half a century or a half century

ソース:Lyndon Johnson on Welfare & Poverty

The ambitious “Great Society” agenda begun half a century ago continues to touch nearly every aspect of American life.

ソース:Rekindling JFK’s Commitment to Freedom

As we remember his life, ask yourself the same question JFK asked a half-century ago: “Will you join in that historic effort?”

half a century → Page 10 of about 50,400,000 results

a half century → Page 10 of about 7,940,000 results

結果を見てもこれはhalf a centuryが圧勝で、ネイティブはこっちの方を好むみたいです。語呂がいいみたいな感じらしいです。どっちも正しいらしいですけど