The moon

In our daily lives, we give little thought to the moon, Earth’s only natural satellite. However, when it is full or during a lunar or solar eclipse, the moon calls particular attention to itself. On the night of April 14-15, a total lunar eclipse will occur.

A generally accepted definition for moon, not just Earth’s moon, is a natural satellite revolving around a planet. Moons within our solar system are numerous, even plentiful. The planets Saturn and Jupiter have more than 60 each.

There are several theories which attempt to explain the origin of Earth’s moon. Currently, the one in most favor suggests that a Mars-sized object crashed into Earth while the solar system was very young. A portion of the resultant ejected material coalesced over time to form the moon in orbit. Magnetic analysis and moon rocks brought back to Earth indicate that the moon is quite poor in iron. These data constrain the mass of the object that presumably hit Earth, meaning that the object probably did not penetrate to Earth’s iron core.

Interestingly, our moon is very slowly spiraling away from us. A laser target placed on the moon during the Apollo Program confirms that the moon is slowly moving away. The time required for a laser beam to travel to the moon and back to Earth is increasing, as well. Calculations indicate that the effect will lessen with separation from Earth and become inconsequential when the moon reaches a distance of about 330,000 miles. We happen to inhabit Earth at a time when the apparent diameters of the sun and moon are very nearly the same, producing spectacular solar and lunar eclipses.

The moon presents the same face to Earth as it orbits our planet:

• New moon: The moon’s lit half is pointing away from Earth. The moon is not seen.

• First quarter moon: Only the right half of the lit portion of the moon is seen.

• Full moon: All of the lit half is seen.

• Last quarter moon: Only the left half of the lit portion is seen.

— Kent Fairfield is a volunteer with Pine Mountain Observatory and a lifelong amateur astronomer. He can be reached at Other PMO volunteers also contributed to this article.