When I look at the tor channel - the entrance point is always 184.108.40.206 (Great Britain) - even if I ask tor to choose a new channel. I thought the route through tor is randomly selected. But if the entrance point is always the same - how could I be sure that it is not observed?
From the FAQ: https://www.torproject.org/docs/faq.html.en#EntryGuards
each Tor client selects a few relays at random to use as entry points, and uses only those relays for her first hop. If those relays are not controlled or observed, the attacker can't win, ever, and the user is secure. If those relays are observed or controlled by the attacker, the attacker sees a larger fraction of the user's traffic — but still the user is no more profiled than before. Thus, the user has some chance (on the order of (n-c)/n) of avoiding profiling, whereas she had none before.