Sky watchers can once again see a sight that has not been seen for three years: a complete lunar eclipse. The eclipse is expected to begin shortly before midnight next Monday, December 20th, and will continue on until just after 5:00 in the morning on the 21st.
A lunar eclipse occurs when the moon passes opposite from the sun, through Earth’s shadow. While there are other types of partial lunar eclipses where only portions of the moon are hidden in shadow, a total lunar eclipse can only occur during the full phase of the moon. This is because the moon has to line up just right on the other side of Earth, exactly opposite of the sun.
So what does this have to do with “blue moons?” “Once in a blue moon,” is commonly treated as something that is a rare occurrence. Blue moons really are not all that rare, actually, and refer to that “extra” moon phase that sometimes works its way into the calendar year. Despite all the mathematical workings of man, a calendar has yet to be invented that works out evenly and has the same number of X, Y, and Z occurrences in astronomy every year. A full moon is approximately every 29.5 days, which is roughly one per month. Over time, though, the moon cycles catch up with our calendar and we may end up having two full moons in one month. When that occurs, that usually happens to be when the next lunar eclipse is expected to occur. On average, a blue moon occurs every 2 ½ years.
So why call it a blue moon? The term blue moon refers to the bluish color the moon sometimes appears to be, due to dust in earth’s atmosphere. The phrase is thought to be derived from the 1883 volcanic eruption at Krakatoa that placed an enormous amount of dust in the air and made the moon actually appear blue for a time. There have been several different, older, definitions of the phrase “blue moon,” but the term is generally understood to be the second full moon in the same calendar month.
You can forget seeing blue, though. During a complete lunar eclipse, the moon actually takes on a reddish color. While, normally, the moon would have light from the sun on the other side of earth shining on it, lighting it up in its usual grey, during an eclipse the light is coming in more at an angle, and the dust in our atmosphere filters out most of the blue light that we normally see. The only colors that are left for us to see are the colors on the other end of the light spectrum (the reds, greens, and resulting yellowish browns). If earth had no atmosphere at all for light to be filtered through, the moon would look black and disappear completely during a total eclipse. Be sure to bundle up as you watch it this year!
Credit: NASA and Obliquity.com.