Scientists have found a way to weigh the Earth. Though the interior of the Earth cannot be directly probed, the gravity on the surface can be measured. The mass of the Earth can be estimated from its gravity.
The most commonly used technique to measure the mass of a planet, is to observe a body orbiting, or passing close to the planet. The path of the body is affected by the gravity of the planet. A heavy planet attracts its moon strongly, and makes it move faster. Astronomers can calculate the mass of a planet after they have observed the motion of one of its moons for a period of time.
Mercury and Venus have no moons, so their exact masses were not known until a few decades ago. Before space flight was developed, the only way to measure their gravity was to estimate their effect on the orbit of other planets. This method was unreliable.
Al this changed with the launch of spacecraft to Venus. Scientists could easily measure the mass of Venus by tracking how strongly these spacecrafts were attracted while passing by
Mercury and Venus have no moons, so their exact masses were not known until a few decades ago. Before space flight was developed, the only way to measure their gravity was to estimate their effect on the orbit of other planets. This method was unreliable.
Al this changed with the launch of spacecraft to Venus. Scientists could easily measure the mass of Venus by tracking how strongly these spacecrafts were attracted while passing by
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