Apple (AAPL) is scheduled to report its fiscal third quarter earnings on July 19, a week from today, and in preparation for our quarterly earnings smackdown we’ve been gathering estimates from the small army of analysts — profession and amateur — who follow the company.

The biggest mystery this quarter — and the biggest discrepancy in unit sales estimates — is the iPad. The question is not so much how many Apple was able to sell (it sells them as fast as it makes them), but how many it was able to build and ship before the quarter ended on June 25.

Among the 39 analysts we’ve polled so far, the estimates ranged from a high of 9.5 million iPads sold (offered by Navin Nagrani, one of the growing team of amateurs who post their estimates on The Mac Observer’s Apple Finance Board) to a low of 6 million (offered by three different professionals: Gleacher’s Brian Marshall, Hudson Square’s Daniel Ernst and Think Equity’s Mark McKechnie).

The average estimate, 7.92 million, represents a 142% increase from the same quarter last year, and would bring the total number of iPads sold in 15 months to nearly 27.4 million. The average amateur estimate is 8.38 million. The  Wall Street consensus is 7.72 million.

We’ll find out who was closest to the mark next Tuesday at about 4:30 p.m. EDT, when the quarterly results hit the business wires. Apple is scheduled to discuss those results in a 5 p.m. EDT conference call with analysts that the company will webcast here.

Below: The estimates we’ve gathered so far and the date they were submitted. The Q2 rank numbers show how accurate each analyst’s estimates were last quarter (the lower the better). [UPDATE: Ticonderoga’s Brian White raised his iPad estimate on Tuesday to 8.5 million, which pushed the consensus to 7.93 million and estimated total iPad sales over the 27.4 million mark.]