What does this actually mean?

Amos 8:9 And it shall come to pass in that day, saith the Lord GOD, that I will cause the sun to go down at noon, and I will darken the earth in the clear day:

I realize that it is simlar to:

Zechariah 14:7 But it shall be one day which shall be known to the LORD, not day, nor night: but it shall come to pass, that at evening time it shall be light.

But it is not the same as what is mentioned in Amos.

Will the LORD not cause the sun to go down at noon, like he said?

Didn't he stop it, once?

Joshua 10:13 And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies. Is not this written in the book of Jasher? So the sun stood still in the midst of heaven, and hasted not to go down about a whole day.

Didn't he make it go backwards, once?

Isaiah 38:8 Behold, I will bring again the shadow of the degrees, which is gone down in the sun dial of Ahaz, ten degrees backward. So the sun returned ten degrees, by which degrees it was gone down.

So, why can he not make it go down at noon?