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Daily Motion of the Sun. Daily motion (diurnal motion) along a circle with center on the Earth’s axis Clockwise in the north temperate zone Counterclockwise in the south temperate zone Up and down in the tropics Don’t worry about CW or CCW in the tropics. Yearly Motion of the Sun.

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Daily Motion of the Sun

  • Daily motion (diurnal motion) along a circle with center on the Earth’s axis

  • Clockwise in the north temperate zone

  • Counterclockwise in the south temperate zone

  • Up and down in the tropics

  • Don’t worry about CW or CCW in the tropics


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Yearly Motion of the Sun

  • Yearly motion (annual motion) along the ecliptic

  • The ecliptic great circle is the intersection of the celestial sphere with the ecliptic plane

  • The Sun drifts eastward (counterclockwise when viewed from above the NCP) with respect to the stars in the course of the year



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Yearly Motion of the Sun 3

  • We cannot see the ecliptic, but we deduce its position by considering which stars are close to the Sun at the time of sunrise or sunset


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Heliacal Rising

  • Heliacal (/hɪˈlaɪəkəl/) rising is the first visible appearance of a star on the eastern horizon before sunrise

  • On the previous morning, sunlight made the star invisible


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Dog Days

  • Sirius is the brightest star in the night sky

  • In ancient Egypt, the helical rising of Sirius coincided with the annual flooding of the Nile

  • Sirius is also known as the Dog Star because of its prominence in the constellation Canis Major (Big Dog)


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Dog Days 2

  • The Dog Days refers to the hottest days of summer

  • At the time of the ancient Greeks this were the days when star Sirius rose just before or at the same time as sunrise (heliacal rising)


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Equinoxes and Solstices

  • The equinoxes are the intersections of the celestial equator and the ecliptic

  • The solstices are the points on the ecliptic furthest from the celestial equator

  • These geocentric definitions agree with the heliocentric ones given earlier



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Equinoxes and Solstices 3

  • The spring equinox is both a time and a place!

  • The spring equinox is the moment when the Sun is at the spring equinox

  • You finish the race when you reach the finish!

  • The spring equinox is the intersection point where the counterclockwise motion is up


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Equinoxes and Solstices 4

  • At the time of the equinoxes, day and night are equally long everywhere on Earth

  • Equinox = equal night

  • At the time of the solstices, the rising position of the Sun reaches its extreme northern or southern positions, so the Sun stands still before turning

  • Solstice = Sun standing

  • Tropic = (Sun) turning


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Equinoxes and Solstices 5

  • Apparent diameter of the Sun is 0.5°

  • At the equinox the centre of the Sun is above the horizon for 12 hours

  • Sunrise is defined to be when the upper limb of the Sun crosses the horizon, so day is actually longer than night at the equinox

  • In addition, refraction bends light over the horizon


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Equinoxes and Solstices 6

  • The change in rising position of the Sun along the horizon is fastest around the equinoxes, and slowest around the solstices

  • Around the equinoxes, the declination (distance from the celestial equator) will change by 0.5° per day

  • Near the solstices, it will stay fixed for almost a week


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Celestial Coordinates

  • In order to set up a coordinate system on a sphere, we need a great circle and a base point on it

  • On the Earth, there is only one natural great circle, on the Celestial Sphere there are three: horizon, celestial equator and ecliptic

  • There are therefore three coordinate systems on the celestial sphere


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Horizon Coordinate System

  • Base circle: horizon, base point: north point

  • Altitude and azimuth

  • Azimuth is measured clockwise from 0° to 360° and tracks the daily path of the Sun in the north temperate zone

  • 90 – altitude is called the zenith distance



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Equatorial Coordinate System

  • Base circle: celestial equator, base point: vernal equinox

  • Declination and right ascension

  • RA is measured counterclockwise in hours from 0h to 24h and tracks the time difference in meridian passage of stars



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Ecliptic Coordinate System

  • Base circle: ecliptic, base point: vernal equinox

  • (Celestial) latitude and (Celestial) longitude

  • Longitude is measured counterclockwise from 0° to 360° and tracks the Sun’s annual motion

  • Celestial latitude and longitude is not relative to the celestial equator!


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Comparing Coordinate Systems

  • Azimuth is measured in the opposite direction from RA and longitude

  • Horizon system is local, the two other are global

  • For stars, the horizon coordinates change with time, while the two other are fixed

  • Horizon for simple observations

  • Equatorial for tracking stars

  • Ecliptic for tracking Sun, Moon and planets


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Comparing Coordinate Systems 2

  • The latitude of the Moon is always less than 5°

  • For the planets, the latitudes are less than 3° except for Mercury, which can go up to 7°

  • The ex-planet Pluto could go up to 17°