Mapping the Night Sky Presenter: Dr Keith Treschman
The origin of much of Western Astronomy is in Mesopotamia from 5000 years ago. Of a sample of 250 names of bright stars, 70% are Arabic, for example, Betelgeuse, Rigel, Aldebaran. The oldest records of constellations are on inscribed stones and clay tablets from Sumeria in 3000 BCE. The zodiac can be traced to Babylon before 2000 BCE. This was passed to the Greeks around 500 BCE. Hipparchus, a Greek astronomer in what is now Turkey, compiled a catalogue of constellations and at least 850 stars from Babylonian sources in the second century BCE. His work is referenced by Ptolemy around 150 CE in his treatise on Astronomy, which when translated into Arabic was called Almagest (The Greatest).
Islam moved across North Africa in the 7th century and crossed into Spain. The Catholic north met the Islamic south and during the 10th to 13th centuries much of the Arab literature was translated into Latin, including Ptolemy’s treatise. This was the avenue where much of Astronomy was discovered or rediscovered by the West and its eventual incorporation into English.
Following European exploration of the Southern Hemisphere, more constellations were added from the late 16th century to late 18th century. The International Astronomical Union (IAU) was formed in 1919. One of the first orders of business was to bring some order to the constellations. A list was compiled and given a three-letter code. Boundaries were drawn in 1928 and 88 constellations were published in 1930. These included 47 of Ptolemy’s list of 48.
Sumeria is also the source for our current use of time, angles and geographical coordinates. The Sumerians used a base 60 (sexadecimal system) for numbers in the 3rd millennium BCE. It was passed onto the Babylonians. 12 could be counted on one hand with the thumb to 3 parts of 4 fingers. With 5 fingers on the other hand, 12 x 5 = 60. 60 is divisible by 12 factors, including the first 6 digits: 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60. 60 x 6 = 360. This gives 360° in a circle. 1/60 of 1° = 1´ (arcminute). 1/60 of 1´ = 1“ (arcsecond). For time 1/60 of 1 hour = 1 minute. 1/60 of 1 minute = 1 second. (Latin: minuta a minute portion; secundus second).
When one looks up to the sky, it appears as an inverted dome with the stars attached. This has led to an imaginary celestial sphere with Earth at the centre. It appears to us that we are stationary and that this sphere is rotating. The north and south celestial poles are extensions of Earth’s rotational axis, and the celestial equator is concentric with Earth’s equator.
Latitude on Earth is measured as an angular distance north or south of the equator. Similarly, declination is the angular distance of a star from the celestial equator with + and – used respectively for north and south. Longitude on Earth is the distance in degrees from the meridian of Greenwich, which is defined as 0°, with measurements 180° east and west. The equivalent on the celestial sphere is right ascension (RA). It is measured from its zero point easterly, and runs from 0 hour to 24 hour, where 15° is equivalent to 1 hour RA.
Now, to define the zero point. If Earth did not orbit the Sun, a star above the meridian of Greenwich would rotate at the same rate as Earth. However, the system requires a zero point in the sky independent of a place on Earth.
The Sun, in a year, appears to pass through the 12 constellations of the zodiac. This line is referred to as the ecliptic, the line where eclipses occur. As the rotational axis of Earth is tipped 23.4° from the vertical to the orbital plane, the plane of the ecliptic and the orbital plane are inclined 23.4° to each other. Hence, the Sun in a year will cross the celestial equator twice. On the occasion that it is crossing from south to north, this was selected as the zero point for RA. Hipparchus, 2000 years ago, called this the First Point of Aries as in his time, the Sun was in the constellation of Aries. This arc became 0 h RA. It happened in the month of March. As this was an important occasion, March was the beginning of the year. At this crossing, there is 12 hours daylight everywhere on Earth, and it is referred to as an equinox.
The Sun reaches 6 h RA for its furthest northerly sojourn at +23.4° declination in June. This corresponds on Earth to a latitude of 23.4° N. As this occurred when the Sun was in Cancer, this gave the name to the tropic. As the Sun was moving north up to this point and then moves south, it is referred to as a stationary point, from where the name solstice was applied. At 18 h RA, the Sun is at -23.4° declination. The latitude of 23.4° S is another tropic. This solstice occurred in December when the Sun was in the constellation Capricornus.
When the IAU were deciding on boundaries for the constellations, they kept the major outline of the pattern, and drew straight lines along lines of declination and RA. As a result, the 12 members of the zodiac are no longer equal along the ecliptic, and the ecliptic now passes through a 13th constellation near Scorpius, namely, Ophiuchus the serpent-bearer.
Hipparchus compared some of his positional measurements of stars with those of others 200 years before him. He indicated that the First Point of Aries had moved at the rate of 1° per century towards the west. The cause is due to precession. The result of the gravitational pull of the Moon and Sun on the equatorial bulge of Earth is a wobble similar to that of a spinning top where its rotational axis traces out a cone. A complete cycle takes 25 772 years. This corresponds to 1° every 71.6 years (against the century of Hipparchus). Thus, the position of a star on the celestial sphere is constantly changing due to precession as well as their proper motion.
After Ptolemy wrote about astronomy, he devoted Tetrabiblos (4 books) to astrology. He made a distinction between the actual zodiac constellations and the signs of the zodiac. The latter were assigned to the equinoxes and the solstices. Thus, the Sun enters Aries at the March equinox. In 2020, the respective dates are March 20, June 20, September 22 and December 21.
At approximately 26 000 years divided by 12 constellations of the zodiac, each 2000 years corresponds to 1 actual constellation. It is close to 2000 years since the time of Ptolemy. If we take actual constellations, you were born in one actual constellation to the west of your astrological birth sign. The First Point of Aries is now in Pisces, the other equinox has moved from Libra into Virgo, and the solstices have shifted from Cancer to Taurus (2 constellations) and Capricornus to Sagittarius.
