It takes a year for the earth to make a circuit of the sun. 

It is a universal principle which is not apt to change (unless there is another theory more convincing)

It's used in reference to an unspecified time before the very moment you are speaking. An example would be: "I have ridden a bike." It's represented by the word "had/have" + the past tense of the verb. The tense can be used to express experience or achievements such as these sentences: "I have won many championships." "I have climbed mountains." "I have completed school." Another use is for an action we expected to have happened, but has not yet happened. "He has not taken out the trash yet." "He has not graduated college yet." Generally this structure implies that we are waiting for the action to be completed. Another way it can be used is to express recurrences of an event at different times in the past. "The student has failed this class 3 times." "This dog has attacked me 3 times this week." Finally you can also use the present perfect tense to describe a development. "The boy has gotten much better at baseball." This means that "over some period of time in the past" a change has happened to lead to the present result. I hope this helps :)

okay, it's a great help...

For a correct and professional explanation, please see this page compiled by the British Council: http://learnenglish.britishcouncil.org/en/english-grammar/verbs/present-tense/present-perfect