The vedas are considered to be the ultimate authority by the Hindus, much like the New Testament and the Quran by the Christians and Muslims, respectively. The Rigveda is generally believed to be the oldest of all the four Vedas.
The Rishi Yaska applauds the Moon for its ability to reflect the sunlight as seen from the Earth (Nirukta 2.6), a fact duly reiterated by the Rishi Gotama, son of Rishi Rahugana (Rigveda 1.84.15). In those days, the Full Moon was referred to as Raka and the New Moon as Sinivali (Sayana Bhasya, and Rigveda 2.33.8). The cause of the eclipses is ascribed to a demon, called Svarbhanu, striking the Sun with darkness. When the gods could not discern the Sun (hidden as it was by darkness), they invoked Rishi Atri who repelled its darkness by chanting four Rik mantras (Rigveda 5.40.5-6) Before he could chant the four Riks, which must have taken at least 50 seconds, Atri must have had to be informed by his disciples or his sons about the sudden darkness that fell upon the Earth; in which case, the actual process must have lasted a few minutes--the typical duration of a total solar eclipse. In the ninth Rik of the same sukta, it is clearly mentioned that only the students and descendants of Atri knew how to dispel the sudden darkness which befell the Sun. It can perhaps be interpreted that only these people knew how to predict the duration of a total solar eclipse.
Even more revealing passages exist in the Pancavimsa Brahmana which describes Atri's dispelling of the darkness by the four Riks in four steps: the first part of the darkness he repelled became a reddish sheep (solar chromosphere), the second part he repelled became a silvery sheep (solar corona!), the third part again became a reddish one, and the fourth a white sheep (regaining the original colour.)
One of the six Upavedas or Vedangas, namely the Vedanga Jyotisa, supposedly composed by Rishi Lagadha around 1300 B.C., makes no mention of the eclipse calculations, even though the concepts of astronomical time divisions are present in great detail.
It is also to be noted that Rahu is nowhere mentioned in the Rig, Sama and Yajurveda. In the Atharvaveda, (13.2.16-18,28) the description of Ketu is more suited for what we today call a comet. In the primitive form of astrology, there was no place for Rahu and Ketu, in particular in the manner in which the division of the twelve houses among the five planets, the Sun and the Moon was conceived. However, Rahu has been earmarked for a quota of twelve years in the astottari dasa system, in addition to the quotas for the seven other regularly wandering celestial objects.
In the original version of the Valmiki Ramayana, a vivid description of a total solar eclipse is given in the first fifteen slokas of the twenty-third sarga of the Aranyakandam, but there is indeed a mention of Rahu as the cause. The description, which appears in the context of a ravaging war between Khara and Lord Rama is worth quoting:
In the proximity of the Sun there appeared a dark reddish disc; at an odd hour of the day the evening dusk approached at an extremely fast pace to be followed by a sudden nightfall; nothing could be discerned anymore; the birds and the animals became awestricken and started howling at the top of their voice; the Sun was totally engulfed by Rahu and appeared to be totally dull, but a big halo was seen around the dark disc of the Sun; some stars and planets became visible....
In the Navagrahastotrom, supposedly composed by Vyasadeva, the description of Rahu fits quite well with that of its Pauranika version. However, the description of Ketu is more akin to that of a comet devouring stars as well as planets. No eclipse can come in the way of the stars and planets exclusively, except for the bodies themselves.
It is suggested in the Mahabharata that during the 18- day long war between the Pandavas and Kauravas, there were only thirteen days between a Full Moon and a possible total solar eclipse presumed to have been seen over the battle field of Kuruksetra. Now, as we noted earlier, a similar condition was invoked in the Holy Quran for prophesying the return of Muhammad in the future. It is therefore, the expected rarity of such an event that must have been known to the authors of such holy scriptures.
A sizable portion of the 35th chapter of the Bhagvata Purana is devoted to narrating a fable as to why Rahu and Ketu are responsible for all the solar and lunar eclipses. Initially, at some stage,the gods and the demons were engaged in a great war over a very long period. Lord Visnu persuaded both the gods and demons for a temporary peace so that the ocean in its entirety could be churned and both devas and the asuras could benefit from what the sea had to offer. The gods under the leadership of Indra grabbed the tail of the king of snakes, Vasuki, who was used as a rope for churning the ocean, while the demons under the leadership of Vali held the mouth of Vasuki. The great mountain Mandara acted as the churner. As a result of this churning, the first to come out was the terrible poison, halahala. It was voluntarily sucked in by the Lord Siva, but he held it in his throat, the reason why Siva is also known as Nilakantha. Then out came Surabhi, Kaustubha, Parijata, Laksmi, the Moon, Varuni (the goddess of wine). And finally emerged Dhanvanatari, the originator of medicine, with a pot of amritam, the nectar. The demons ran away with the pot. Lord Visnu acting in the interest of the gods transformed himself into Mohini, a beautiful woman. Dazzled by her beauty, the demons offered the pot to Mohini and asked her to distribute the nectar amongst themselves as she was found to be an appropriate person. Of course, she gave all the nectar to the gods. One of the demons, Rahu, saw through the trick and sat in the line in disguise, where the nectar was being distributed. But before he could swallow the divine nectar, the Sun and the Moon detected his masquerade and reported it to Lord Visnu, who then chopped off Rahu's head with his sudarsan cakra. Because Rahu had already drunk the nectar, he remained alive in spite of his being reduced to a trunkless body. Since then Rahu has not forgiven the Sun and the Moon. And this is the reason why, as has been depicted in the style of Pauranik description, every once in a while Rahu gobbles up the Sun or the Moon and we witness the solar and the lunar eclipses. Of course, being trunkless he cannot hold either the Sun or the Moon for long, and they come out safely after a while. With time the headless trunk of the demon came to be known as Ketu, and the earlier version of Ketu gracefully turned into what is known as Dhumketu.
Since then, Rahu has become acceptable as the eighth planet, and in the astrological formulations, we find it in the astottari dasa system. Later on, another dasa system, called the vimsottai dasa system, was introduced which referred to nine ruling planets, the ninth being Ketu.
Sometime in the third century A.D., the era of Siddhantas was ushered in. The Surya Siddhanta gives detailed methods for making ecliptic calculations. It was around this time that Rahu and Ketu were astronomically defined to be the ascending and the descending nodes of the lunar orbit, intersecting the plane of the Earth's orbit. The astronomical significance of Rahu and Ketu was reduced to mere imaginary points of intersection between the lunar orbit and the plane of the ecliptic.
As we have noted earlier, evidence for observation of eclipses is found in the ancient Indian texts. A solar eclipse finds mention in the Mahabharata, where Lord Krisna skillfully uses his knowledge of eclipse predictions to save the life of Arjuna, the great warrior. The accuracy with which eclipses have been predicted is incredible. In the Indian almanacs, the time at which an eclipse begins, at which it peaks, and when its ends have all been given. This entire period of an eclipse is called the parvakala.
Thorough prescriptions of the customs which an orthodox Hindu is supposed to observe on the eclipse days are given in the Manusmriti, Grahalaghava, Nirnaya Sindhu, and a number of similar textbooks including Atharvaveda.
It is suggested that the inauspicious period actually begins about four praharas before a solar eclipse and three praharas before a lunar eclipse, (prahara being the unit of time equivalent to one-eighth of a day). Since food prepared during this period is not supposed to be consumed at any time, Hindus generally observe a fast. The water, clothes, food grains, etc., are sprinkled with the sacred tulasi leaves. People take a bath as soon as the eclipse begins and also perform rituals like tarpana, sraddha, homa, prarthana and give donations. After the eclipse is over, one is suppose to take a bath once again. To erase the unholy shadow of Rahu that has fallen on one and all and avoid its ill-effects, another bath is prescribed after the eclipse is over. However, married women are not allowed to wash their hair.On the other hand, it is believed that during an eclipse all the water becomes as pure as the water of the holy river Ganga. Any surging water (say, waterfall or a fountain) or water in the lakes, river and the seas is regarded as a good omen, its auspiciousness increasing in the above sequence. The more pious may go to some sacred place or to the nearest seashore for taking a holy bath. There is a strong belief that any donation given during the eclipse hours is most auspicious and valuables such as money, clothes, cows, horses, land and gold are to be gifted away. The very rich might weigh themselves against brass or copper utensils or even gold or silver, and the same amount is recommended for donation. During this period, circling around a cow giving birth to a calf is equivalent to walking (pradaksina) around the earth.
If the death anniversary of the clan ancestors falls during this period, the rites are not performed in the usual way by feeding the Brahmanas; instead, the grahansraddha should be performed with ordinary or uncooked food or, more simply, by donating gold to the priest. Such rituals can be performed even at night as there are no restricted hours. This in itself is contrary to most of the conventional practices which are performed during the daytime, and shows to what extent our ancestors were awestruck by these celestial phenomena.
It is believed that if the eclipse falls on the third, sixth, eleventh or tenth rasi house from the zodiacal sign of birth (solar or lunar, depending on the parts of India one belongs to), it is auspicious and brings good luck to the native; a solar eclipse falling on the second, seventh, ninth or the fifth place of the natal rasi is regarded as moderately auspicious; but if it falls on the fourth, eighth or twelfth place of the natal rasi, it is considered to be inauspicious and brings bad luck to the person born under these signs.
Those whose birthdays fall on eclipse days should not look at the Sun, while others can look at the eclipsed Sun, but not directly. Such a person is permitted an indirect view of the eclipsed Sun, such as its mirror image from a pot full of pigmented water, or through a cloth. Besides these, there are injunctions against shaving, and cutting one's hair or nails on the eclipse day. This is also the day when no initiative or effort of profound consequence should be taken, and nor should students engage in serious studies.
Thus eclipses are fascinating not only because they have astronomical significance, but also ecause they give us insights into the culture, tradition and beliefs of people belonging to different periods.
An annular eclipse is when the sun is visible like a ring of fire, with the moon darkening its centre. This will be visible only in 
