The constituent driven by the Sun, with a period of exactly 12 solar hours. Because its period differs slightly from M2, the two drift in and out of step every 14.77 days: in step they add to give spring tides, out of step they partly cancel to give neaps. The ratio of S2 to M2 amplitude therefore sets how strong the spring-neap cycle is at a station.
S2 is nearly half of M2 at this station, so springs and neaps look completely different.
Also heard as
- principal solar semidiurnal
Related terms
M2
The principal lunar semidiurnal constituent, period 12.42 hours, usually the largest part of the tide.
Spring tide
A tide of larger than average range, occurring around new and full moon when Sun and Moon pull in line.
Neap tide
A tide of smaller than average range, occurring near first and last quarter moon when Sun and Moon pull at right angles.
Harmonic constituent
One of the regular cosine waves, each with a period fixed by astronomy, that together make up the tide.