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Notes on Stonehenge and Seasons
2b. The Ecliptic
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The Babylonians determined the exact path
of the sun through the zodiac constellations
Stonehenge (2800 – 1500 B.C.)
2b.1 Ecliptic is the dashed line on your Starwheel
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2b.2
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Obliquity of the Ecliptic
The Ecliptic is tilted
23½° to the equator
(“obliquity”)
Its NOT the same as the equator!
2b.3
Obliquity of the Ecliptic
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2b.4 From Earth’s point of view
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This is because the earth’s axis of rotation is tilted
by 23½ degrees relative to the axis of its orbital
revolution around the sun.
Plane of the
Earth’s orbit
This is what gives us seasons.
Around the sun
Yellow
Red is equator
Black
Blue is ecliptic
Fig 1-6, p.24
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2b.5 Ecliptic on Mercator Map
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2c.1 Ecliptic Longitude
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Ecliptic Longitude is measured eastward along the ecliptic,
starting at 0 degrees at the First Point of Aries.
Solstitial Colure
Ascending Node of Sun (blue) is start of spring
90°
0°
2c.2 Ecliptic Longitude on Polar Map
The sun moves
about 1 degree
east along the
ecliptic each day.
Equinoctial Colure
3a. The Seasons, and what causes them
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The Earth’s axis of rotation is tilted 23° with respect
to the Earth’s orbital plane.
0°
0° Spring Equinox
90° Summer Solstice
270°
180° Fall Equinox
Solstitial Colure
270° Winter Solstice
90°
Equinoctial Colure
North Ecliptic Pole
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Solstitial Colure
270°
Equinoctial Colure
180°
The orientation of the tilted axis remains the
same as the Earth revolves around the Sun
180°
Fig 3-4, p.64
C.1b Local Horizon
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C.1c
Local Horizon System
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Prime Meridian is line from
North to South through Zenith
Fig 1-1, p.20
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C.2a
Daily Path of Sun
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C.2b The Equinoctial Sun
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•Spring (and Fall) Equinox, the sun is on the equator
•Sunrise is due East
•Sunset is due West
•Transit is when sun crosses prime meridian
•Sun Transits at “local noon”, at 52° above the horizon
C.2b The Summer Sun
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•Sun is on Tropic of Cancer, highest declination 23.5°
•Sunrise is in North-East
•Sunset is in the North-West
•Transit is at 52+23=75° altitude angle (above horizon)
•Length of day is around 15 hours
C.2b The Winter Sun
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•Sun is on Tropic of Capricorn, lowest declination -23.5°
•Sunrise is in South-East
•Sunset is in South-West
•Transit is at 52-23=29° altitude angle (above horizon)
•Length of day is about 9 hours
Tropic of Cancer
Tropic of Capricorn
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2c.1 The Analemma
2c.2 Transit Times
•
Note Sun transits 12:08 pm on
average at Santa Clara, because
we are 8 minutes west of the center
of the pacific time zone.
•
Equation of Time:
Sun is as much as 20 minutes
early/late due to elliptical orbit of
earth, and obliquity of ecliptic.
•
Analemma: is the figure 8 plot of
declination of sun vs equation of
time
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2c.3 Sun is a poor timekeeper
•
Sun moves further in
Right Ascension near
solstices than at
equinoxes, makes sun get
behind clock after both
solstices
•
Also the day is longer
than 24 hours when we
are near the perihelion
(sun moves faster on
ecliptic). This is why the
lower loop of the figure 8
is bigger in the analemma
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3. Archeoastronomy
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Stonehenge (2800 – 1500 B.C.)
Fig 3-11, p.70
3a.1 Rising and Setting Points
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Ancient
astronomers
would naturally
put a rock on the
ground to mark
the extreme
points on the
horizon where
the sun rises/sets
each summer and
winter
3b.2 “the avenue” points towards summer sunrise
3b.1 Stonehenge 3100 BC
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• The stone
circle was
added 1000
years later!
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3b.3 Heelstone in the Avenue
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3b.4 Summer Solstice Sunrise
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b). Stone Circles
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Stone circles often have 29 stones + 1 xtra one
off to side. Originally there were 30 “sarson
stone” in the outer ring of Stonehenge
3b. Lunar Standstill
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• Winter Full moon at major standstill will rise one arch to the
north of the where the sun rises at summer solstice
• At “Minor Standstill” it will rise in the arch to the right!
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