Sky watchers should not miss the chance the see the blue moon this Easter weekend. The next blue moon will rise again on Halloween night in 2020.

The blue moon of March or the Sap moon is considered significant. It will be the last blue moon for the next two and a half years based on the calendar of lunar events.

Not Really Blue

The moon doesn't really turn blue on a blue moon. Blue moon is the term used to call the second full moon of the month. Based on lunar phases, a full moon occurs every 29 days on the average, or almost 12 times a year.

Blue moons or two full moons in a month are rare occurrences that happen only every three years.

"Normally we get one full moon in a month but the lunar cycle - the lunar months if you like - are slightly shorter than the solar ones, and so we have an extra moon every now and then," says Bill Boyd of the School of Environment, Science and Engineering at Southern Cross University.

Blue moons look pale grey and white and indistinguishable from any other moon.

"Squeezing a second full moon into a calendar month doesn't change the physical properties of the moon itself, so the color remains the same," says NASA.

Actual blue moons are often seen after volcanic eruptions. When ash plumes from an erupting volcano fill the sky, particles strongly scatter red light, allowing blue light that acts like a blue filter pass through the ash clouds.

Blue-colored moons were seen after the eruption of Mount Krakatoa in Indonesia, the eruptions of El Chicon in Mexico, Mt. St. Heles, and Mt. Pinatubo. 

Rare Blue Moons

There are two definitions of blue moons. The seasonal blue moon is the third full moon in an astronomical season that is more commonly known as spring, summer, fall or autumn, and winter. Spring and fall begin on equinox while summer and winter start during the solstice.

The monthly blue moon is the second full moon in a month with two full moons. For 2018, there are two full moons in January, and there are also two in March in most time zones.

On Jan. 31, sky watchers witnessed a rare celestial treat with the super Blue Blood Moon, a second full moon that coincided with a total lunar eclipse. February only had 28 days, so it did not have a full moon this year, and the lunar occurrence spilled over to the following month.

Last March 1, the worm moon graced the sky with the first full moon of the month that signaled the start of the March equinox.

From the year 2020 to 2037, at least eight upcoming blue moons can be expected based on the Metonic cycle, the period of 19 years in which the phases of the moon recur on the same calendar date and within two hours of the same time.

On Jan. 31, 2037, another total eclipse of the blue moon is expected.

How To Watch It?

The blue moon will be visible in the early morning sky on March 31, Saturday. It will peak at around 8:37 a.m. Eastern time in the United States.

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