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.
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