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  • 1. ROTATION The Earth is a sphere that rotates on its axis that passes through the North and South poles. The time it takes for the Earth to make a complete rotation is approximately 24 hours (exactly 23.934 hours). This rotation results in daytime when an area is facing the Sun and nighttime when an area is facing away from the Sun.
  • 2. Mean sidereal day: the period during whichEarth makes a complete rotation on its axis(absolute rotation). The time interval of themean sidereal day is 86164.0905382 seconds.Mean solar day: The period of timebetween two successive transits of the meansun; the standard for the 24-hour daymeasured from midnight to midnight
  • 3. HOW FAST IS THE EARTH SPINNING AROUND ITS AXIS?At the equator, the Earths surface moves 40,000kilometers in 24 hours. That is a speed of about1040 miles/hr (1670 km/hr or 0.5 km/sec).This is calculated by dividing the circumference ofthe Earth at the equator (about 24,900 miles or40,070 km) by the number of hours in a day (24).As you move toward either pole, this speeddecreases to almost zero (since the circumferenceof the spinning circle at the extreme latitudesapproaches zero).
  • 4. REVOLUTIONThe Earth revolves around the Sunonce every 365.25 days. Its velocityin space in its orbit is about 18.5miles per second. The shape of theorbit is an ellipse
  • 5. A tropical year (also known as a solar year) is thelength of time that the Sun takes to return to thesame position in the cycle of seasons, as seenfrom Earth; for example, the time from vernalequinox to vernal equinox, or from summersolstice to summer solstice.The sidereal year is the time taken for the Sun toreturn to the same position with respect to thestars of the celestial sphere. It is the orbital periodof Earth, equal to 365.25636042 The sidereal yearis 20 minutes and 24 seconds longer than thetropical year.
  • 6. HOW FAST IS THE EARTH REVOLVING AROUND THE SUN?The Earth orbits, on average, 93 million miles(149,600,000 km) from the Sun (this distance isdefined as one Astronomical Unit (AU)), takingone year to complete an orbit.The Earth revolves around the Sun at a speed ofabout 18.5 miles/sec (30 km/sec).
  • 7. PRECESSION
  • 8. One consequence of precession is that the north star Polaris is drifting. It is only "north star" by coincidence today. Vega will be our north star for a time in the distant future.Another consequence is that precession introduces adifference between a sidereal (real) year and a tropical(by the sun) year because during the course of one yearthe position of the equinox changes slightly. "precession" was discovered by Greek astronomer Eratosthenes when he compared his own circa 200 BC records with older charts
  • 9. HOW FAST IS OUR SOLAR SYSTEM MOVING WITHIN THEMILKY WAY GALAXY? The sun is about 26,000 light-years from the center of the Milky Way Galaxy, which is about 80,000 to 120,000 light-years across (and less than 7,000 light-years thick). We are located on one of its spiral arms, out towards the edgeIt takes the sun (and our solar system) roughly 200-250million years to orbit once around the Milky Way. Inthis orbit, we (and the rest of the Solar System) aretraveling at a velocity of about 155 miles/sec (250km/sec).
  • 10. HOW FAST IS THE MILKY WAY GALAXY MOVING WITHINTHE LOCAL GROUP OF GALAXIES?The Milky way Galaxy is just one galaxy in agroup of galaxies called the Local Group. Withinthe Local Group, the Milky Way Galaxy is movingabout 185 miles/sec (300 km/sec).
  • 11. Grids an parallels