What is the aperture and how does it impact sunrise/sunset photos? what is generally considered an ideal aperture for sunrise/sunset photos?

Hello there! 

Aperture is defined as being the size of the opening in the lens. It can be set automatically or manually.  If you have a small aperture, less light will reach the camera, while a larger aperture will let in more light. So, the larger the aperture in a sunrise/sunset photo, the brighter it will be. 

Aperture is measured in f-stops. 

The recommended aperture for sunrise/sunset is f-8, f-11, or even higher to help minimise the amount of light that gets into the camera to prevent the image from becoming too bright. 

Hope this helps! 
-Astro, Helper-In-Training 

Its the 3rd one i think