While the saying “once in a blue moon” suggests an uncommon event, a blue moon isn’t an extremely rare astronomical occurrence. There are various definitions of a blue moon, but the one most widely accepted—backed by NASA—refers to the circumstance where two full moons appear within a single month.
In India, during the Purnima of the Shravan month (August 30-31), the full moon coinciding with Raksha Bandhan will be a remarkable event. It will possess the qualities of a “blue moon” and a “super moon,” thus earning the title of a “Super Blue Moon.” This convergence of rare astronomical phenomena is noteworthy.
On August 30, this special moon is set to rise at 6:35 pm in Delhi, slightly later in Mumbai, and about an hour earlier in Kolkata.
What defines a super moon?
The moon’s orbit around Earth is not a perfect circle; it’s an ellipse, a stretched-out circle. The moon completes its orbit around Earth in 27.3 days.
(From one new moon to the next, the duration is 29.5 days. This is due to the additional time required for the sun to illuminate the moon in the same manner as it does at the start of each revolution around Earth. The new moon is the opposite of the full moon, representing the darkest phase when the illuminated side faces away from Earth.)
The point in the moon’s elliptical orbit closest to Earth is called perigee, while the farthest point is called apogee. A super moon occurs when the moon is either passing through or near its perigee, coinciding with a full moon. (This also applies to a new moon, though it remains invisible.)
A full moon arises when the moon is directly opposite the sun from Earth’s perspective, fully illuminating its day side. Appearing as a luminous circle in the night sky, the full moon rises around sunset and sets around sunrise. The moon appears “full” not only on the Purnima but also on the nights before and after.
And what about a blue moon?
Although “once in a blue moon” implies rarity, a blue moon isn’t as infrequent an astronomical phenomenon. Multiple definitions exist, but the one commonly accepted—validated by NASA—pertains to two full moons occurring within a single month.
Due to the 29.5-day new moon cycle, situations arise where a full moon emerges at the start of a month, with sufficient days remaining for another full cycle to conclude. In such cases, a second full moon can appear on the 30th or 31st. NASA states that this occurs every two or three years.
The first full moon of August 2023 appeared on August 1. It was also a super moon, but the super moon of August 30-31 will be even more significant as the moon is closer to perigee.
Will the moon actually display a blue hue?
No. Occasionally, airborne smoke or dust can scatter red light wavelengths, potentially causing the moon to appear somewhat bluish in certain locations. However, this phenomenon isn’t related to the term “blue moon.”