The morning of Jan. 31 will bring a distinctive celestial event to the skies over Hawaii: a lunar eclipse. During the second full moon of the month, commonly known as a “blue moon,” the eclipse will be visible starting at 12:51 a.m. with a very faint shadow beginning to cross the face of the moon. The total eclipse will run from 2:51–4:07 a.m. During the total eclipse, the moon will look red. Because of the red color of a lunar eclipse, popular media started calling the phenomena a “blood moon.”
To understand a lunar eclipse, we need to understand the moon. The moon does not create its own light; it is illuminated by reflected sunlight. The phases of the moon are the appearance of the illuminated portion of the moon as seen by us on Earth. Half of the moon’s surface is always bright, but the part of the illuminated portion that we can see varies. The phases of the moon move through a consistent cycle as the moon orbits the Earth and the relative positions of the Earth, moon and sun change.
New moon occurs when the moon lies between the Earth and sun. The moon rises and sets at roughly the same time as the sun. The sun shines on the part of the moon facing it, which happens to be the side facing away from the Earth. As a result, we don’t see the moon.
Two weeks later, the sun, Earth and moon are again in a line. This time, the moon is opposite the sun, with the Earth between. It rises at sunset and sets at sunrise. The illumination part of the moon’s surface is completely facing the Earth, so we see the whole thing or a full moon.
A lunar eclipse is a special type of full moon. The moon, Earth and sun are in a perfectly straight line. As a result, the moon passes through the Earth’s shadow, temporarily blocking sunlight from reaching the moon’s surface. Because of the Earth’s atmosphere, a small amount of light reaches the moon’s surface.
However, our atmosphere scatters sunlight. Blue light gets scattered more, which is why our sky is blue. The red and orange light from the sun passes through our atmosphere and then bends or refracts back to the surface of the moon, giving it a red color during the eclipse. If the atmosphere is full of particles, like vog, the moon appears redder.
That is the blood moon part of the Jan. 31 full moon, but what about the blue moon? Astronomers measure the moon’s phase cycle as the time from new moon to new moon or every 29.53 days. Many cultures set their calendars based on the phases of the moon, including the Hawaiians. It’s not a coincidence that our months are roughly the same length as the lunar cycle.
This brings us back to the blue moon. The length of our months are no longer set by the phases of the moon, rather they each have a fixed number of days. However, since the length of the month is so similar to the phases, we generally only see one full and one new moon a month. Months with a blue moon are the exception, they are months with two full moons, the second called the “blue moon.” The moon was last full on Jan. 2 and will be full again 29.53 days later, Jan. 31. A blue moon occurs once every 2.7 years.
The blue moon blood moon will likely cause a stir in the upcoming weeks and we highly suggest waking up to see it.