We had a full moon July 1, and we will see another tonight.

The second of two full moons in a calendar month is often called a “blue moon.” The time between one full moon and the next is close to the length of a calendar month, so the only time one month can have two full moons is when the first full moon happens in the first few days of that month. This happens every two to three years. Thus the phrase: “Once in a blue moon.”

Our calendar is designed around the moon’s phases. The approximate average of 30 days per month coincides with the 29 days, 12 hours, 44 minutes it takes for the moon to make a full revolution around Earth (say, from one full moon to the next). With a clear night, the moonlight we’re likely to see tonight is actually sunlight reflected from the moon’s surface.

People have yet to travel to another planet, but 12 U.S. astronauts have wandered around on the moon, Earth’s nearest natural celestial neighbor. About 841 pounds of moon rocks and soil have been brought back for study. Selenologists, astronomers who study the moon, tell us that the moon rocks are about 4.6 billion years old, about the age of the solar system.

The moon’s diameter is about 2,160 miles. Earth’s distance to the moon is about 239,000 miles, and the sun is about 400 times farther away from Earth.


Jim Gilbert’s Nature Notes are heard on WCCO Radio at 7:15 a.m. Sundays. His observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota. He taught and worked as a naturalist for 50 years.