Historical Development of Astronomy

Reference: The Astronomy Timeline link in the PHY250 website

AncientAstronomy.ppt

ConstellationsIntended 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.

ConstellationsThe 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)