When checking the progressed MC, many people notice that the results on Astro.com differ from those on Astrology Readings. This is because the two platforms use different calculation methods:
- Astro.com (default) uses ARMC + 1 Naibod (~0.98564733°/year) to calculate the progressed MC.
- Astrology Readings uses MC from Solar Arc (~1°/year, moving linearly along the zodiac).
Neither method is absolutely right or wrong; they simply approach the calculation differently, leading to a discrepancy of about 2-4° in the progressed MC results. Users can choose the method that best suits their interpretation system.
How Astro.com Calculates the Progressed MC (Default: ARMC + 1 Naibod)
What is ARMC?
ARMC (Right Ascension of the Midheaven) is the astronomical coordinate of the MC in the equatorial coordinate system, rather than directly following the zodiac. In the Placidus house system, the MC does not move linearly along the zodiac but is influenced by Earth’s rotation and the latitude of the birthplace.
To calculate the MC at any given moment, ARMC must first be determined, and then the MC is derived from it based on the chosen house system. This is why Astro.com uses ARMC instead of shifting the MC directly along the zodiac.
Astro.com’s default method applies ARMC + 1 Naibod, where Naibod Key (~0.98564733°/year) is the average daily motion of the Sun along the zodiac, used to progress ARMC.
Naibod Key Calculation Formula:
360° / 365.2422 days/year = 0.98564733°/daySince Earth orbits 360° around the Sun in a year (365.2422 days/year), the Sun moves 0.98564733° per day along the zodiac.
Formula for Progressed ARMC Calculation on Astro.com:
Progressed ARMC = Natal ARMC + (Age × 0.98564733°)
After obtaining progressed ARMC, MC is recalculated from ARMC using the Placidus house system or the selected house system. Since the MC is derived from ARMC rather than directly shifting along the zodiac, the progressed MC in this method may not move exactly 1° per year but fluctuate slightly due to the house system’s influence.
Why Astrology Readings Uses Solar Arc MC
How is the Progressed MC from Solar Arc Calculated?
The progressed MC using Solar Arc is determined by moving the natal MC at the same speed as the progressed Sun.
Formula:
Progressed MC = Natal MC + Solar Arc
Where:
Solar Arc = Progressed Sun - Natal Sun
The progressed Sun is calculated based on the “1 year = 1 day” principle in Secondary Progressions.
Why is the “1 Year = 1 Day” Principle Used in Secondary Progressions?
The “1 year = 1 day” principle is based on astronomical observations and astrological symbolism, reflecting how humans develop over time. It is derived from the relationship between Earth’s daily rotation and Earth’s orbit around the Sun.
- One full Earth rotation (1 day) corresponds to a complete cycle in astrology (24-hour system).
- One Earth orbit around the Sun (1 year) is the main cycle used to measure human growth and maturity.
In Secondary Progressions, each day after birth symbolizes one year of life. For example:
- Day 1 after birth corresponds to Age 1.
- Day 2 after birth corresponds to Age 2.
- Day 30 after birth corresponds to Age 30.
Thus, to determine the progressed Sun’s position at any given age, we must find the Sun’s position on the Nth day after birth, where N equals the person’s age.
The Solar Arc MC method applies this principle by shifting the MC at the same rate as the progressed Sun. This ensures that MC moves linearly at approximately 1° per year, maintaining consistency with the movement of other progressed planets.
How to Calculate the Progressed MC from Solar Arc Using Julian Date
To calculate MC from Solar Arc, it is essential to determine the progressed Sun’s position, which corresponds to the Sun’s location on the Julian Date of the progressed day. Julian Date (JD) is a continuous date-counting system used for precise astronomical calculations without the complications of leap years or different calendar systems.
Steps to Calculate MC from Solar Arc Using Julian Date:
1. Determine the Julian Date for the Birth Date using the formula:
JD = 367Y - ⌊(7(Y + ⌊(M+9)/12⌋))/4⌋ + ⌊(275M/9)⌋ + D + 1721013.5 + (UT/24)
2. Calculate the Julian Date for the Progressed Date by adding the number of days corresponding to the person’s age: JD Progressed = JD Birth + Age
3. Use the Julian Date for the progressed date to determine the position of the progressed Sun, then calculate the Solar Arc: Solar Arc = Progressed Sun - Natal Sun
4. Calculate the Progressed MC using the formula: Progressed MC = Natal MC + Solar Arc
This method ensures that MC moves linearly at approximately 1° per year and is not affected by house systems or geographical coordinates.
The progressed MC reflects career changes, reputation shifts, and the evolving direction of one’s life. Try calculating your MC using both methods to see which one aligns better with your personal experiences.
Check your progressed chart here:
🔗 https://astrologyreadings.online/transits-progression-chart-calculator/
