Yep, exactly. Make sure your image is unlocked in the layers palette. When the black sky is selected, add a layer mask. Go to Layer: Layer Mask: Hide Selection. When you click that, a box should appear next to your image in the layers palette. It will resemble a black and white version of your image. Whatever is black in that box (the one on the right) will be hidden and whatever is white will be visible.
It's a bit to get your head around, but if you use a white paintbrush on that side box, it will reveal elements in your image. If you use a black paintbrush, it will hide elements in your image. This is a way to hide the sky while keeping the earth.
Check out the attached image. I forgot to unlock it at first and did it later on, but for you, first thing to do is double click the layer and select "Ok." This will unlock it. Also, the fourth image is just me goofing around in the layer mask, so you can see how brushing in the mask alters the image you see.