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SCIENCE HISTORY IMAGES/ALAMY STOCK PHOTO

INTERCALATING SOLAR AND LUNAR. This Chinese lunisolar calendar dates from 978 C.E. and incorporates elements from both solar and lunar calendars. Lunisolar calendars try to reconcile both the solar year (used in solar calendars) and the phases of the moon (used in lunar calendars). However, a solar year does not contain a whole number of days or lunar months. To compensate for this, many lunisolar calendars adjust the length of their years and months by adding in extra months when necessary. Without such an adjustment, the seasons will steadily drift through the months.