Have a dialogue with instructions before cropping.

[SebastianKane]

• Give the user an example of a valid crop.
• Tell the user to fill in fields.
• Tell the user to crop the moon if it's not already properly cropped.

[Gavin1937]

Basically verify with user about whether the rectangle box covering the moon is really covering on the moon. If not, ask user to move that box around and select the correct moon position