Today is the first quarter Moon phase. It is a term that confused me as a child when I first started to pay attention to the night sky. The First Quarter Moon is also called, somewhat illogically, the Half Moon because the Sun’s rays illuminate exactly 50% of the Moon’s surface.
At the First Quarter in the Northern Hemisphere, the right half of the Moon is lit up, as shown below. In the Southern Hemishere, the left half is illuminated and near the Equator, the upper part is bright after moonrise, and the lower part is bright before moonset.
As Juliet told Romeo, “O, swear not by the moon, the inconstant moon, That monthly changes in her circled orb.”
You know that ocean tides on Earth are mostly generated by the Moon’s gravitational pull. The largest tidal range is around Full Moon and New Moon when the Moon and the Sun’s gravitational forces combine to pull the ocean’s water in the same direction. These tides are known as spring tides or king tides. And with today’s First Quarter and again at the Third Quarter, the Moon and Sun pull in different directions, producing the smallest difference between high and low tide. These are known as neaps or neap tide.