Humans have known about the existence of our Moon since the time that we first looked towards the heavens. It was unique among the objects in our night sky until the arrival of the telescope in the early 1600's. It was with the telescope that Galileo first discovered moons orbiting the planet Jupiter. The four moons he discovered in 1610 were Io, Europa, Ganymede and Callisto. The moons of Mars survived undetected for almost 300 years following the invention of the telescope.
It wasn't until 1877 that we discovered that Mars had two moons of its own. The discovery of these moons was made by American astronomer Asaph Hall at the US Naval Observatory. He had at his disposal one of the best telescopes of the day and was observing Mars during a favorable opposition, a time when Mars is much closer to the Earth than is usually the case. The two moons were named for two sons of Ares, the God of War: Phobos (Greek for fear) and Deimos (Greek for terror).
There are two reasons why it took so long for us to discover the moons that orbit Mars. The first reason is that these moon are very small. Our Moon has a diameter of 3,474 kilometers. Phobos is so small that gravity isn't large enough to mold the moon into a sphere. Rather, it is shaped more like a potato about 26 kilometers long by about 22 kilometers across. Deimos, the smaller of the two, is only about 15 kilometers long by 12 kilometers across.
The second reason they were so hard to discover is because they orbit so close to Mars. While our Moon orbits at a healthy distance of 384,000 kilometers from the Earth, Phobos is only 9,378 kilometers distant from Mars and Deimos is a little farther out at 23,459 kilometers. This proximity means that when an astronomer looks through a telescope at Mars, the light being reflected by Mars overpowers the light being reflected by Phobos and Deimos.
To give you an idea of the relative scale of the Earth-Moon and Mars-Phobos-Deimos systems, take a look at the graphic to the left of this text. At the top you can see the planets Mars (left) and Earth (right) drawn to the same scale. Immediately below Mars is a dot labeled Phobos. While the pixel you see makes Phobos larger than it really is, it is drawn at the correct distance from Mars. A little further down, you can see a pixel identified as Deimos. Again, the one pixel size makes Deimos appear larger than it really is but accurately shows you the distance of Deimos from Mars.
Now, start scrolling down.
Keep on scrolling.
Keep on scrolling.
Just a little further.
It won't be long now.
Depending on the font size you are using and the width of your browser, you may or may not have already passed the Moon. The size and distance of the Moon as drawn are both to the correct scale. As you can see, someone observing the Earth from Mars would have little difficulty in spotting our Moon, both because of its large size and because of its greater distance from the Earth.
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