Although historic and historical have some overlap in meaning, they are usefully differentiated.
Historical, the more general and common word, means of history, of the nature of history, relating to history, belonging to history, occurring in history, and so on. The Oxford English Dictionary was originally called A New English Dictionary on Historical Principles. We read historical fiction and perform historical research (which, if we are lucky, might become historic).
Historic is used less often, and generally means historically famous, historically important, etc.: the president’s historic speech; preserving historic buildings; a historic moment for the country. Historic can refer to present events if they are obviously momentous enough.
William Safire summarised the distinction thus: “Any past event is historical, but only the most memorable ones are historic.” Yet the distinction is not absolute. In one sense historic is a subset of historical, so blurring inevitably occurs. Historic sometimes carries the broad meaning associated more strongly with historical, while historical can refer to something (or someone*) historically significant, i.e. historic. These crossover usages are not wrong, but by avoiding them you will help maintain a convenient semantic division.
Once again I find myself agreeing with Merriam-Webster, who after reviewing the historical evidence conclude that “bucking [the trend] may be historically justified but is more likely to interfere with the smooth transfer of your ideas from paper to reader.” This is because some readers – including many English language authorities – insist on keeping the words’ meanings separate. In The Complete Plain Words, for example, Ernest Gowers stresses that their “useful differentiation should not be blurred by the use of one for the other”.
A note on pronunciation: In both words /(h)ɪ’stɒrɪk/ and /(h)ɪ’stɒrɪk(ə)l/ the h can be aspirated or not; if it is not, an is the accompanying indefinite article. When the stress falls on the second syllable, the h is weakened, hence some people’s preference for an: compare a(n) historian with a history. Be aware, though, that this preference seems quaint to some, and that a historic is the dominant modern form.
* According to Loose Talk: The GUBU File by Damian Corless, a presenter of Eurovision said to Ireland’s winner, “Johnny Logan, you are historical!” Unfortunately, I can neither remember this nor find video evidence for it.