The Sidereal Calendar is a solar calendar that follows the annual motion of the sun relative to the stellar zodiac, rather than to the equinoxes and solstices.
The start point of the year in this calendar is the point at which the sun passes the ecliptic longitude of Sagittarius A*, the radio source identified at the heart of our galaxy. The equatorial coordinates for Sagittarius A* were RA 17h 45m, Dec -29 d 0 m in year 2000 AD/CE. The ecliptic longitude of that point was 267 d. The sun would have reached that longitude on December 19, 2000, which would be the 1st day of that sidereal year.
As it happens, Sagittarius A* is very close to the constellation boundary between Scorpio and Sagittarius. For this reason, Sagittarius A* is designated the 0 point of Sagittarius on the ecliptic. The ecliptic is then divided into 12 zodiac divisions each 30 degrees of the ecliptic from Sagittarius 0, and each named with the names of the 12 classic zodiac constellations.
| Name | Length | Usual current Gregorian start date | |
|---|---|---|---|
| ♐️ | Sagittarius | 29 (30 in leap years) | December 19 |
| ♑️ | Capricorn | 30 days | January 17 (18 in leap years) |
| ♒️ | Aquarius | 30 days | February 16 (17 in leap years) |
| ♓️ | Pisces | 30 days | March 18 |
| ♈️ | Aries | 31 days | April 17 |
| ♉️ | Taurus | 31 days | May 18 |
| ♊️ | Gemini | 31 days | June 18 |
| ♋️ | Cancer | 31 days | July 19 |
| ♌️ | Leo | 31 days | August 19 |
| ♍️ | Virgo | 31 days | September 19 |
| ♎️ | Libra | 30 days | October 20 |
| ♏️ | Scorpio | 30 days | November 19 |
Leap rules[]
The leap year rules for the sidereal calendar are as follows:
- There is a leap year every fourth year, through the 36th.
- Then the 39th year is a leap year (10 leap years in 39 years).
- Then the cycle resumes after the 39th year
- The 39 year cycles repeat 4 times through the 156th year, then the 160th year is a leap year. (41 leap years in 160 years)
- The 160 year cycle repeat 5 time through the 800th year (205 leap years in 800 years, 9 more than in 800 Gregorian years)
These rules result in 292,205 days in 800 sidereal years, with an average year length of 365.25625 days.
Those who read their horoscopes regularly will notice that the zodiac months in this calendar start almost one month later than the traditional zodiac months. This is because this calendar is adjusted for the precession of the equinoxes that has occurred since the traditional zodiac was defined 2,000 years ago.
In order to keep the month lengths close to the time for the sun to move 30 degrees, the month lengths should be shifted by one month every 2,100 years.
Accurate Sidereal Year is about 365.256363004 days (J2000) (365 Days 6 Hours 9 Mins 9.763545 secs)
If we add 10 leap days in 39 years, the length of the year comes around 365.25641025641025641 which is nearest to the accurate sidereal year.
- 10 leap days in 39 years ⇒ 365.25641 (365 Days 6 Hours 9 mins 13.8461538sec → 4 secs error per year)
- 7 leap days added in Julian calendar about 1100 years ⇒ 365.2563636363 (365 days 6 hours 9 mins 9.8181 secs → 0.05 secs per year)
- 41 leap days in 160 years gives the length of 365.25625 (365 days 6 hours 9 mins) ⇒ gives 9.763545 secs per year.
So the best method to approximate the sidereal year is 10/39 method.