Today is our last chance to view the “blue moon”… before it occurs again in 2015. And I thought, may be once in a blue moon, I should recycle one of my blog posts. So here’s it goes — I am reproducing (a modified version of) a short blog post from 2007:
The time it takes for Moon to complete its orbit around Earth is 29.5 days. (The Moon would have needed only about 27 days, but due to Earth’s own orbit around the Sun, it needs a few more hours to catch up.) Since the length of one moon cycle (moonth?!) is pretty close to our calendar month, the Moon usually completes 12 orbits in a year. Hence, we normally see only one full moon in each calendar month. But because the moon cycle is few days shorter than an average calendar month, once in every two-three years two full moons would fall within a single month. When that happens, the second full moon is known as the blue moon. The phrase ‘once in a blue moon’ owes its origin to the rarity and inconsistency (in terms of which calendar month it will occur) of such event. Tonight (August 31st) is our once-in-a-blue-moon opportunity to enjoy an “extra” full moon!
And while you’re at it, take a moment to observe the Sea of Tranquility where Neil Armstrong took the legendary ‘small step for [a] man’ in 1969.
PS: I think the concept of extra month – called adhik māsa – in the Hindu calendar is created to fix this mismatch between lunar months and solar year. Because the months in Hindu calendar are based on lunar phase, they won’t completely align with the solar year and approximately every three years it will have to play catch up, i.e. adhik māsa.