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By Dayrius Tay
If you had looked to the skies the evening of 31 January, you would have been greeted with the awe-inspiring sight of a crimson red moon. The celestial event is basically a lunar eclipse where the Earth obscures the sun’s light, casting a shadow on the moon.
In this eclipse, the moon is within the Earth’s full shadow, known as the umbra. However, it is still visible due to Rayleigh scattering in the Earth’s atmosphere. Rayleigh scattering affects short wavelength radiation, which we perceive as bluish (also explaining why the sky is blue), more than it does radiation with longer wavelengths. More red light is refracted through the atmosphere, resulting in the moon’s orange hue.
At this point, you might be thinking that the name ‘Blue Blood Moon’ is oxymoronic. The blue in the name arises not from the colour of the moon, but from it being the second full moon in a calendar month, adding to the rarity of the phenomenon as Blue Moons, occurring once every 2.7 years on average. The term ‘blue moon’ was used originally to refer to the moon literally appearing blue due to dust or smoke particles in the atmosphere. When these particles are similarly sized compared to the wavelength of light, Mie scattering is dominant, scattering long wavelengths (red light) and causing the moon to appear blue.
On the other hand, the ‘super’ in the name is no coincidence. During a full moon, the Sun, Earth and Moon line up. This exerts increased gravitational force on the Moon in the direction of the Earth, causing it to appear up to 14% larger as the moon is physically closer to Earth. The term ‘Supermoon’ was coined by an astrologer (a pseudoscientific fortune teller) and has the esoteric technical name of perigee syzygy. This occurrence is often associated with calamities such as earthquakes and volcanic eruptions, though such claims have been thoroughly debunked. Supermoons generally occur once every 14 months. The last time all these events occurred simultaneously in the Western hemisphere was in 1866.
The blood moon has played a fascinating role in history, with the most notable example being the lunar eclipse in March 1504. Christopher Columbus and his crew were stranded in Jamaica, eating a great deal of the locals’ food, causing the inhabitants to threaten to cut off the food supply. In desperation, Columbus searched through tomes of astronomical tables and noticed an impeding lunar eclipse. He promptly declared to the locals that his god was angry and would make the moon ‘inflamed with wrath’. The blood moon appeared on schedule, prompting waves of alarmed locals to rush to Columbus’ ships with supplies and offerings. The moral of this story? Always have a list of shocking predictions on hand when stranded ashore an island with no food supplies..
This lunar eclipse caused a surge of public interest in astronomy, with 5,000 folks showing up at the Science Centre to catch a glimpse of the blood moon through a telescope. Multiple educational institutes including NUS and NJC organised viewing sessions to raise public awareness about astronomy. It is these cosmic events that spur prospective stargazers to join the Astronomical community and be mesmerized by the vastness of the universe.