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Trekking the terminator

Savor the sights of a First Quarter Moon.

In the past, I’ve railed about the inconsistency in lunar phase nomenclature. Why do people call a seven-day-old Moon “First Quarter” and a 14- to 15-day-old Moon “Full”? If we label the evening phases according to the sunlit fraction seen from Earth, a seven-day-old Moon would be a Half Moon, and a 14- to 15-day-old Moon would be Full. But by the portion of orbit covered, they become First Quarter and Half.

The first way of thinking makes more sense to me. Most people call a seven-day-old Moon “Half” anyway, and we all agree that the 14- to 15-day-old Moon is Full. We could complete the scenario by calling a three- or four-day-old Moon “Quarter” and the 10- to 11-day-old Moon “Three Quarters.” The latter would eliminate “gibbous,” a word that bewilders many beginners.

Relax, purists. I’m not going to petition astronomers to rename the phases of the Moon. Instead, I’m going to petition you to go outside with your telescope and spend an evening exploring the First Quarter Moon (or Half Moon, if you’re more poet than astronomer). This month’s First Quarter Moon occurs April 3, with subsequent ones May 2, June 1, June 30, and July 30.

A perfect First Quarter Moon would see the Moon’s line of 0° longitude aligned precisely with the terminator, the dividing line between lunar day and night. Because the Moon wobbles side to side as it orbits Earth (an effect called libration), this rarely happens. As a result, features positioned along the terminator vary from one First Quarter Moon to the next.

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