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AncientAstronomy.ppt
Historical Development of Astronomy
Reference: The Astronomy Timeline link in the PHY250 website
Constellations
Intended to identify stars in the sky conveniently
First identification of “constellations” found in cuneiform texts and artifacts
dating back roughly 6000 years
Remnants found in the valley of the Euphrates River, suggest that the
ancients observing the heavens saw the lion, the bull, and the scorpion in
the stars.
Constellations
The earliest references to the mythological significance of the Greek constellations may be found
in the works of Homer, which probably date to the 7th century B.C. In the Iliad, for instance,
Homer describes the creation of Achilleus's shield by the craftsman god Hephaistos:
On it he made the earth, and sky, and sea, the weariless sun and the moon waxing full, and all the
constellations that crown the heavens, Pleiades and Hyades, the mighty Orion and the Bear,
which men also call by the name of Wain: she wheels round in the same place and watches for
Orion, and is the only one not to bathe in Ocean (Iliad XVIII 486-490).
At the time of Homer, however, most of the constellations were not associated with any particular
myth, hero, or god. They were instead known simply as the objects or animals which they
represented--the Lyre, for instance, or the Ram. By the 5th century B.C., however, most of the
constellations had come to be associated with myths, and the Catasterismi of Eratosthenes
completed the mythologization of the stars. "At this stage, the fusion between astronomy and
mythology is so complete that no further distinction is made between them"--the stars were no
longer merely identified with certain gods or heroes, but actually were perceived as divine
(Seznec, 37-40). Despite the many mentions of the stars in Greek and early Roman texts, by far
the most thorough star catalogue from ancient times belongs to the Roman Ptolemy of
Alexandria, who grouped 1022 stars into 48 constellations during the 2nd century A.D. Although
Ptolemy's Almagest does not include the constellations which may only be seen from the
southern hemisphere, it forms the basis for the modern list of 88 constellations officially
designated by the International Astronomical Union (Pasachoff, 134-135). The influence of both
the Greek and Roman cultures may be plainly seen; the myths behind the constellations date
back to ancient Greece, but we use their Latin names.
The Big Dipper – North America
The Plough - England
The Celestial Bureaucrat - China
Charles’ Wain – Medieval Europe
Ursa Major (the Bear) – Greeks and Native Americans
The Celestial Sphere
From the perspective of the
earth, the stars rotate around
the earth once every 24 hours
(approximately).
The Celestial Sphere fixes the
locations of the stars with
respect to each other.
The Celestial Sphere
Celestial Equator
The Celestial Sphere
From the perspective of the
earth, the projection of the
earth’s equator on the
celestial sphere is called the
Celestial Equator.
The Celestial Sphere
Ecliptic
The Celestial Sphere
From the perspective of the
earth, the sun’s position in
the sky changes over the
course of the year.
The apparent path of the sun
during the year with respect
to the fixed starts on the
celestial sphere is called the
Ecliptic.
The Celestial Sphere
Equinox
The Celestial Sphere
From the perspective of the
earth, the location of the
intersection of the Ecliptic
with the Celestial Equator is
called the Equinox.
There are two of these
intersections, and each
occurs when line connecting
the earth to the sun is
perpendicular to the earth’s
axis of rotation.
The Celestial Sphere
Autumnal Equinox
Vernal Equinox
The Celestial Sphere
The Autumnal Equinox
occurs when the sun passes
from the earth’s Northern
Hemisphere to the Southern
Hemisphere. Fall begins at
the Autumnal Equinox.
The Vernal Equinox occurs
when the sun passes from
the Earth’s Southern
Hemisphere to the Northern
hemisphere. Spring begins
at the Vernal Equinox.
The Celestial Sphere
Summer solstice
The Celestial Sphere
The point on the Ecliptic
where the Sun is at it’s
Northernmost point above
the Celestial Equator (ie, it is
highest in the sky in the
Northern hemisphere) is
called the summer solstice.
Summer begins at the
summer solstice, and the
length of daylight is the
greatest in the Northern
Hemisphere.
The Celestial Sphere
Winter solstice
The Celestial Sphere
The point on the Ecliptic
where the Sun is at it’s
Southernmost point below
the Celestial Equator (ie, it is
lowest in the sky in the
Northern hemisphere) is
called the winter solstice.
Winter begins at the summer
soltice, and the length of
daylight is the smallest in the
Northern Hemisphere.
The Celestial Sphere
The location of an object
measured in hours along the
Celestial Equator is called the
Right Ascension of the object.
The Celestial Sphere
One circulation around the
Celestial Equator takes 360o.
Rather than using degrees,
the Celestial Sphere is divided
in 24 hours with
corresponding minutes, and
seconds.
Since 360/24 = 15o, each hour
corresponds to 15o along the
Celestial Equator.
The hour 0 h is defined by a
line connecting the North
Celestial Pole and the Vernal
Equinox.
The location of an object
measured in hours along the
Celestial Equator is called the
Right Ascension of the object.
The Celestial Sphere
The location of an object
measured degrees is called
the declination of the object.
The Celestial Sphere
The location of an object
measured degrees is called
the declination of the object.
Star catalogs quote the
location of celestial object on
the basis of the object’s right
ascension (r.a.) and
declination (dec.)
The Zodiac
The constellations that lie
along the ecliptic are
collectively called the Zodiac.
The Zodiac
The constellations that lie along
the ecliptic are collectively
called the Zodiac.
The constellations of the Zodiac
are Cancer, Leo, Virgo, Libra,
Scorpio, Sagittarius,
Capricornus, Aquarius, Pisces,
Aries, Taurus, Gemini.
From the perspective of the
Earth, The sun would be in
different constellations of the
Zodiac at different times of the
year (if you could see the stars
in daylight).
What’s your sign?
Footnote: This is a good time to review the
Stellar Magnitudes and the Sky Maps link in
the PHY250 website.
Megalithic structures at
Carnac (4500 BC)
Newgrange Tomb (3200 BC)
Stonehenge (3000 BC)
Why?
Why?
Agriculture and Hunting
* Summer and Winter Solstice
* Vernal Equinox
Why?
Agriculture and Hunting
* Summer and Winter Solstice
* Vernal Equinox
Different constellations at different times of the year
More accurate (?) sun rises over specific rocks at solstice, equinox
Why?
Agriculture and Hunting
* Summer and Winter Solstice
* Vernal Equinox
Different constellations at different times of the year
More accurate (?) sun rises over specific rocks at solstice, equinox
Could be used to identify planting and harvesting schedules and predict the
coming of the seasons
Could be used to predict the return of migrating animals
Why?
Navigation
* Stars at fixed locations (celestial sphere)
Why?
Navigation
* Stars at fixed locations (celestial sphere)
Stars at fixed locations could be used to locate position on the earth
The octant and sextant
Why?
Navigation
* Stars at fixed locations (celestial sphere)
Stars at fixed locations could be used to locate position on the earth
The octant and sextant
Astrolabe (~400)
Octant (1731)
Sextant (1759)