Rahu and Ketu are the nodes of the Moon. When l lie Moon crosses the ecliptic while going from south to north of the ecliptic, it is the ascending node of  the  Moon which is called Rahu. The latitude of the Moon at Rahu is zero and is on the increase from the negative (south) to the positive (north). While  crossing the ecliptic going from the north to the south i. e.  the descending node of the Moon is called Ketu. In figure 15, while there appear four points of … Continue reading


Nodes are the points at which the orbit of the Moon, or any planet cuts the plane of ecliptic. During the course of its movement (i.e. of the Moon or the respective planet) when the said heavenly body crosses the ecliptic plane, the crossing points are called nodes of the Moon or of the respective planet. When the Moon or the respective planet crosses the ecliptic while going from north to south, the crossing point is called descending node. In the former case, the latitude of the Moon or the … Continue reading

Phases of Moon

The Moon has no light of its own but it reflects the light received from the Sun. It revolves round the Earth and its path is inclined at an angle of 5° approximately to the ecliptic. So, the eclipse cannot take place on every amavasya and purnima. (It will be explained in the next chapter.) The Earth is in the centre with O as its centre and there are eight positions of the Moon shown around the Earth (see figure 14 on the next page). The sunrays are coming from … Continue reading


Zodiac has been divided into twelve rashis each  of  30° and their Indian names have been given   according to the shape of the stars in it. 1) Aries = Mesha              (7) Libra = Tula 2) Taurus = Vrisha          (8) Scorpio = Vrischika 3 )  Gemini = Mithuna    (9) Sagittarius = Dhanus 4) Cancer = Karkata      (10) Capricorn = Makara 5 )  Leo = Simha              (11) Aquarius = Kumbha 6 )  Virgo … Continue reading


Zodiac is an imaginary belt of about 9° North or  South  of the ecliptic within which the Moon and all the planets (except Pluto) remain in the course of  their  movement. The fixed zodiac is one in which the first point of  Aries is always fixed in the nakshatras i.e. always at an angle of 180° to Chitra star. The longitudes measured with reference to this fixed first point of Aries which has a permanent position on the ecliptic fixed among the stars are called Nirayana longitudes. They are divided … Continue reading


The effect of Sun’s and Moon’s attraction is not constant. Moon is sometimes above and sometimes below the ecliptic and therefore its pull on the equatorial bulge of the Earth is not always in the same direction as that of the Sun which results in the nodding of the celestial pole to and from the pole of the ecliptic. This nodding is called nutation . The  result  is that the precession is sometimes more and  at other times less than its mean value by about 9 fronds of arc to … Continue reading

Precession of Equinoxes

By continuous observation our rishis found out that the   longitudes of stars are increasing. Later on, the same phenomenon was noticed by the Greek astronomer Hipparchus (190-120 BC). They  considered two possible explanations for this: (1) The stars  are moving but the movements of all the stars  were mostly identical which was impossible. So they  shiftded it. (2) The first point of Aries (which is the intersection of ecliptic and celestial equator) is shifting backward. They also observed that there was no appreciable change in the latitudes of the … Continue reading


First of all we take up the case of an inner planet. Let it be Mercury. The Sun is in the centre around which all the planets, including the Earth, are moving. Mercury is nearer to the Sun and it completes one revolution in 88 days. The Earth is away from the Sun and completes one revolution in 365V4 days. So, the angular velocity of Mercury is faster than that of the Earth. The arrows in the figure are showing the direction in which Mercury and the Earth are moving. … Continue reading

Kepler’s Laws

The laws according to which the planets move around the Sun were discovered by John Kepler (1571-1630) which are given below. 1    Each planet moves in an elliptic orbit with the Sun in one of the foci. 2    Equal areas are covered in equal times by the radius of the planet i.e., by the line joining the planet and the Sun. 3    The squares of periodic times of the planets are to one another as the cubes of their mean distance from the Sun. Though the three … Continue reading


Meteorites: These are relatively larger bodies, big rocks etc., which do not get completely burnt up in atmosphere before reaching the Earth’s surface, and which produce craters etc. or get buried deep. Continue reading