I've noticed an interesting phenomenon and would like to ask for everyone's thoughts on it.

Each year, when the Sun enters Libra in the chart, it appears to repeatedly enter certain houses.

For example, considering the chart for Washington D.C. when the Sun enters Libra:

Regiomontanus house

- In 1980, the Sun was near the 8th house.
- In 1981, the Sun was near the 5th house.
- In 1982, the Sun was near the 2nd house.
- In 1983, the Sun was near the 11th house.
- In 1984, the Sun was near the 8th house again.
- In 1985, the Sun was near the 5th house again.

This pattern continues in the sequence 8-5-2-11.

I recall that Mary Fortier Shea mentioned the reason for this in her book *Planets in Solar Returns: Yearly Cycles of Transformation and Growth*, but I don't have the book with me at the moment.

I'm curious about the principles and astronomical phenomena that cause this pattern.

I hope someone can help me unravel this mystery. Thank you!

This can only relate to the proportion of the revolution of the Sun to the rotation of the earth. You know that one year is approximetely 365.25 days long (more exactly 365.256363), thus ca. 1/4 day longer as 365 full earth rotations. That means approximately after 4 years the Sun is in the same house again. It’s that easy.

Georgius,

Thank you very much for your response. Although I must admit that I had some difficulty understanding it.

Perhaps I didn’t express myself clearly. What I actually observed is that rather than the Sun's position in the houses showing a regular annual change, it seems that it is the Midheaven (MC) that exhibits a regular annual pattern. I suspect this might be related to the precession of the Earth's axis, commonly referred to as the "precession of the equinoxes" or "axial precession".

No, it's not to do with precession (which is a much slower process). Georgius is right: because the solar year is approximately 365.25 years (that is, 365 full days and around 6 hours), the Sun will return to the same place in the zodiac at approximately the same time of day every four years (because 4 x 0.25 = 1 full day/rotation of the earth). This is equally true of annual revolutions of nativities. And when you have the same time of day at the same place on the same calendar date of any year, the MC and the house position of the Sun will also be the same.

Thank you, Georgius and Martin Gansten.

I understand that you are referring to the concept of leap years. I believe it might be clearer to illustrate this phenomenon using a chart.

Actually, the solar ingress chart is a kind of the solar return chart, which shows the Sun returning to the same position every year.

Below are the solar Libra ingress charts, all based on Washington, D.C. as the location.

Hi Georgius and Martin Gansten.

You are correct! I have verified it, and the difference is exactly 23 hours, 24 minutes, and 13 seconds over 4 years.

Thank you !

The Earth’s orbital period, or tropical year, is 365.2422 days—not a neat 360 days—which creates a predictable shift in solar return charts over time.
Here’s the breakdown: each year, the solar return occurs 5 hours, 48 minutes, and 43 seconds later than the previous year (0.2422 days × 24 hours ≈ 5.813 hours). Since the Right Ascension of the Medium Coeli (RAMC) shifts at 15 degrees per hour, this delay moves the MC forward by 87.195 degrees annually (5.813 × 15). That’s nearly three zodiac signs per year—or about one sign every 4.12 months. Over four years, the MC shifts 348.78 degrees (87.195 × 4), coming close to a full 360-degree cycle. This pattern holds as long as you’re at the same latitude and longitude.