NOAA’s tide and tidal current predictions take into account astronomical considerations due to the position of the moon and the sun. Large amounts of water must flow long distances around uneven land masses. The Moon’s distance from Earth varies from 356,400 to 406,700 kilometers (221,457 to 252,712 miles). Water level highs and lows lag the tide causing forces by 1-3 hours. Neap tides occur during the first and third quarter moon, when the moon appears "half full." Lunar Tides Lunar tides occur at every 12 hours and 48 minutes, alternating between Moon side and far side high tides with lunar low tides in-between as reference. This produces moderate tides known as neap tides, meaning that high tides are a little lower and low tides are a little higher than average. When this happens, the bulge of the ocean caused by the sun partially cancels out the bulge of the ocean caused by the moon. Seven days after a spring tide, the sun and moon are at right angles to each other. These occurrences are often called 'perigean spring tides.'. This NASA image from the Apollo 8 mission shows the Earth viewed over the horizon of the moon. Often between 6-8 times a year, the new or full moon coincides closely in time with the perigee of the moon the point when the moon is closest to the Earth. High tides do not coincide with the location of the moon. Rather, the term is derived from the concept of the tide "springing forth." Spring tides occur twice each lunar month all year long, without regard to the season. A perigean spring tide occurs when the moon is either new or full and closest to Earth. These are called spring tides, a common historical term that has nothing to do with the season of spring. This means that high tides are a little higher and low tides are a little lower than average. This drags the ocean bulge in the eastward direction of the Earths rotation. There is therefore friction between the ocean and the seabed as the Earth turns out from underneath the ocean tidal bulges. In both cases, the gravitational pull of the sun is "added" to the gravitational pull of the moon on Earth, causing the oceans to bulge a bit more than usual. Similar basic arguments have been invoked in the context of tide-planetary wave interactions and in explaining other elements of the tidal spectrum in the MLT. The Earth rotates faster than the Moon orbits the Earth (24 hours compared to 27 days). The moon appears full when the Earth is between the moon and the sun. The moon appears new (dark) when it is directly between the Earth and the sun. Tides are long-period waves that roll around the planet as the ocean is "pulled" back and forth by the gravitational pull of the moon and the sun as these bodies interact with the Earth in their monthly and yearly orbits.ĭuring full or new moons-which occur when the Earth, sun, and moon are nearly in alignment-average tidal ranges are slightly larger.
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