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Does the Force Felt by a Person on the Surface of the Earth Vary with Lunar Phase?

-- Ryan | September 12, 2021

Question:

Hi! Im doing research into cardiovascular disease and the lunar cycle. My question is regarding the gravitational force of each lunar phase, categorising them into New Moon, First Quarter, Full moon and Third Quarter. For how long would you consider the gravitational effects of each lunar cycle to last. For example, there is a point each month where the moon is full. How many days surrounding the full moon would you suggest the gravitational effects of the full moon are at force, for example on tides?

-- Ryan

Answer:

If you ignore the small variations in the Moon’s orbit (which amount to about 10% of the distance to the Moon), the force felt by a person on the Earth’s surface does not vary with lunar phase. This is due to the fact that the force felt by a person on the Earth’s surface due to the Moon’s gravity is proportional to the mass of the person multiplied by the mass of the Moon divided by the distance to the Moon squared. Since none of these quantities change with lunar phase, there is no change in the force due to the Moon with lunar phase. Furthermore, note that the forces felt by a person on the Earth’s surface are completely dominated by the force we feel due to the mass of the Earth. The force felt by a person due to the Sun’s and Moon’s gravity are 300,000 and 1700 times smaller, respectively, than the force we experience due to the Earth’s gravity.

-- Jeff Mangum

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