What date?
What latitude?
What longitude? (It will be different at the extremes of the time zone.)
I hope it isn't cloudy.

In addition to what DrBob1 said, we also need to know the height of the object whose shadow length is to be determined.

And we also need to know if the ground is flat or what angle the slope is. Also we need to know if the object is vertical or leaning in any particular direction!