While it might seem counterintuitive that a "once-in-a-decade" event happens twice in two years, it's important to understand that the terminology can be a bit misleading.
Understanding the Terms
Supermoon: This occurs when a full moon is at its closest point to Earth in its orbit, making it appear larger and brighter. These happen several times a year.
Blue Moon: Traditionally, this referred to the third full moon in a season with four full moons. However, a more popular definition is the second full moon in a single calendar month. This is less frequent but still not exceptionally rare.
Why the Confusion?
The term "once-in-a-decade" for a Supermoon Blue Moon is a generalization. While it's true that the combination of both events is relatively uncommon, the exact timing of these events can vary significantly.
Supermoons: As mentioned, these occur multiple times a year.
Blue Moons: While less frequent, they can happen every two or three years.
When the timing of these two events aligns, we get a Supermoon Blue Moon. This alignment can happen more frequently than once every ten years, as the individual occurrences of each event are not strictly tied to a decade-long cycle.
In essence, while the combination of a Supermoon and a Blue Moon is a special event, the individual components occur with more regularity than the term "once-in-a-decade" might suggest.