Most of us know the myth of Rahu Ketu eating sun,moon as the reason for eclipse.This makes us think that Hinduism is all about myths and superstition.
To break one such myth or superstition,let me tell you the story of rahu ketu was for comman man.
The real scientific meaning of Rahu is North Node,and Ketu - South node.The Moon's orbit and the earth's orbit intersect and these two intersecting points are known as North Node (Rahu) and the South Node (Ketu).

We observe eclipses when there is such an intersection of the orbits at these nodes.

adding some more to the same topic:
The moon's apparent path intersects the ecliptic obliquely at two points called nodes.This is similar to the sun's path or the ecliptic intersecting the equator at an oblique angle.The point where the moon crosses the ecliptic from south to north is called the ascending node or rahu. Where it crosses the ecliptic from north to south is called the descending node or ketu.These two points are six signs or 180 degrees apart. Just as the equinoctial point shifts westwards on the ecliptic at a constantly shifting point.Thus rahu and ketu go on receding or shifting westward along the ecliptic. Their movement is therefore constantly retrograde.They complete one round of the zodiac in approximately eighteen years and ten days.
Rahu and ketu , though only astronomical points have a special status in vedic astrology. They are treated as graphs or planets, like any other planet.

Rahu is not an actual planet-ball, as the other planets are, but a point in space that is of great influence. The nodes Rahu and Ketu are always opposite each other in the Zodiac. They are calculated from the orbits of the Sun and Moon in relation to the Earth. They are gravitational line up points. They cause the Moon to actually wobble in its orbit.

It is a scientific fact, that when the Moon gets close to the nodes in its real orbit, that it wobbles due to the gravitational pull of the point. So, when in a chart we see that the person was born at a time when the Moon was close to one of the nodes (Rahu or Ketu) that the mind is disturbed, since the Moon rules the mind. This is a fact which you will see played out in the lives of those who have this. Since the nodes occupy two signs, roughly one sixth of the population has this.

However, it is strongest when the Moon is really close to the Node it conjoins- or in other words, when the two are close by degrees within the sign- say within 5 degrees of each other. Then the effects become pronounced.

Rahu and Ketu are also known as the Shadowy Planets or Chhaya Graha, so what does this mean??

All of us know that the Earth Rotates around the Sun. In Astrology we are considering a birth on the planet Earth!!! so we consider ourselves in a steady state while SUN is rotating around US!!!

So naturally the Earth casts a shadow somewhere in space!!!

We also know that the MOON rotates around the EARTH!!! Now the point where Lunar Path intersects the Solar Path