Lunar Days
This calculation module is designed to compute the exact dates and start times of lunar days within a specified time range. Historically, astrological practice has developed several approaches to defining exactly what constitutes a “lunar day”. The Zeus program supports four different algorithms, allowing you to flexibly configure calculations for any astrological tradition (Western, Vedic, Babylonian, or symbolic).
Calculation Settings
At the top of the window, you can set the necessary parameters:
- Geographical location: Since some algorithms (like the classic one) are strictly dependent on the observer’s coordinates, ensure the correct city is selected.
- Calculation Algorithm: A drop-down list in the upper right corner allows you to select the method for computing the lunar day (see below for a detailed description of each algorithm). The screenshot shows the “By Moon risings” method selected.
- Time Range: Set the start date and search span (in years and days).
Results Table and Context Menu
After clicking the “Calculate” button, a chronological list is generated where each row contains:
- The sequential number of the lunar day (from 1 to 30).
- The exact date and time it starts.
- The day of the week (Mon, Tue, Wed, etc.).
- Additional astronomical markers (e.g., if the start of the day coincides with Moon phases — First Quarter, Full Moon, etc.). Right-clicking any row in the table brings up a menu that allows you to instantly Apply to dial (1-4): the program will rebuild the selected working chart dial to the exact date and time the highlighted lunar day begins.
Lunar Day Calculation Algorithms
The choice of algorithm fundamentally affects exactly when a new lunar day begins and its duration.
1. By Moon Risings (Classic Western) This is the most popular algorithm in modern Western astrology.
- How it works: The first lunar day always begins at the exact moment of the New Moon (conjunction of the Sun and Moon). However, the second and all subsequent lunar days begin at the moment the Moon rises above the horizon for the selected geographical location.
- Feature: Because Moonrise depends on geographical coordinates (latitude and longitude), lunar days according to this algorithm are tied to a specific location. As a result, the first and last lunar days of the month can be very short (lasting only a few minutes).
2. Tithi (Vedic Algorithm) This is an ancient Indian calculation system used in Jyotish (Vedic astrology). Unlike the risings method, Tithis do not depend on the observation location or geographical coordinates.
- How it works: The lunar month (29.5 days)