Transcript USING AN ELECTRONIC DISCUSSION FORUM IN TEACHING

Daily Motion of the Sun
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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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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
Yearly Motion of the Sun 2
Yearly Motion of the Sun 3
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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
Heliacal Rising
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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
Dog Days
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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)
Dog Days 2
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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)
Equinoxes and Solstices
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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
Equinoxes and Solstices 2
Equinoxes and Solstices 3
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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
Equinoxes and Solstices 4
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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
Equinoxes and Solstices 5
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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
Equinoxes and Solstices 6
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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
Celestial Coordinates
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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
Horizon Coordinate System
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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
Horizon Coordinate System 2
Equatorial Coordinate System
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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
Equatorial Coordinate System
2
Ecliptic Coordinate System
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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!
Comparing Coordinate
Systems
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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
Comparing Coordinate
Systems 2
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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°