ANN ARBOR—Michigan skywatchers will enjoy two full moons during the month of June, but just barely. The first will appear on the first evening in June; the second will take place just one minute before the month ends at midnight on June 30, according to University of Michigan astronomer Richard Teske.

“The official time of full moon is determined by the exact moment each month when it is opposite the sun,” Teske says. “Two full moons can take place in one month, because the interval from one full moon to the next is about 29 days and 13 hours or just a little shorter than the length of most months. This means that February with its 28 or 29 days can never have two full moons, and occasionally doesn’t have any.”

Teske adds that a two-full-moon-month comes along once every 33 months on average. The next occurrence will be in January of 1999. It is common to refer to the second full moon in a month as a “blue moon” to suggest it seldom happens, as in the phrase “once in a blue moon.”

June’s full moons will rise in the far southeast and traverse the southern sky quite low down, following nearly the same track taken by December’s sun in the daytime sky. The sun is farthest north on June 21, and is nearly as high all month long. The full moon, opposite the sun on the sky, is then farthest south. In December, positions are reversed and the full moon rides highest all night long.

“When it is highest in the sky, the full moon looks brilliant and has an undoubted yellow-white color,” Teske says. “Yet the moon is a very poor reflector of sunlight, with about the reflecting power of an asphalt parking lot. Its brilliance and apparent yellow-white color in the night sky are an illusion caused by dark adaptation of one’s eyes, together with the fact that the black sky provides no background illumination for comparison. Astronauts who have circled the moon and walked on its surface report it is almost colorless. Some describe the color as a dull grey; others say it is a dull grey-tan.”

The light and dark areas we see as the face of the “man in the moon” are the result of regional differences in surface reflectivity. Early astronomers believed the darker regions were “seas,” the brighter ones “highlands.” Samples of lunar material brought back by American astronauts and by unmanned Russian spacecraft have revealed the true nature and ages of these regions.

“The bright highlands are cratered and pitted moon crust dating back to the very origins of our satellite 4.4 billion years ago,” Teske says. “The darker seas are vast plains of hardened lava that emerged from the moon’s interior to flood parts of its surface a billion years later. These plains also are pockmarked with craters from meteor impacts. Except for small outcrops of rock on Earth, the whole surface of the moon is older than the oldest parts of the Earth’s surface.”

With its entire past exposed to view on its surface, the moon is ideal for examination by planetary scientists who probe the history of the solar system, according to Teske. “When you look up at the moon on a warm summer’s night, you are gazing upon the most ancient planetary surface visible to the naked eye.”

University of Michigan