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If you post a letter or an email, you don't see it travelling, but you can measure the time it took to get there. If you know how fast a horse can run, you can calculate when it will finish a 2-mile race (please gamble responsibly).
If I quickly turn on and off a switch I get a ''drip'' of electricity
I can see the Sun and I can also see the Earth but I do not see anything travelling from the Sun to ''time''
Quote from: TheBoxI can see the Sun and I can also see the Earth but I do not see anything travelling from the Sun to ''time''To time something, you need to know the time it left, and the time it arrived.Unfortunately, it takes an enormous amount of rocket fuel to deliver scientific instruments to the surface of the Sun - and they would vaporize long before they got there. So measuring the departure time is tricky!However, in 2006 a pair of Sun-observation satellites (STEREO-A and STEREO-B) were launched into orbits where one drifted ahead of Earth, and the other drifted behind the Earth. Because these were not actually approaching the Sun, it only took a gravitational slingshot around the Moon to get them into their desired orbits.These satellites need to have a good idea of their position in 3-dimensional space, so they can point their antennas at Earth. These satellites also need to have an accurate idea of the time on Earth, so they can transmit their results at the right time. Their radio transmissions need to arrive at Earth at the right time that the deep-space tracking antennas are looking for their signal, and pointing in the right direction. In 2015, STEREO-A passed behind the Sun. While it was on a fairly close line-of-sight to the Sun, the microwave signals would have travelled about twice the Earth-Sun distance, and taken twice the Earth-Sun delay to reach the Earth.This is a case where we have been able to place scientific instruments in a location that lets us measure the Earth-Sun delay of the speed of light fairly directly. See: ...sorry, you cannot view external links. To see them, please
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