The alignment of a full moon with Friday the 13th creates a rare astronomical and cultural intersection. This event requires the precise synchronization of two independent cycles: the Moon’s monthly phase progression and the Gregorian calendar’s structure. The Moon completes its cycle from new moon to new moon, known as the synodic period, in about 29.5 days. For the full moon phase to land on the 13th of a month, and for that specific date to fall on a Friday, the timing must be exact. This combination of celestial mechanics and calendar structure makes the event an infrequent occurrence.
The Date of the Last Coincidence
The last time a full moon coincided with Friday the 13th was on September 13, 2019. For observers in the Eastern time zone of the United States, the Moon reached its peak illumination at 12:33 a.m. on Saturday, September 14th. However, the Moon appears visually full for about three days centered around this peak, meaning the lunar disk was completely illuminated throughout the night of Friday the 13th for all of North America.
This particular full moon was also notable for its appearance as a “Micro Moon.” This occurs when the full moon phase aligns closely with the Moon’s apogee, the point in its elliptical orbit farthest from Earth. At this distance, the Moon appears approximately 14% smaller and 30% dimmer than a “Supermoon.” The September full moon is traditionally known as the Harvest Moon.
The Frequency of the Event
The infrequency of this coincidence is rooted in the mathematical requirements of aligning the lunar and solar calendars. The Moon’s synodic period of 29.5 days does not divide evenly into the number of days in a year, causing the date of the full moon to shift by roughly 11 days each year. Simultaneously, the 13th of any month has a one-in-seven chance of falling on a Friday, as the day of the week for a specific date progresses one day later each year, or two days in a leap year.
The alignment of a full moon on Friday the 13th is statistically rare. While a full moon on any given date is an approximately one-in-thirty occurrence, the requirement for it to land on Friday the 13th averages out to roughly once every 20 years. This average, however, masks a wide variability; the actual gap between occurrences can sometimes be as short as a few years or extend to more than 30 years. The irregular spacing is due to the non-repeating nature of the solar and lunar cycles.
Anticipating the Next Date
The next confirmed date for the full moon to officially align with Friday the 13th is much farther in the future. Astronomers project that the next time the Moon reaches its exact point of full illumination will be on August 13, 2049. This long interval highlights the complexity of the required orbital and calendrical alignment.
The nearly 30-year gap between the 2019 event and the projected 2049 date is a clear example of the event’s highly variable frequency. While the Moon will appear full on many other 13th days, the moment of peak illumination must be precisely timed. The August full moon is traditionally named the Sturgeon Moon.