A-RIDDLE: WHO AM I?

In spring I do well,
And in June, I excel,
All summer, my output is keen.
When colder it grows,
My uncertainty shows,
And in autumn, I'm one for fourteen.
*
In April I soar,
Through July, my friends score,
All summer, I'm high as the sky.
Then comes the post,
When I'm needed the most,
And in autumn, not one RBI.
*
I rumble through June,
And they pay me the moon,
All summer, my teammates show faith.
Then the leaves start to fall,
And my stick becomes small,
And in autumn, they bat me at eighth.

From "Mrs. Goose Goes to Washington," best book of the year.