Rotation of Mercury
In the past, astronomers thought that the Earth was the center of the Universe and everything revolved around it. But then they realized, thanks to Copernicus, that the Sun was the center of the Solar System, and all the planets or orbiting it. The reason the stars, Moon, Sun and planets move in our sky is because the Earth itself is turning on its axis. A day is one rotation of the Earth's axis, and takes 24 hours.
Once they realized the Earth was turning, astronomers wanted to know if the other planets were rotating as well.
The planet Mercury turned out to be one of the most difficult planets to determine how fast it was turning. In the 1880s, Ginovanni Schiaparelli mapped the surface features of Mercury very carefully, and estimated that the planet's rotational period was 88 days. This theory, that Mercury was tidally locked to the Sun, became widely held. This means it would complete one rotation for every orbit around the Sun in 88 days. One side of the Mercury would face the Sun, and the other would always face away from it.
This turned out to be incorrect.
In the 1960s, with the invention of radio astronomy, scientists put this question to rest, once and for all. Soviet scientists in 1962 were the first team to ever bound radar signals off of Mercury and receive them again. In theory, if Mercury were tidally locked to the Sun, one side would be extremely hot, and the other side would be extremely cold. The difference would be detectable in the radio spectrum.
It wasn't until 1965, when US astronomers using the Arecibo Observatory would provide conclusive evidence that the planet's rotational period was about 59 days.
Since then, astronomers have measured it much more accurately, giving the current value of 58 days and 15.5 hours.
Filed under: Astronomy
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