What is the communication delay between earth and mercury?

Answer 1

Here is how you work it out.

Communication by radio effectively travel at the speed of light, which is about 186,000 miles per second.

So the question becomes: how far apart are they? Unfortunately the answer is: it depends.

For part of the year they are on opposite sides of the sun. When that happens it doesn't matter how far apart they are. We can't send a radio signal through the sun, so we cannot communicate at all.

Even when they are on the same side of the sun the answer is still: it depends. The reason is that neither one of them stands still, and neither one is the same distance from the sun. Planets don't travel in a circle around the sun. They travel in an ellipse.

So what is an ellipse? Think of it as a circle that someone sat on. There is another, a mathematical, definition. But it is easier to show you. For this you need a piece of wood, a pen or pencil, 2 nails (and a hammer), and two loops of string of different sizes.

Place the smaller loop (let's call it Mercury), on the board. Next, hammer the 2 nails partway into the wood inside of Mercury. Take the pencil, pull the string tight, and mark the wood around the nails with the pencil as you draw it around the 2 nails with the string as tight. Now do the same using the larger string (let's call it Earth).

Now pull out one of the nails.

The remaining nail is the sun.

Earth and Mercury go around the sun. Mercury has a shorter distance than Earth does, plus Mercury moves a lot faster than Earth does. In the time for Earth to go around the sun Mercury would have done it 4 times and be working on its 5th circuit.

If you look at it you can easily see that Mercury and Earth are hardly ever the same distance apart. As well, communication is usually a two-way exercise. One person says something, the other person hears it, says something back, and the first person hears what the second person says.

So the answer is not easy, and changes from moment to moment.

Let's say that they are 70 million miles apart. When we divide 70,000,000 by 186,000 the answer is a little bigger than 37. In other words, when person 1 talks to person 2 it would be 37 seconds, or a hair more than 6 minutes, before person 2 heard what was said.

Heck, texting is faster!