That's a first. / That's the first.

[Wookie]

Ruth : Did you go to work earlier than Russ in the morning?
Ross : Yeah.
Ruth : That's a first.

How come "a first" instead of "the first" is used?
I thought "the" comes before ordinal numbers like "first".
And I want to know the difference in meaning and usage between "a first" and "the first". Thanks.

 

[Naamplao]

Ruth : Did you go to work earlier than Russ in the morning?
Ross : Yeah.
Ruth : That's a first.

How come "a first" instead of "the first" is used?
I thought "the" comes before ordinal numbers like "first".
And I want to know the difference in meaning and usage between "a first" and "the first". Thanks.

"That's a first" is slang meaning that you are surprised or excited at somebody doing something. In this context, Ross is always late compared to Russ. Ruth cannot believe that Ross actually came to the office earlier than Russ so she sarcastically says "That's a first". Note the stress on the first word.

"That's the first" is not a complete thought. "First" here is used as an adjective but what it is modifying is not there.

That's the first time that Ross arrived at the office before Russ.

The above is not said in a sarcastic manner. It is a simple statement of fact.

That's the first time she flew on an airplane.
That's the first male child in that family.
That's the first artist to visit our gallery.