When I first started with photography, I had the hardest time getting the horizon straight in my landscape photographs. I have come along way in getting things lined up the first time; however, I still often need to correct a crooked horizon in Lightroom. I also obsess over getting straight lines near the edge of my images aligned and parallel, like masonry walls or poles. Lightroom has a very quick tool for rotating your images very precisely to a known edge, such as a horizon. Here is how it works:
Begin by selecting the picture you want to edit. Then, enter the Develop Module by clicking on the develop icon in the upper right part of the screen (1), or press Opt-Cmd-2
In the Pane running down the right side of your screen, click on the Crop Overlay tool. (2)
The Crop Overlay menu will drop down revealing more options. click on the straighten tool (3).
This will enable a ruler icon at your mouse. Begin by clicking on the first point of your straight line reference (4). In our case, this is one end of the horizon we are trying to straighten. Hold down your mouse button and drag the pointer to the second reference point.
Hold down your mouse button and drag the mouse pointer to the second reference point (5), and then release the mouse button.
Lightroom then rotates the picture so that the line between the two reference points we established is now horizontal. In this case, the resulting image is rotated clockwise 1.79 degrees. If you desire, you can make an adjustment from this point by clicking on the slider and dragging right (to rotate clockwise) or left (to rotate counterclockwise). Additionally, you can click on the angle number indicated in the window (6) and change this value by pressing the up or down arrow buttons, or by typing in a new angle value.
The straighten tool in Adobe Lightroom works great on horizons, but don't forget that you can use it anytime you have a straight line reference you want to align to zero. Additionally, the tool also works on vertical lines.