a satellite of a satellite
The largest satellite orbiting the Earth is the Moon.
The only natural satellite orbiting Earth is the moon.
It is the moon.
It is the moon.
If the Moon is to a satellite, then Earth is to a planet. The Moon is a natural satellite orbiting Earth, just like artificial satellites orbit Earth. Similarly, planets are natural satellites orbiting stars, like Earth orbits the Sun.
Assuming no typo on your part regarding the "A" satellite, it would be that the moon was the first satellite orbiting earth. If you meant "why is the moon known as a satellite", it's because it is a satellite. Anything that orbits is a satellite.
A moon is a natural satellite, as opposed to an artificial satellite, which is a man-made object in orbit.
The moon is considered to be a satellite of the Earth. But from any place far away, the Earth and Moon might appear to be a double planet orbiting the sun.
A Satellite (like our moon) Orbiting bodies are in fact orbiting each other, so that there are binary stars (two stars of about equal size) that are orbiting each other, or even binary planets.
By orbiting the earth about once a month.
A satellite is not always a robotic module constructed on Earth and launched into orbit. A satellite, in fact, is any object which is orbiting another. The orbiting object is the satellite of whatever it orbits. By now you ought to be able to guess it, but if not, it is the moon.