First, make your clips and transitions a little easier to see in the Timeline.

1. Choose Windows > Window Options > Timeline Window Options. Then, select the medium-sized Icon Size if it is not already selected. Click OK.
2. From the Time Zoom Level pop-up menu at the bottom of the Timeline window, select 1 Second so you can see the action clearly, as it happens.
3. Hold the Control key (Windows) or the Command key (Mac OS) as you click first on the Solar1.mov icon and then on the Earth.mov icon in Bin 1 of the Project window to select both clips.
4. Choose Project > Automate to Timeline to bring both clips to the Timeline.
5. In the Automate to Timeline dialog box, change the Contents setting to Selected Clips, change the Ordering setting to Selection Order, and change Clip Overlap from 15 frames to 30 frames. Verify that Sequentially, Beginning, Use Default Transition, and Ignore Audio are selected in the dialog box. Click OK.

The Cross Dissolve transition appears in the Timeline window between the two clips.

6. In the Timeline window, click the Track Mode button to the right of the Video 1 track label to display the Video 1 upper track, the Transition track, and the Video 1 lower track. This reveals the substructure of the Video 1 track in Single-Track editing mode. Use this expanded view of the tracks for the entire lesson.

Now the Timeline window contains two clips, starting with Solar1.mov in the upper track of Video 1 track and ending with Earth.mov in the lower track of the Video 1 track. The default transition, Cross Dissolve, has been inserted in the transition area between them.

You could also create a storyboard with the clips, use Automate to Timeline using the default transition, then modify or replace selected transitions as needed later. For additional information about creating a storyboard.