Why 57 B.C an Important event? (Final) By: s r siddanthy Ananthuni INTRODUCTION We are using two common calendars in India ,one is shali vahana shaka which begins from 78 A.D and second one is Vikram shaka which starts from 57 B.C, though Vikramadity widely accepted as Chandra gupta ii of Gupta dynasty ,during the period 375 A.D--415A.D /there another version as Vishnugupta alias chanikya belongs to 123 B.C who referred Jyothisha content in his Ard Shstra also worked in Vikramaditya period , was the person behind the king Vikramaditya in establishing his kingdom . To establish such periods ,we should know chronology of historical periods .Here 57 B.C has got not only historical importance but also astronomical and astrological importance too .For this purpose we must focus on ancient Indian astrology ,Indian history apart from the astronomical fact . To resolve this event we should discuss three major issues, i.e. one Astronomical event, second one Astrological event, lastly third one is Historical event. According to Bharatiya Jyothish Scientific Samakya (ICAS) Mr. K.N.Rao made Yuga vibhjana based on great personalities. As 1. Veda yugam 23,750 -8350 B.C 2. Purana yugam 8350 B.C -3000 B.C 3. Parashara yugam 3000-57 B.C 4. Varaha Mihira 57 B.C ---1900 A.D 5. Teacher's yugam 1900 A.D --Present Nakshatra is the term for lunar mansion in Hindu astrology .A nakshatram is one of 27 (some times 28 also) sectors along the ecliptic. Their names are related to the most prominent asterisms in the respective sectors .The number of nakshatras reflects the number of days in a sidereal month (value is 27.32 days ) ,the width of nakshatra traversed by the moon in about one day .Each nakashatra is further sub divided in to four padas . The nakashatras of traditional Bharateeya astronomy are based on a list of 28 asterisms found in the Adharvanaveda (AVS 19.7) and also in the Shatapatha Brahman the vedanga Jyothisham . (1) 57 B.C an Astronomical Event : **: This part of the article is about the astronomical event when the sun is at zenith over the equator. We can also observe the same event happening on other planets and setting up a celestial co-ordinate system .During an equinox; the earth is not tilted toward or away from the sun and the length of the day is same at all points on the earth's surface. An equinox occurs twice a year around March 20and 22nd September, when the plane of the earth's equator passes the center of the sun .At this time the tilt of the earth's axis is inclined neither away from nor towards the sun .The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name equinox is derived from the Latin aqueous (equal) and nix (night), because around the equinox, night and are about equal length. 1 At an equinox the sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e. declination 0) and ecliptic intersect .these point of intersections are called equinoctial points .classically, the vernal point (Ra= 00h 00m 00s and longitudes 0 degrees) and autumnal point (Ra=12h 00m 00sc and longitude 180 degrees) .By extension, the term equinox may denote an equinoctial point. The equinoxes are the only times when the sub-solar points (the place on the earth's surface where the center of the sun is exactly over head) is on the equator. The sub solar points crosses the equator moving northward at the March equinox and moving southward at the September equinox. since the sun's ecliptic latitude is not exactly zero it is not exactly above the equator at the moment of equinox ,but the two events usually occur less than 30sec apart ) The equinoxes are the only times when the terminator is perpendicular to the earth's equator .thus the northern and southern hemisphere are illuminated equally .(At the solstices ,that angle reaches its minimum of 66.5 degrees ,corresponding to 90 degrees minus earth's axial tilt) . O denote the positions of objects in the sky, astronomers use system based on the celestial sphere. The use two measurements, right ascension and declination. Right ascension (abbreviated RA) is similar to longitude and is measured in hours, minutes and seconds eastward along the celestial equator. The distance around the celestial equator is equal to 24 hours. The right ascension of the vernal equinox is 0h 0m 0s. Declination is similar to latitude and is measured in degrees, arc minutes and arc seconds, north or south of the celestial equator. Positive values for declination correspond to positions north of the equator, while negative values refer to positions south of the equator. The declination of the north celestial pole is 90° 0' 0" and the south celestial pole's declination is -90° 0' 0". Declination at the equator is 0° 0 2 ' 0". Figure- 1 Locating a star with RA and DEC The position of an object is stated with the right ascension first, then the declination. For example, the bright star Sirius' position is RA: 6h45m8.9s Dec: -16°42'52.1". The position of Betelgeuse is RA: 5h55m10.3s Dec: +7°24'25.4". The advantage of the equatorial coordinate system is that it expresses the position of a star or galaxy in a way that is independent of the observer's position on Earth. However, the right ascension and declination of a given object change slowly over time, mainly due to a phenomenon called precession. This happens because both the ecliptic and the equator are slowly moving, as a result of tidal forces from the Sun, Moon and planets. The main effect is from the Moon and (to a lesser extent) the Sun, which makes the celestial pole orbit around the ecliptic pole once every 26,000 years. So along with the 3 RA and Dec of an object, you will usually see the date, expressed in years, when those coordinates were approximately valid. This date, or "epoch", defines the précising equator and equinox used to construct the star catalog. Common examples are B1950.0 and J2000.0, where the B and J stand for slightly different sorts of year. The changes to the coordinates happen slowly enough that successive generations of star catalog are 50 years apart. However, the most recent star catalogs, which are equinox J2000.0, will probably be the last in the sequence: there are unlikely ever to be equinox J2050.0 catalogs, because of the adoption of the International Celestial Reference System (ICRS). The ICRS broke the connection between catalog positions and the Earth's motion, and is defined instead by a set of quasars. For continuity, the ICRS was set up to be a good approximation to the equinox J2000.0 system, so in effect the catalog RA, Dec system has been frozen at J2000.0. Another meaning of equinox is the date when the day and night are the same length .The equinox is not exactly the same as the day, when day and night are of equal length for two reasons -Firstly, because of size of the sun, the top of the disk rises above the horizon (constituting sun rises which is the start of day time) When the center of the disc is still below the horizon. secondly ,the earth's atmosphere refracts sun light which means that an observer can experience light (day time )even before the first glimpse of the sun's disc has risen above the horizon .To avoid this ambiguity the term equinox sometimes used in this sense .Time of the sun set and sun rise vary with an observer's location (longitude and latitude ),so the dates when day and night are of exactly equal length likewise depend on location .For places near the equator the day time is always longer than the night ,so they would never experience an equinox by this definition .During the earth's motion around the sun four important events were known in Indian astronomy since the ancient times .These events were Vernal and Autumnal equinoxes , and winter and summer solstices.( Fig. 1) given below shows the orbit of the earth around the sun where it shows the position and the orientation of the earth’s spin axis on June 21 . At this time, the North Pole is tilted toward the sun, and there is summer in the northern hemisphere. Similarly, on December 21, it is the South Pole which is tilted toward the sun. The earth’s spin axis maintains same orientation with respect to a three dimensional co-ordinate system defined at the center of the sun (heliocentric). The X and Y axes of this co-ordinate system lie in the plane of the ecliptic. The center of the earth moves in this plane as it traverses around the sun. 4 Figure 2: This coordinate system for equinoxes is illustrated in the following figure (for which you should imagine the earth to be a point at the center of the sphere). The celestial coordinate system The position of the equinoxes is determined at points on the orbit when both poles of the earth are illuminated by the sun’s rays. It happens when this axis becomes perpendicular to a line drawn from the origin of the co-ordinate system to the center of the earth. Even though the orientation of the earth’s axis remains almost the same in a given year ( more likely 50 seconds angular distance in an year ) yet, its orientation changes over longer time period, i.e., the orientation change is cyclic in nature.