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As far as I've known, sea tides are higher on a Full moon day. And that is due to the "gravitational pull" of the moon.
But we know that moon is visible because of reflection of sun's rays. So, even on a new moon's day, the moon is still there, but it's not visible.
So, why doesn't it affect the gravity on earth?

Thanks in advance.

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    $\begingroup$ In a new Moon day the tides are even higher. In both cases the tides are not higher because of reduced distance but because Sun, Earth and Moon are roughly aligned. $\endgroup$
    – Alchimista
    Feb 20, 2018 at 11:55

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There are two main gravitational causes of tides: the Moon, and to a lesser extent the Sun.

When the moon is full or the moon is new, the Earth, Moon and Sun are roughly aligned, and the Lunar tide combines with the Solar tide to give a "Spring tide" that has a larger range.

When the moon is at first quarter, or third quarter (ie a half moon), the solar tide and the lunar tide are acting against each other, and you get a "neap tide" with a lower range than a spring tide.

The size of the tide on a particular day at a particular location is strongly affected by the local shape of the coast and sea floor, Some places only have one tide each day, or tides of different size, due to these effects.

Note the name "Spring tide" is not from the season. You get Spring tides every New and every Full moon. It comes from an old English word meaning "to bulge" The tide "bulges" during New and Full moon.

So you are not correct to say that tides are higher on the day of Full moon. The New Moon's gravity does create tides, and tides at New moon are as large as the tides at Full moon, and larger than the tides at half moon.

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    $\begingroup$ With another minor correction for full/new moon at apogee or perigee. $\endgroup$
    – Carl Witthoft
    Feb 20, 2018 at 16:49
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    $\begingroup$ There are lots of minor corrections, which is why tide tables are normally based on measurments of the tide; not on modelling of gravity. $\endgroup$
    – James K
    Feb 20, 2018 at 16:58
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    $\begingroup$ Actually, not just the local shape. The tides are large-scale standing waves on the surface of the world's oceans. The Sun and the Moon provide a forcing function that drives the oscillations, and the amplitude of the wave at any given point depends on many details including how well an entire ocean "basin" resonates with the forcing function. $\endgroup$
    – Solomon Slow
    Feb 21, 2018 at 23:54
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You are right. Highest monthly tides are during a new or full moon since the sun and moon combine their gravitational attraction in line with the earth. The highest tides of the year are middle of Northern Hemisphere winter because that's when the earth's orbit is closest to the sun.

The usual simple modeling of tides assumes an oceanic earth with no continents and uniform depth of oceans. This gives the typical diagram that explains diurnal tides:

enter image description here

However, Earth is nothing like this hypothetical planet and real tides behave nothing like the model.

There are numerous points on earth which experience no tidal range, called Amphidromic Points. Tides radiate out from these points, getting larger the further the radius from the nearest AP. These radiating lines of equal tidal phase connect with those from the next AP. The lines rotate, intermeshing with each other like gears.

enter image description here

Red dots = AP locations. The video link below shows how complex tides are. See if you can find any resemblance at all to the standard “tidal bulge” model.

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