Aryabhatt Vedic Lession
The Panchanga
Knowledge of astrology is useful for daily use in indian homes. All the daily rituals and even day to day pursuits make use of astrology. While predictive astrology was mainly restricted for the kings in ancient times, practical astrology in the form of what we call today as electronic astrology was of concern to the layman as well.

The Indian almanac which details information about the festivals, rituals and planetary combinations for the purpose of election of a suitable moment,and has been in use since times immemorial, is called a Panchanga .

A panchanga consists of five parts :

1. Thithi  or the lunar date.
2.Vaara  or the day of the week.
3.Nakshatra  or the lunar asterism
4. Yoga
5. Karana

While vaara or the day of the wek is the function of the sun alone, the other four parts of the panchanga depends on the desposition of the moon alone or the moon-sun duo.The moon thus has a special significance in vedic astrology, besides the sun.
Lunar months
The moon goes round the earth once in a lunar month. Like other planets, it moves from west to east along the zodiac although the rotation of the earth makes it appear to be moving in the reverse direction. One revolution of the moon around the earth produces what is called a lunar months :
(a) The sidereal month : This is the period of time when the moon takes one round of the zodiac, as observed from the earth. It's duration is equal to 27.3217 mean solar days(or 27 days 7 hours 43 minutes approximately). During this period the moon when observed from a fixed star,  moves once round the zodiac and returns to the same star.
(b) The symbolic month : This is the period of time which elapses between the new moon and the next. It's duration is 29.5306 mean solar days (or 29 days 12 hours 44 minutes approximately). The synodic month is also called a lunation. A new moon indicates a conjunction of  the sun and the moon. The synodic month is larger than a sidereal month because it indicates the relation of the moon with the sun. During one revolution of the of the moon,the sun too moves along the zodiac by slightly less than one sign. To catch up with the sun (in order to complete the synodic month) the moon, thus has to take some extra time.
(c) The nodical month : The intersection of the ecliptic by the moon's path results in the formation of the ascending and descending nodes of the moon, respectively known as rahu and ketu. They move in the reverse direction along the zodiac.A nodiacl month is the time taken by moon to complete one round from rahu to rahu. Since rahu moves in the reverse direction the rahu meets the moon slightly earlier along the zodiac.The duration of the nodical month is approximately 27.2122 mean solar days.
(d) The anomalistic month : This is the duration of time that the moon takes to complete one revolution around the earth, in it's orbit from perigee to perigee. It's duration is approximately 27.5546 mean solar days.
The moon loops around the earth in an elliptical orbit just as the earth does around the sun. The orbit itself is in constant flux due to perturbations caused by the sun and other planets. Perigee is the point at which a body (the moon , in this case)in orbit around the earth is at least distance from the earth. Whereas apogee is the point when the body is farthest.

An anomalistic lunar month is the lunar equivalent to the solar anomalistic year which indicates the passage of the sun around the earth from perigee to perigee.

Perihelion and Aphelion are the equivalents of perigee and apogee, with the sun as the center and the planets orbiting around it.
Cycles of moon
A lunar year generally considered for astrological purposes, is a synodic year consisting of tweelve synodic months amounting to approximately 354 days. It consists of 360 tithis i.e. thirty tithis in a synodic month.This year falls short of a solar year of about 365.25 days by eleven days.

If totally lunar calendar was followed , the various seasons would fall to coincide with the lunar months. Since a lunar year would end eleven days before a solar year.This would mean a difference of over a month every three solar years. In order to compensate and make the solar and lunar calendars work side by side a luni-solar concept has been developed. The extra lunar month is considered every third year or before.This extra month is called an intercalary month. The use of various yuga cycles consisting of three year cycles, five year cycles, eight year cycles, eleven year cycles, nineteen year cycles and thirty year cycles etc., are signified brilliant attempts by Indian pre-vedic astronomers to harmonise the solar and lunar years.The nineteen year cycle appears to be the most accurate yuga or cycle. It consists of seven intercalary months over a period of nineteen solar years. It implies that in a period of 228 solar months , there are 235 lunar months(new moons or full moons).
The metonic cycle: Consistent with the above observations is the discovery by Meton (433 BC) that there occur 235 lunations in a period of nineteen solar years. It will be seen that total number of days in nineteen years come out to be 6939.60 days.Total number of days in 235 lunar synodic months come out to be 6939.69 days.The two figures are remarkably close. It means that 228 solar months is equal to 235 lunar months.The nineteen year luni-solar cycle is so accurate that the tithis or lunar days fall on the same days after nineteen years. Even such astronomical phenomenon as the eclipses recur after nineteen year intervals with accuracy.
Adhika maasa or the intercalary month : The sun changes it's sign or rashi every month.The day it enters a sign is called as it's ingress into that sign. A lunar month in which there is no solar ingress into a sign is considered as intercalary month. An intercalary month occurs in 32 solar months and 16 days.This means that an intercalary month occurs in every three years and this year has thirteen lunar months.
Kshaya maasa or omitted month : This happens when there are two solar ingresses(i.e. sun enters two signs) during one lunar month.This happens very infrequently. When there is an month omitted , there occurs two intercalary months during one year.
Paksha : A paksha consists of fifteen lunar dates or we can say two paksha make one lunar month. A krishna paksh extends from Poornima (full moon) to Amavasya (new moon).  A shukla paksha extends from new moon to full moon.
Moon's nodes
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 weestwards 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.

The orbit of the earth around the sun and that of the moon around the earth. Rahu and ketu are formed where the moon's orbit intersects the apparent path of the sun around the earth.