Moon Phase

Fig. 1: NOAA Battery NY tidal records for a typical month, illustrating spring and neap tides and their relation to the phase of the moon. Water levels are shown relative to Mean Lower Low Water (MLLW). Also shown are the mean high water (MHW) and mean low water (MLW) datums.

*Note: The Stony Brook Storm Surge Research Group uses the mean higher-high water (MHHW) benchmark as the local “normal” high tide, which is based on the long term (18.6 yr) mean of the higher of the two semi-diurnal (twice-daily) high tides, neglecting the lower of the two daily high tides. Known as the diurnal inequality this daily variation in the height of high tide is due to the inclination of the Moon’s orbit (28.5 deg.) with respect to the equatorial plane of the earth. Using the MHHW datum gives a better benchmark for triggering off alert signals than using say MHW.