May 1, 2007 A blue moon is when two full moons occur within the same month, as dictated by the satellite's 29.5 day orbit around the earth. A full moon is that point in the moon's orbit when it's on the back side of the earth, as seen from the sun.
The term "once in a blue moon" means a rare occurrence, but what does it really mean? On May 31st, there will be a real blue moon in the sky, but many people still don't know what it means.
Kelly Beatty, the executive editor of Sky & Telescope Magazine, offered some insight to DBIS. Beatty said, "A blue moon is when you have two full moons in the same month." So, a blue moon really means an extra full moon. It takes 29 and a half days to go from one full moon to another. But most months have either 30 or 31 days, so those extra days add up and eventually, about once every three years, we get a second full moon in the same month.
Beatty explains why we see the moon as full, "A full moon is that point in the moon's orbit when it's on the back side of the earth, as seen from the sun. So as we look behind us into the night, we see the moon fully illuminated by sunlight."
The expression "once in a blue moon" is not a reflection of what a blue moon really is. Beatty explained it is possible, with certain atmospheric conditions, to have a moon that may be tinged blue, but the expression is just a popular phrase.
The American Astronomical Society and the American Meteorological Society contributed to the information contained in the TV portion of this report.
BACKGROUND: The month of May will have two full moons in the same month, a phenomenon that has come to be commonly known as a "Blue Moon." An older definition applies to an extra full moon that occurs in one quarter of the year, which would normally have three full moons. The third of those four full moons is deemed the extra one, or the "blue moon." According to folklore, when there is a blue moon, the moon has a face and can talk to objects bathed in its moonlight.
ABOUT THE MOON: The moon is Earth's only natural satellite, a cold, dry orb whose surface is studded with craters and strewn with rocks and dust. The moon's gravitational force is only 17% of the Earth's gravity. For example, a 100 pound (45 kg) person would weigh only 17 pounds (7.6 kg) on the Moon. The temperature on the Moon ranges from daytime highs of about 265F (130C) to nighttime lows of about -170F (-110C). The moon has no atmosphere. On the moon, the sky is always appears dark, even on the bright side (because there is no atmosphere). Also, since sound waves travel through air, the moon is silent; there can be no sound transmission on the moon. The phases of the moon are caused by the relative positions of the earth, sun, and moon. The moon goes around the earth, on average, in 27 days, 7 hours, and 43 minutes. The sun always illuminates the half of the moon facing the sun (except during lunar eclipses, when the moon passes through the earth's shadow). When the sun and moon are on opposite sides of the earth, the moon appears "full" to us, a bright, round disk. When the moon is between the earth and the sun, it appears dark, a "new" moon. In between, the moon's illuminated surface appears to grow (wax) to full, then decreases (wanes) to the next new moon.
SEASONAL VARIATIONS: Native American tribes in the northern and eastern United States provided a number of names for various full moons. They used them to keep track of the seasons. For instance, the Full Harvest Moon -- occurring this year on September 26 -- is the nearest full moon to the fall equinox, indicating the best time for the harvest. On the Full Beaver Moon, it was time to set beaver traps before the swamps froze, ensuring a good supply of pelts for winter furs. Other tribes knew it was time for spring planting during the Full Corn Planting Moon, occurring this year on May 2.