# Difference between solar annual and solar daily earth tides

I understand there is a semi-daily and daily solar earth tides (S1,S2) because the earth is rotating around its axis.

I understand there is an annual and semi-annual solar earth tides (Sa, Ssa) because the distance and inclination of the earth relative to the sun change during the year.

Now, my logic says that S1 and S2 should also change amplitudes slightly during the year since the distance to sun is changing. Is that same thing as the annual solar tide? To be clear, I'm not asking if its the same amplitude, I'm asking if its the same thing. Or is there a change in the daily tide amplitude and an annual tide?

I really hope that's clear

• Have you read something like this ? hyperphysics.phy-astr.gsu.edu/hbase/tide.html . The tide observed is a vectorial addition of forces coming from various gravitational attractions, and there will be a time dependence . The annual just affects the size of the added forces. – anna v Jun 18 '18 at 15:51

The tidal components are a way of decomposing the complicated tidal effects into a set of sinusoidal oscillations each of which has a fixed amplitude (by definition).

The changing amplitude of the solar tide during the year is represented by the sum of several components, with nearly equal periods, which show "beats" in amplitude when they are summed.

The general theory is based on the mechanics of the solar system, and is based on 6 "fundamental" frequencies, plus multiples of them and sums and differences of the multiples. The most general theoretical classification scheme has included almost 400 resultant components, but in practice the typical number for practical tidal prediction is limited to around 40 by the accuracy of available measurements and the "noise" effects of wind, etc, on the sea level.

See https://en.wikipedia.org/wiki/Theory_of_tides for a table of the most significant periods, and typical amplitudes for a few coastal locations.

The six "fundamental frequencies" are described in https://en.wikipedia.org/wiki/Arthur_Thomas_Doodson#Doodson_numbers.

• This question is about the earth tides rather than the ocean tides, but the similar concepts apply -- except for the wind, of course. The vertical component of the tidal forcing function has little effect on the ocean tides, but is a key driver of the earth tides. The horizontal component drives the ocean tides; it's what makes water flow. The horizontal component has little effect on the earth tides; the solid earth doesn't flow. Another factor: The ocean tides complicate the earth tides because the shifting oceans result in stresses on the solid earth. – David Hammen Jun 18 '18 at 16:12
• I missed the word "earth" - but as you say the general principles are the same. Also, earth tides don't have the complication of changing water depth around the coast, which requires higher multiples of the fundamental frequencies to account for it - up to 10x the fundamental in some river estuaries. – alephzero Jun 18 '18 at 16:19
• So if I observe a physical phenomenon which has an annual period and I think its caused by the earth tides, Saying 'its caused by the annual earth tide' rather then 'it's caused by the annual change in the daily tide' is better? – user4421975 Jun 18 '18 at 20:14