A full moon graced the night sky on Sunday. This one was special – it was the smallest full moon of the year and also the second full moon of May, commonly known as a "blue moon."

While most calendar months have only one full moon, a second one can occur sometimes. This is because the interval between full moons is about 29.53 days, and most months have 30 or 31 days. Over time, this creates the rare situation of a single month with two full moons. When that happens, the second one is called a blue moon. This May, full moons fell on the 2nd and the 31st.

Despite its name, a blue moon has nothing to do with its color. The moon does not turn blue or emit blue light.

On average, a blue moon appears once every two to three years, according to Yang Jing, a member of the Chinese Astronomical Society. The next one is expected in January 2029.

It's rarer for a blue moon to also be the year's smallest full moon. The last time this happened was in 2020.

How does a smallest full moon occur?

It happens when a full moon coincides with the moon being near its apogee, the farthest point from Earth in its orbit, and the time difference between the two is usually less than 24 hours, Yang explained.

This year, the full moon fell at 4:45 p.m. on May 31, while the moon reached apogee at 12:33 p.m. on June 1 – less than 20 hours apart. This made it the smallest full moon of 2026.

Just as there is a smallest full moon, there is also a largest one.

The largest full moon of this year will appear on December 24.

The moon's orbit around the Earth is roughly elliptical, making its distance ranging from about 360,000 kilometers to 400,000 kilometers. The apparent diameter of the largest and smallest full moons differs by about 14%, and their apparent area by about 30%, said Yang, adding that the difference is difficult to detect with the naked eye, but will be clear by using photographic equipment.

If you captured the smallest full moon yesterday, try photographing the largest one with the same settings, and you'll be able to compare the two and see the difference.