another planet--what does it refer to?

Hello!

I can't understand the following paragraph fully. Help me, please!

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If two astronomers observe a planet at the same time, they can measure the angles indicated as A and B and measure the distance between the telescopes. This give them enough information to be able to calculate the distance to another planet. The greater the distance is between them, the more accurate their calculations can be.

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1. What is the another planet? ........This is my hump (Can I use this word?) ; Two astronomers caculate the distance between the earth and a planet . I'll use some symbols.( E for the earth, E1, E2 for the spots of two astronomers, S1 for a planet, S2 for another planet, S3 ...for another planet and so on..)

They can calculate the distance E1 E2 and the angles <S1 E1 E2 and <S1 E2 E1.

And then, they can calculate the distance E S1. So, I thought, from the contaxt, the another planet should be S1. But, It sounds like referring to another planet S2. What do you think? Is the another plannet the first star from the earth or any other star?

2. Who / what are them? ............Considering the context, it would be 'the two astronomers'. But is there any chance it could be the distance between the eart and the star?