Dominic DeGrazier 3:37 p.m., Sept. 21
The Moon, the Tides, and the Vernal Equinox
The Moon, just past full phase, rises into the darkening eastern sky at around 7:25 p.m. on Saturday, March 19, some 25 minutes after the sun sets. About twelve hours later, you can watch the moon sink into the Pacific Ocean while the sun is about to come up on the other side of the sky. This out-of-phase, rise-set synchronicity is characteristic of every full moon. The 12-hour difference on this occasion is explained by the fact that both the sun and the moon will be near opposing equinoxes in the sky -- in other words, both will lie nearly over Earth's equator
Highest Tide this month, a rather moderate +6.0 feet, occurs at 10:33 p.m. on Sunday, March 20. Lowest tides are -0.9 feet on two occasions: 2:30 p.m. on Thursday, the 17th, and 3:05 p.m. on Friday, the 18th — both good times for tidepool discoveries. These low tides mark the end of a months-long series of extreme low tides occurring during afternoon hours. For the next several months, extreme low tides will only occur in the early-morning hours.
Vernal Equinox on Sunday, March 20 at 4:21 p.m. Pacific time heralds the beginning of the spring season for Earth's northern hemisphere. At the instant of vernal equinox, the sun lies in the plane of Earth's equator. As a consequence, days and nights are of equal length (12 hours each) everywhere on our planet. Another consequence is that the sun rises due east along the horizon and sets due west. During the next three months, as the sun shines more and more directly on our hemisphere, daylight hours will lengthen and the rise and set positions of the sun will gradually shift toward the northeast and northwest, respectively.