By Michel Charles | Updated March 24, 2022
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Astronomers measure a solar year from Earth’s orbit around the Sun, whereas a lunar year is derived from the Moon’s phases. While most cultures employ the solar calendar, lunar calendars still govern key religious observances. The discrepancy between the two cycles is quantified by the “epact”—a difference of roughly 11 days per year.
A lunar year consists of twelve lunations, each lasting about 29.53 days—the time between successive new moons. In contrast, a solar year represents the period required for Earth to complete one full orbit around the Sun, approximately 365.25 days. While calendar months are adjusted for convenience, these definitions underpin the difference in length.
Consequently, a lunar year totals roughly 354 days, whereas the solar year spans 365 days. This 11‑day gap, known as the epact, accumulates over time, leading to a one‑year offset between solar and lunar calendars every 33 years.
Globally, the Gregorian solar calendar is the standard for civil purposes. However, several faith traditions rely on lunar or lunisolar systems. The Islamic (Hijri) calendar is purely lunar, with 12 months that shift through the seasons. The Jewish calendar, though based on lunar months, is synchronized to the solar year through intercalation. China employs a lunisolar calendar that blends both cycles.
To reconcile the 11‑day annual discrepancy, lunar calendars insert an intercalary month every three years, producing a 13‑month cycle. Solar calendars address the fractional day by adding a leap day to February every four years, as seen in the Gregorian system.
