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Old 03-03-2015, 06:37   #1
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Phases of moon/Tides

I have 2 RYA training booklets Day Skipper & Yachtmaster and they have similar statements in them regarding tides.

Day Skipper - "Moon orbits the earth over a 28 day cycle creating spring and neap tides".

Yachtmaster - "The tidal cycle is 28 days - the period of one full moon to the next".

I live in UK and have Reeds almanac as well as local tide table for my area and the RYA Training almanac.

Analysis of this information indicates that 28 days is not correct. Full moon to full moon is more along the lines of 29.5 days. Very difficult to determine exactly but a google revealed the average presumably over a very long time Full to Full is 29 days 12 hours 44 mins & 3 seconds.

I emailed the RYA pointing this out and they have sent a very lengthy reply which has left me confused.

They say the tidal cycle is approx 28 days (contary to the evidence in my opinion) and although spring tides to neap tides is generally around 7 days it can be between 5 to 9 days.

They suggest the factors that should be taken into account when determining the tidal cycle are the SYNODIC month 29.5 days, the TROPICAL month 27.3 days and the ANOMALISTIC month 27.5 days.

Take an average of these and you 28.1.

Please can someone explain.

Thanks

Mike
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Old 03-03-2015, 07:02   #2
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Re: Phases of moon/Tides

The Moon appears to move completely around the celestial sphere once in about 27.3 days as observed from the Earth. This is called a sidereal month, and reflects the corresponding orbital period of 27.3 days

The moon takes 29.5 days to return to the same point on the celestial sphere as referenced to the Sun because of the motion of the Earth around the Sun. This is called a synodic month (Lunar phases as observed from the Earth are correlated with the synodic month).

There are effects that cause small fluctuations around these values.
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Old 03-03-2015, 07:40   #3
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Re: Phases of moon/Tides

Excerpted from here ➥ The Ocean's Tides Explained

As the Earth spins on its own axis, ocean water is kept at equal levels around the planet by the Earth's gravity pulling inward and centrifugal force pushing outward.

However, the Moon's gravitational forces are strong enough to disrupt this balance by accelerating the water towards the Moon. This causes the water to 'bulge.' As the Moon orbits our planet and as the Earth rotates, the bulge also moves. The areas of the Earth where the bulging occurs experience high tide, and the other areas are subject to a low tide.

The same forces are at play as the Earth revolves around the Sun. The Sun's gravity pulls ocean water toward the Sun, but at the same time, the centrifugal force of the combined Earth-Sun revolution causes water on the opposite side of Earth to bulge away from the Sun. However, the effect is smaller than the Moon, even given the greater mass of the Sun (greater mass means greater gravitational force). Why? Simply because The Sun is so far away over 380 times farther away from the Earth than the Moon.

Because the tides are influenced by both the Moon and the Sun, it's easy to see that when the Sun lines up with the Moon and the Earth, as during a New Moon or Full Moon (a configuration also called "syzygy"), the tidal effect is increased. These are known as spring tides, named not for the season, but for the fact that the water "springs" higher than normal.

On the other hand, if the Sun and the Moon are 90 degrees apart in relation to an observer on Earth as during the First Quarter Moon or Third Quarter Moon (sometimes called half moons), then high tides are not as high as they normally would be. This is because despite its greater distance, the Sun's mass allows it to exert enough gravitational force on the oceans that it can negate some of the effects of the Moon's pull. This phenomenon of lower high tides is called a neap tide.
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