE SOLAR DAY  The difference between clock noontime and astronomical noontime (when the sun is at it’s highest point) is as much as 16 minutes  EX. astronomical noon in Yew York occurs earlier than it does in Philadelphia  Time Zones were established in the late 19 century o Time zones are based on the same time at 0 degrees longtitude in Greenwich, England, a location called the prime meridian o With some exceptions, every 15 degrees of longitude around the globe begins a new time zone, therefore resulting in 24 time zones on Earth in total o Going from one time zone to the next usually changes the clock time by exactly 1 hour  Along with solar day, there is also Sidereal Day, which is the length of time from when a star is in one place in the sky until the next time that it is in the same place o The length of the solar day compared to sidereal day differ because the way that earth rotates AND revolves, brings the stars back to their original location 4 minutes earlier each day therefore making the sidereal day 23 hr and 56 min Reading 1-5  Ecliptic is the straight path the sun makes on the celestial sphere o The term “ecliptic” has a second use, Earth orbits the sun in a plane also called the ecliptic  An Equinox is when the ecliptic and celestial equator (two different circles) intersect o There are only 2 points where they intersect, which are each called an equinox o When the sun appears at either of these 2 points it is directly above the equator resulting in 12 hours of daytime and 12 hours of nighttime o The Vernal Equinox happens around march 20, and is the prime meridian of the celestial sphere  Solstice is the point on the ecliptic farthest south or farthest north of the celestial equator o Winter solstice is the day that the sun rises to the lowest height at noon o Summer solstice is the day that the sun rises to its highest height at noon  Because the southern hemisphere is mostly covered by oceans, when earth is closer to the sun, the southern oceans scatter more light and heat directly back into space than occurs when the sun is higher over the northern hemisphere o Therefore, had the extra energy sent back into space when we are closer to the sun been absorbed by our planet, earth would indeed heat up more during this time of year  During winter months, the farther north you are, the less daylight is provided  The Sun takes one full year to complete a trip around the ecliptic o This motion is actually caused by earths orbit around the sun)  The seasons do NOT a
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