The Motions of the Earth and the effect of Sidereal, Solar and Synodic Time Periods

There are 3 kinds of motion associated with the Earth namely which are:
- Rotation
- Revolution
- Precession

It is to be noted that the Earth rotates around it's own axis constantly along with also revolving around the Sun. There is also a third motion which happens very minutely that is known as procession. The Earth wobbles around it's axis which tilts it's relative position to the background stars by approximately 50 seconds a year. This effect of the wobble causes the earth to shift about 1 degree in a span of 72 years (1 degree = 3600 seconds divided by 50 seconds wobble). Applying some mathematics, we can infer that the Earth's axis gets shifted by 180 degrees in 12960 years (72 years for 1 degree multiplied by 180) and to return back to it's original position of 360 degrees takes roughly 25920 years (72 years for 1 degree multiplied by 360). 

This wobbling effect though very minor in nature since it is only barely 50 seconds for a year is not considered within the parlance of Tropical Zodiac and that is the whole reason why sun sign dates differs between the Indian and Western system. The Indian Vedic system adopts the Sidereal Zodiac that takes into account the Wobble effect whereas the Western system takes only the Tropical Zodiac not factoring the Wobbling impact of the Earth. The debate is however endless with respect to the time period in which the wobbling effect started to take place which gives rise to the phenomenon called 'Ayanamsa' which is defined as the angle by which the sidereal ecliptic of a celestial body is less than its tropical ecliptic longitude.

The consensus currently is that Earth has shifted around 24 degrees from its initial position and that is the reason why we see about 24 days difference between the two zodiacs. For instance, the common belief is that the Sun enters the sign of Sagittarius around 22nd of November but the month of Margazhi which is Sun's movement into the sign of Dhanus or Sagittarius occurs only around 16th December as per the almanacs published. The difference will only expand as time keeps travelling but the thing to be noted is that the dates of the Tropical Zodiac will remain more or less constant whereas it's only the dates in the Sidereal Zodiac that will undergo a change of roughly about a day once in every 72 years.

Another important point to keep track is the time taken by the Earth to revolve around the Sun. We know that one year constitutes of 365 days and a leap year is added once every 4 years to make the tally as 366 days. This is because the Earth takes about 365.2422 days to traverse 360 degrees around the Sun. This extra quarter of 0.25 over and above 365 days is added as an extra day once every 4 years (0.25*4 = 1 day) to bridge for this gap. For all practical purposes, a year cannot consist of days in decimals and hence a conventional year is accounted only as 365 days with a leap year of 366 days thrown in once every 4 years. Also it can be noted that Earth revolves about a degree a day around the Sun (365 days for 360 degrees) with the average mean motion of the Earth around the Sun being accounted for about 59 Min 8 seconds in a day.

Lets take a look at some important terminologies with respect to Earth's motion:

Sidereal Day: A Sidereal Day is the time taken by Earth to rotate around it's axis with respect to the stars. This takes about 23 hours 56 minutes and 4.09 seconds.
Sidereal Day = 23 Hours, 56 Minutes, 4.09 Seconds or 0.997270 Days

Solar Day or Tropical Day: A solar day is the time it takes for the Earth to rotate about its axis so that the Sun appears in the same position in the sky. We know a day consists of 24 hours with the time taken by Earth to rotate around it's axis being 23 hours, 56 minutes and 4 seconds. We also know that the Earth revolves around the Sun. To complete a solar day, Earth must rotate an additional portion equal to 1/365 of a full turn. The time required for this extra rotation is 1/365 of a day is approximately close to about 4 minutes. Thus a Solar Day is recorded as 24 hours.
Solar Day = 24 Hours or 1 Day

Synodic Day or Lunar Day: This basically equates to a 'Tithi' within an Indian parlance and denotes the angular distance formed between the Sun and Moon. The Earth revolves around the Sun and the Moon traverses across the Earth's orbit. Since the motions of the Sun and Moon are always varying in speed, the length of a tithi constantly changes and can alternate between 19 hours to 26 hours. However the mean length considered is 24 hours based on the solar time.
Synodic Day = 24 Hours or 1 Day

Sidereal Month: A Sidereal month is the time needed for the Moon to return to the same place against the background of the stars. The Moon revolves around the Earth and takes approximately a little over 27 days to return back to it's orbit. The average daily motion of the moon is said to be about 13 degrees 10 minutes and 35 seconds and to transgress 360 degrees around the Earth is said to take about 27.32 days (360 degrees divided by 13 degrees, 10 minutes and 35 seconds). In Vedic Astrology, the constellations are divided into 27 types of Stars or Nakshatras with each having a time span of 13 degrees 20 minutes. Thus it could be configured that on an average, a Nakshatra span lasts a little over one day considering the daily mean motion of Moon is 13°10'35'' whereas time duration associated with a Nakshatra happens to be 13° 20''.
Sidereal Month = 27 Days, 7 Hours, 43 Minutes, 11.5 Seconds or 27.321661 Days

Solar Month: A solar month is derived based on a Calendar year which consists of 365 days and a leap year added once every 4 years making it 366 days. We know there are 12 months in a year and dividing it by 365 or 366 results in about 30.5 days per month. However for practical purposes we do not normally round the days in a month and the standard days in a month is taken as 30. However some of the months do also have 31 days with February having the least with 28 in normal years and 29 during a leap year.
Solar Month = 30 Days

Synodic or Lunar Month: A Lunar month is considered to be the time period between one full moon to another full moon or one new moon to another new moon. This is based on the relative reference of the Sun and Moon with Earth being the binding factor. Taking the Sun and Moon to be in the exact same position, we already know that a Sidereal month takes approximate 27.32 days. By the time the Moon takes this time to make one path on the Earth's orbit, the Earth would have motioned around 27 degrees towards the Sun. (Since Earth's motion around the Sun is about a degree a day or more precisely 59 min 8 seconds per day). While the Moon is orbiting Earth, the Earth is progressing in its orbit around Sun. After completing a sidereal month, the Moon must move a little further to catch up with the new position having the same angular distance from the Sun. Therefore, a Lunar month takes about 2.2 days longer than a sidereal month making a Lunar Month approx 29 and a half days in length.
Synodic Month = 29 Days 12 Hours 44 Minutes 2.9 Seconds or 29.530589 Days

Sidereal Year: A Sidereal Year is the time that Earth takes to orbit the Sun and return to the same position in relation to the background of the stars. It takes into account the precession caused by the wobbling of the earth and hence it takes about 20 minutes longer than a Tropical year which does not factor into account the effect of precession.
Sidereal Year = 365 Days, 6 Hours, 9 Minutes, 9.5 Seconds or 365.256364 Days

Solar Year or Tropical Year: A Solar Year or Tropical Year which is also known as the Year of the Seasons is the time it takes the Earth to complete a full orbit around the Sun. However while deriving the measure of a Tropical year, the axial tilt of the earth caused due to it's wobbling nature is not considered into account making it slightly shorter in duration than a Sidereal Year.
Solar Year = 365 Days, 5 Hours, 48 Minutes, 46.43 Seconds or 365.242189 Days

Synodic or Lunar Year: A Synodic or Lunar Year is based on the monthly phases of the Moon's cycle which constitutes the relationship based upon the angular distance between Sun and Moon. We have seen that a Lunar month lasts about 29 and a half days and hence to cover 12 cycles will take on an average of about 10-11 days less than a calendar year of 365 days. The number of Lunar months in a Solar Year is said to be about 12.36827.
Synodic Year = 354 Days, 8 Hours, 48 Minutes, 34 Seconds or 354.367060 Days  

Based on the motions of the Earth, the following can also be summarized briefly:
Rotation of the Earth causes the Day and Night Phenomena
The Earth rotates from West to East and one side of the Earth faces the Sun causing day light and the other side gets obscured from the rays of the Sun causing darkness. The Rotation of the Earth also determines the Ascendant in a Horoscope which is the point on the eastern horizon when an individual takes birth. The Ascendant has a direct correlation with the Sun. The Sun rises in the East which dawns a new beginning. In the default Kalapurusha chart, the first sign Aries which also happens to be the exalted sign for Sun is said to denote the Eastern direction. Also the Sages have attributed Sun to be the significator for the first house of birth manifestation.

Revolution of the Earth causes Seasons
As the earth rotates on its axis producing day and night, it also moves about the sun in an elliptical orbit that requires about 365 1/4 days to complete. The earth's axis is tilted with respect to its orbital plane which is what causes the seasons. When the earth's axis points towards the sun, it is summer for that hemisphere. When the earth's axis points away, winter can be expected. Since the tilt of the axis is 23 1/2 degrees, the North Pole never points directly at the Sun, but on the summer solstice it points as close as it can, and on the winter solstice as far as it can. Midway between these two times, in spring and autumn, the spin axis of the earth points 90 degrees away from the sun. This means that during this period, day and night have more or less about the same length of 12 hours.
                           

-> Summer Solstice : Jun 20-22 - Sun reaches it's northern most point when North Pole tilts closest to the Sun - Northern Hemisphere experiences longer days & shorter nights - Southern Hemisphere experiences shorter days & longer nights

-> Winter Solstice : Dec 21-22 - Sun reaches it's southern most point when South Pole tilts closest to the Sun - Northern Hemisphere experiences shorter days & longer nights - Southern Hemisphere experiences longer days & shorter nights

-> Vernal or Spring Equinox : Mar 20-21 - Sun moves northward across the celestial equator with the North Pole starting to tilt towards the Sun indicating climate starting to get hotter in the Northern Hemisphere. Northern Hemisphere experiences Spring with days & nights being equal - Southern Hemisphere experiences Autumn with days & nights being equal.

-> Autumn Equinox : Sep 22-23 - Sun moves southward across the celestial equator with the South Pole starting to tilt towards the Sun indicating climate to start getting hotter in the Southern Hemisphere. Northern Hemisphere experiences Autumn with days & nights being equal - Southern Hemisphere experiences Spring with days & nights being equal.

Note: Autumn always comes after summer season and spring follows winter season. In autumn, the temperature gradually decreases and in spring the temperature starts increasing. The Autumnal Equinox is related to the autumn season and the term vernal equinox is related to the spring season

Additionally, it would also be a subject matter of interest to know about the concepts of Perihelion and Aphelion. Perihelion or Periapsis occurs when Earth moves closest to the Sun which takes place around 1st week of January every year whereas Aphelion or Apoapsis is when Earth moves furthest away from the Sun and this takes place around the first week of July every year.
                               

Precession of the Earth causes Ayanamsa or Precision of Equinoxes
As the Earth wobbles around it's axis, it leads to an axial tilt creating the effect of Ayanamsa which leads to the formation of Sidereal Zodiac based upon the backdrop of the stars as opposed to the Tropical Zodiac which causes seasons and does not factor the wobbling effect into it's equation.

Interlink in Astrology - Grouping or Pairing of Houses

Whenever one views or analyses a house result in Vedic astrology, observing it purely just from a singular bhava does not always yield the rightful result. There are 12 houses and in one way or other each and every house establishes a relationship with each another in producing a result. The pairings or groupings of like minded houses should always be at the back of the mind before interpretation. There are basically 12 houses in a horoscope and we know that the number 12 is divisible by 6,4,3 and 2.

Let us look at the different groupings or pairings of houses:

-> 6 House Grouping:

- Visible and Invisible Houses:
1-2-3-4-5-6 : Represent first 6 houses covering the invisible portion
7-8-9-10-11-12: Represent the last 6 houses covering the visible portion

In general the houses 1,2,3,4,5,6 are considered as the invisible portion and the remaining houses 7,8,9,10,11,12 are taken to be part of the visible half of the zodiac. The first house represents the rising sign on the eastern horizon and the 7th house represents the setting sign on the western horizon. The houses from the 7th are the ones that have already occurred before birth and hence said to represent results of the material world. The Lagna or the first house is basically the one you're born with and yet to experience the results. It is due to this reason that the base of Dharma trikona (1st house) and Moksha trikona (4th house) are in the invisible half and the base of artha trikona (10th house) and kaama trikona (7th house) fall in the visible half. The first 6 houses generally represent more of the private matters whereas the latter 6 houses depict more of the public matters. If more planets are placed in the visible half as opposed to the invisible half it can be interpreted as a native working more for his own self and development than for the good of a common cause and vice versa.

- Odd and Even Houses:
1-3-5-7-9-11: Odd Houses denoting more of internal perspective
2-4-6-8-10-12: Even Houses denoting more of external perspective

The odd houses in general are more self and relationship oriented by nature whereas all even houses denote material pursuits which involve wealth, reputation etc. The odd houses cover all the houses from the Dharma and Kama Trikona and the even houses take into account the Artha and Moksha significations. More planets in the even houses signify a native more focused towards his goals/objectives, achieving a status and looking towards comforts whereas a native with more planets in the odd houses works hard towards building relationships and enjoying bodily pleasures. A person with more planets in even houses can be said to be less emotional as opposed to one with more planets in odd houses as odd bhavas involve a lot of interaction and connection.

-> 4 House Groupings:

- Kendra, Panapara and Apoklima Houses
1-4-7-10: Kendra houses are the center houses denoting physical manifestation (Physical)
2-5-8-11: Panapara houses are money houses where emotions and creations are involved (Emotional)
3-6-9-12: Apoklima houses are expense houses where intellect and thoughts are involved (Mental)

The Kendra houses are like a foundation providing support to one other. All kendras will be like a pillar or support to each other and comprise of different element - Fire, Earth, Air, Water. They all will also belong to the same mobility (movable, fixed or dual) and influence one another. The Panapara as the name sounds denotes all the money related houses and also termed as the 'Anukendra'. The Apoklima houses are the houses towards which we expend or use for achieving our objectives and these are also called as 'Upakendra'. As per the Kala Purusha, these denote the planets Mercury and Jupiter who respectively rule the 3,6,9,12 houses where intelligence is conveyed. Thus more planets in Kendra can mean acquiring one's life goals whereas in Panapara can mean to attain material significance and in Apoklima can mean focus more on learning and growth. In an overview, we can say houses 1-4-7-10 to be houses of power whereas 2-5-8-11 to be houses of wealth and 3-6-9-12 denoting growth with 3-6 being more towards physical growth and 9-12 bordering towards spiritual growth. 

- Sthana, Dhana and Dur Houses
1-4-7-10: Sthana
2-5-9-11: Dhana
3-6-8-12: Dur 

The 'Sthana' Bhavas are root of one's existence. These are considered as pillars or stable houses and known as four petals of lotus. The 'Dhana' Bhavas are means through which we gain wealth and knowledge with wealth houses primarily being 2,11 and knowledge houses being 5,9. These are known as food of the lotus. The 'Dur' houses causes losses and misfortune, said to be unstable and shaky and known as prana of the lotus. From the above classification compared to the Kendra wise grouping, the only change happens to be the replacement of the 8th with 9th houses. Though 8th also happens to be an indirect money house, it is more associated with negative shades whereas the 9th despite being an expense related house is said to be more auspicious and fortunate by nature and hence for the Dhana to prosper one would also require the contribution of the 9th house.

- Part of Life Houses
1-2-3-4: Early Part of Life
5-6-7-8: Middle Stage of Life
9-10-11-12: Ending chunk of Life

These are basically at a very high level. The 1st represents the native himself whereas the 2nd denotes primary resources, the 3rd being neighbors, friends, younger siblings and the 4th is the early home environment. During the middle stages, the native acquires children (5th), incurs health issues (6th), gets into relationships (7th) and caring for longevity (8th). The last stage incurs higher level learning (9th), one's inherent karma (10th), attaining one's wishes and desires (11th) and going towards exit/emancipation/moksha (12th). The points through Ashtakavarga can also give a fair idea about which stage of life will be more beneficial to the native.

-> 3 House Groupings:

- Dharma, Artha, Kama and Moksha Trikona Houses
1-5-9: Dharma Trikona (Duty Bound, Rights, Knowledge)
2-6-10: Artha Trikona (Resources, Wealth, Possessions)
3-7-11: Kama Trikona (Desires, Pleasures, Excess)
4-8-12: Moksha Trikona (Comforts, Liberation, Release)

The Trikona Group denotes the aim of life. They work with each other towards a common cause. The 5th from any bhava originates as a product from the 1st and the 9th results as a creator of the first. Best way to understand is that the 5th is the off spring which is the product or creation of the first whereas the 1st is the self (you) which arose from the 9th (father). In this way the trikona bhavas are interlinked. The 2nd house (wealth) is a resultant or creation of the 10th house of profession which incidentally is a by product of the 6th that denotes day to day environment. Based on the planets placed in the Trikona Bhavas one can say whether one is more oriented towards Dharma or Artha or Kama or Moksha. An ideal mix of all the four trikonas is ideally preferred. In an outline however they can be classified as houses 1-5-9 being houses of influence, 2-6-10 as houses of material security, 3-7-11 houses gyrate towards a social connection and 4-8-12 houses indicating happiness and liberation.

-> 2 House Groupings:

- Axis
1-7: Self & Spouse
2-8: Family & Inheritance
3-9: Skills & Knowledge 
4-10: Home Environment & Work Environment
5-11: Intelligence & Desires
6-12: Health & Expenses

As per the Axis group pair, each house and its opposite house is connected in one way or other. The 7th house from any house can be called as the significant 'other'. It can also complement the house placed opposite to it. Eg. 7th denotes spouse complements the Self and vice versa. It also shows the native's interest towards forging various kinds of relationships. Likewise the 4th denotes all our internal comforts enjoying the home life whereas 10th is working towards performing one's responsibilities to keep life moving. That is also one of the reasons 4th house is generally considered as homeland and 10th is about travelling away from one's comfort zone and hence denotes an external perspective. Hence it can be said that every planet or house has an influence on the 7th house from it.

- Pairs
1-1: Forming part of same house
7-7: Forming part of opposite houses
2-12: Maintains a for and against placement
3-11: Friendly placement
4-10: Kendra placement
5-9: Trikona placement
6-8: Obstruction placement

These are different pairs of houses which is checked especially for matching and synastry purposes. A 1-1 placement means planets placed in the same sign so there could be some similarity in behavior and traits. A 7-7 relationship denotes complementary one to each other and this is what is termed as 'Samasaptaka'. A 3-11 usually denotes a friendly understanding as the elements of the signs always happen to be friends. A 2-12 can be beneficial from one side but not from the other. A 5-9 rapport can said to be auspicious but belonging to the similar element. A 6-8 is more of a collision and in general not auspicious.

- Argala
Primary Argala
2-12
4-10
3-11
Secondary Argala
5-9
8-6
Other Argala
7-1

Argala means planetary or sign intervention. It means a bolt or lock. Any sign or planet placed in a house from a particular house has an obstructing influence. For instance if any planet is placed in 2nd from a particular house it boosts the signification of the planet but this can be counter effected if a planet is placed in the 12th to it. Hence more favorable planets need to be placed in the 2nd as compared to the 12th for the event to pass in a good manner. As a special argala rule, it is said that a malefic placed in 3rd does not cause any intervention irrespective of the Argala rule defined.

- Bhavat Bhavam:
1-1 (1st house itself expands result of 1st house because 1st from 1st is 1st)
2-3 (3rd house expands result of 2nd house because 2nd from 2nd is 3rd)
3-5 (5th house expands result of 3rd house because 3rd from 3rd is 5th)
4-7 (7th house expands result of 4th house because 4th from 4th is 5th)
5-9 (9th house expands result of 5th house because 5th from 5th is 9th)
6-11 (7th house expands result of 6th house because 6th from 6th is 11th)
7-1 (7th house expands result of 7th house because 7th from 7th is 1st)
8-3 (7th house expands result of 8th house because 8th from 8th is 3rd)
9-5 (7th house expands result of 9th house because 9th from 9th is 5th)
10-7 (7th house expands result of 10th house because 10th from 10th is 7th)
11-9 (7th house expands result of 11th house because 11th from 11th is 9th)
12-11 (7th house expands result of 12th house because 12th from 12th is 11th)

This is a house to house concept with the resulting bhava being the fruit of the bhava enhancing the effects of the initial house. For instance to check the results of the 5th house of children it is not enough if we only verify the 5th house matters. The 5th house from the 5th which is the 9th should also be checked as the 9th bhava happens to be the fruit yielding house for the 5th bhava. In the concept of Bhavat bhavam all the odd houses attain prominence as they happen to be the resultant bhava for the house being checked.

-> Special Category of House Classification / Other Unever Groupings etc:
In few cases there can also be an unequal grouping such as 8 of them belonging to one classification and another 4 belonging to another grading. One of the examples being the Apachaya/Upachaya group where houses 3,6,10,11 houses are said to be part of Upachaya (viz houses of growth) and all others forming part of remaining Apachaya houses. Similarly for marriage we take primarily the 7th of spouse, 2nd of family and 11th of desire fulfillment. All these may come under different grouping mechanism but a combination of all these three houses are key for marriage establishment along with few other houses as well. Like this in few other cases also a few bhavas can be clubbed when checking a signification.

In general, the houses of Vedic Astrology are viewed under this common parameter:
- The 1st, 5th and 9th houses form a triangle and they are known as “Konas” or “Trikonas” or “Trines”.
-  The 1st, 4th, 7th and 10th houses form a rectangle and called “Kendras” or “Quadrants”
-  The 2nd, 5th, 8th and 11th houses are called called “Panaphara” or “Succeedents” 
-  The 3rd, 6th, 9th and 12th houses are called “Apoklima” or “Cadents”
- The 3rd, 6th, 10th and 11th houses are called “Upachayas”. These are called Growth houses and said to improve with time
- The 1st, 2nd, 4th, 5th, 7th, 8th, 9th, 12th are called "Apachayas". These are the remaining houses not forming part of the Upachaya classification
- The 6th, 8th and 12th houses are called “Trik Sthanas” or “Dusthanas”. Generally these are termed as the bad, evil or negative houses
- The 3rd, 6th, 8th and 12th houses are called “Maraivu” or "Leena". These are the hidden houses. Here the 3rd is added over and above the 6th, 8th and 12th. The reason is all these bhavas become the 12th house of negation from the key houses of 4th (mother), 7th (spouse), 9th (father) and 1st (self)
- The 3rd, 6th and 11th houses are known as "Trishadaya". Here the lords of the houses are termed to be most malefic by nature
- The 4th and 8th houses are called “chaturasras” or the corner houses.

                                                           P.S: Picture pulled from internet

-> Conclusion:
Thus, on the whole when checking for a Bhava, it would be prudent to check the resultant bhava also that affects the bhava in question. Out of these the most powerful happen to be checking the odd/even houses in the 6 group category, the Kendra/Panapara/Apoklima in the 4 group category, the different Trikona houses in the 3 group category and the opposite pair of axis in the 2 group category. Apart from the number of planets and influence in the houses specified, it would also be wise to look at the Ashtakavarga points on an individual/combined based to understand which area of life the native is more inclined towards.