How this solar arc is calculated
First, the calculator converts the recorded local birth time to one UTC instant using the offset you enter. It calculates a tropical natal chart for that instant. For the chosen target date, it uses a secondary-progression key: one day after birth in the ephemeris stands for one tropical year of life (365.24219 days). The Sun’s tropical longitude at that symbolic ephemeris moment minus its natal longitude is the true solar arc. That same arc is added to every natal planetary longitude, wrapping around the 360-degree zodiac.
Under the true method, the directed Sun therefore lands on the secondary progressed Sun. The other directed planets are transformed natal coordinates, not their astronomical positions on the symbolic date. With coordinates, the natal Ascendant and Midheaven each receive the selected longitude arc. These are rotated natal axis points; the calculator does not recompute an Ascendant from the new Midheaven or directed house cusps. Its two-ring wheel fixes 0° Aries at the left, shows natal points inside and directed points outside, and traces the closest major directed-to-natal aspects within a one-degree orb.
What changes with another method
The mean/Naibod key used here is 0.98564733° per year. The mean arc multiplies that constant by elapsed tropical years; the true arc measures the Sun’s actual change in tropical longitude at the symbolic ephemeris moment. The result shows both arcs, their signed difference, and both sets of directed positions. Choose which method draws the outer wheel and filters aspects. The difference can grow over a lifetime. Other solar-arc methods advance sidereal time or the Midheaven and then recalculate the Ascendant and houses. Those angle conventions differ from this page’s rotated natal axes.
Input and interpretation limits
An unknown birth time does not define an exact birth instant or reliable angles. This page withholds a precise solar-arc result when the time is unknown rather than silently substituting noon. A wrong historical UTC offset also changes the base moment. At the 1800–2100 edges, a target date can map to a symbolic ephemeris moment outside the supported range; the calculator reports that condition. One-degree aspects are geometric filters, not evidence that an event will happen. Read the individual natal chart and the quality of the source birth record before assigning meaning.