Rahu [North Node] & Ketu [South Node] –
Allow me to explain in very simple and layman terms. For an observer on planet Earth, the Sun appears to move around the Earth and complete one rotation in one Solar Year.
The Moon on the contrary completes one rotation in one Lunar month. Naturally when the Sun appears on the horizon, the Earth will cast a shadow in space [the other end]. The point where Moon’s trajectory cuts across Earth’s shadow in space is denoted by the Nodes. So, every month the Moon will cut across this shadow, so, technically we should experience a Lunar eclipse. However, this is not so, because of vertical displacement of Moon. Same rule applies to Solar Eclipse. Check the pic attached with this explanation.
An alternate explanation will be –
To the observer on earth, the paths of the sun and the moon appear to be two great circles projected on the celestial sphere (see the diagram). The sun’s path, the solar ecliptic, makes a complete revolution in one year. At the same time, the moon’s circular path is completed in about one month. Every month the moon will overtake the sun which moves more slowly. This is called new moon or in Sanskrit, amavasya. Usually the moon’s path passes above or below the sun’s path and no eclipse occurs. But, periodically the moon overtakes the sun at the place where their paths intersect. This causes the sun or the moon to be hidden from the earth’s view and is thus called a solar or lunar eclipse. These places of intersection are the north and south lunar nodes, or as they are referred to in Hindu mythology, Rahu and Ketu. Therefore, in the symbolic language of mythology, Rahu and Ketu are said to “swallow up” the Sun and the Moon. The ancient Hindu observers of the sky were aware of the cause of the solar and lunar eclipses and
so described the process in the language of metaphor.
To understand how Rahu and Ketu are interpreted in Vedic Astrology, we need little bit of mythology – so, here it is below –
In Hindu mythology there is a wonderful story that describes how the gods and the demons once formed an alliance to produce a nectar that could give them immortality. This is the story of the churning of the milk-ocean and the descent of Lord Visnu as the Kurma avatara, the divine tortoise. When the nectar that was churned from this ocean was being served to the gods, a demon, disguised as a god, sat between the Sun and the Moon in an attempt to procure the nectar. When he was detected by the Sun and the Moon, Lord Visnu immediately severed his head from his body. Unfortunately, it was not fast enough, for the demon had already tasted a small quantity of the nectar and had become immortal. Ever since, this demon is said to wreak vengeance on the Sun and Moon whenever they come near. The head of this great demon is known as Rahu and his tail is known as Ketu.
In Hindu astrology Rahu and Ketu are known as two invisible planets [Chhaya Graha OR Shadowy Planets]. They are enemies of the Sun and the Moon, who at certain times of the year (during conjunction or opposition) swallow the Sun or the Moon causing either a solar or a lunar eclipse. In Sanskrit this is known as grahanam or seizing.