# Solar day and sidereal day | Science

What will be the gravity on the surface of Mercury, knowing that its diameter is 4,880 kilometers and its density is almost equal to the earth? we were wondering last week. The calculation is very simple, because for two stars of the same density, gravity is proportional to the radius. And since the radius of the Earth is 2.6 times greater than that of Mercury, the gravity of the planet smaller and closer to the Sun is 2.6 times less than the Earth, that is, 3.7 m / s^{2}. A person of 70 kilos in Mercury would only weigh about 27.

The explanation of this direct proportionality between radius and gravity is equally simple: according to the law of universal gravitation, the attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. If we consider a unit mass body located on the surface of a planet of radius R, the force of attraction is proportional to the mass of the planet and inversely proportional to the square of R, which is the distance from the body to the center of the planet. But the mass of the planet, in turn, is proportional to its volume - that is, the cube of its radius - and its density, and, therefore, to two planets of the same density, such as Earth and Mercury, the only one variable is R; and as the attraction is proportional to R^{3} and inversely proportional to R^{2}, is ultimately proportional to R.

Everyone knows that we call "day" while it takes Earth to make a complete turn around its axis, that is, between two consecutive sunrises ... Or not?

. (tagsToTranslate) day (t) solar (t) sidereal (t) period (t) rotation (t) earth (t) time (t) to elapse (t) two (t) consecutive sunrise (t) (t) be ( t) same

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