'Bystander' Vs 'Spectator'?

A bystander is there by accident or coincidence.
A spectator is there on purpose.

Thanks for answering. In the following sentence which would be the correct word?:
There was an accident on the street and bystanders/spectators circled the site.
There were lots of bystanders/spectators present to see the eclipse.

There was an accident on the street and bystanders/spectators circled the site.

Bystanders. Some bystanders were there and saw the accident happen. They are witnesses.

Thanks for the reply. Does the same logic apply to the second sentence too? Are onlookers and bystanders one and the same? In news I hear the reporters using the word onlooker/lookers- on, how does it change the meaning of the word if we replace the words with onlooker/lookers- on?

I changed my mind after thinking about it.

Onlookers and bystanders are the same, except the bystanders probably saw the accident, and the onlookers didn't.

Could you please elaborate on it some more?

OK.

There was a bad car accident. Three people were walking along the road when the accident happened, so they saw it. These three people are bystanders.
http://www.macmillandictionary.com/dictionary/american/bystander

After the accident, the police came. Ambulance